Metabolism of Lipids : Medical Biochemistry

                                                                     ? Chemistry & Definition of Lipids

• Definition: Lipids are a heterogeneous group of organic molecules that are insoluble in water (hydrophobic) but soluble in organic solvents like chloroform, ether, benzene. They include simple fats, oils, waxes, phospholipids, glycolipids, sterols, and their derivatives.

• Basic elements: Primarily carbon (C), hydrogen (H), and oxygen (O); sometimes nitrogen (N), phosphorus (P), and sulfur (S).

• Energy content: 1 g lipid provides ~9 kcal, compared to 4 kcal from carbohydrate/protein → makes them efficient energy stores.

Chemical Nature

• Lipids are esters of fatty acids with alcohols (e.g., glycerol, cholesterol, higher alcohols).

• Some are amphipathic molecules → contain both a hydrophilic head (polar, e.g., phosphate group) and a hydrophobic tail (non-polar fatty acid chain).

  • This amphipathic property is fundamental for membrane structure (phospholipid bilayer).
    

                                                                             ? Classification of Lipids

Lipids are classified into four main groups:

1. Simple Lipids

These are esters of fatty acids with various alcohols.

a) Neutral fats (Triglycerides / Triacylglycerols, TAGs):

• Formed from 1 glycerol + 3 fatty acids.

• Main storage form of energy in adipose tissue.

• Stored in anhydrous form (without water) → compact energy source.

• Adipose tissue functions as a dynamic reservoir for energy balance.

b) Waxes:

• Esters of fatty acids with long-chain monohydric alcohols (not glycerol).

• Found in protective coatings (skin, ear wax, plant cuticles).

• Biologically less important in humans.

2. Compound (Complex) Lipids

Contain additional chemical groups besides fatty acids + alcohol.

a) Phospholipids:

• Contain phosphate group + nitrogenous base in addition to glycerol/fatty acids.

• Glycerophospholipids: Lecithin (phosphatidylcholine), cephalin (phosphatidylethanolamine), phosphatidylserine.

• Sphingophospholipids: Sphingomyelin (important in myelin sheath).

• Functions:

  • Structural framework of membranes.

  • Surfactant (lecithin) in alveoli reduces surface tension.

  • Cell signaling (phosphatidylinositol → IP₃, DAG).

b) Glycolipids (Glycosphingolipids):

• Fatty acid + sphingosine backbone + carbohydrate moiety.

• Types: Cerebrosides, gangliosides.

• Found in brain and nerve tissue → critical in neuronal membranes and myelin.

c) Lipoproteins:

• Complexes of lipids + proteins (apolipoproteins).

• Function: Transport lipids in blood.

• Types: Chylomicrons, VLDL, LDL, HDL.

3. Derived Lipids

Products of hydrolysis of simple and complex lipids; include biologically active compounds.

• Fatty acids: Saturated (palmitic, stearic) and unsaturated (oleic, linoleic).

• Steroids: Cholesterol (precursor for steroid hormones, bile acids, vitamin D).

• Eicosanoids: Prostaglandins, thromboxanes, leukotrienes (derived from arachidonic acid, act as local hormones).

• Ketone bodies: Acetoacetate, β-hydroxybutyrate, acetone (produced in liver during fasting).

4. Miscellaneous Lipids

Do not fit in other categories but are biologically important.

• Terpenes: Carotenoids, dolichol, ubiquinone (Coenzyme Q).

• Fat-soluble vitamins: A, D, E, K.

? Biological Functions of Lipids

• Energy storage: TAGs store excess calories.

• Structural role: Phospholipids and glycolipids form biological membranes.

• Insulation & protection: Subcutaneous fat insulates; adipose cushions organs.

• Signaling molecules: Steroid hormones, prostaglandins, leukotrienes.

• Vitamin absorption: Fat is needed for absorption of fat-soluble vitamins (A, D, E, K).
  

                                                                  ? Applied Clinical Aspects of Lipids

1. Disorders of Fatty Acid Metabolism

• Carnitine deficiency: Prevents transport of fatty acids into mitochondria for β-oxidation → hypoketotic hypoglycemia, muscle weakness.

• MCAD deficiency: Inability to oxidize medium-chain fatty acids → hypoglycemia, sudden infant death syndrome (SIDS).

2. Disorders of Cholesterol and Lipoproteins

• Atherosclerosis: Excess LDL cholesterol deposition in arterial walls → CAD, stroke.

• Familial hypercholesterolemia: LDL receptor mutations → markedly elevated cholesterol and premature CAD.

• Abetalipoproteinemia: Deficiency of ApoB → absence of chylomicrons and VLDL → fat malabsorption, failure to thrive.

3. Inherited Sphingolipidoses (Lysosomal Storage Diseases)

• Tay–Sachs disease: Hexosaminidase A deficiency → GM2 ganglioside accumulation → neurodegeneration, “cherry-red spot” in retina.

• Gaucher disease: Glucocerebrosidase deficiency → lipid-laden macrophages (“crumpled tissue paper” appearance).

• Niemann–Pick disease: Sphingomyelinase deficiency → hepatosplenomegaly, neurodegeneration, “cherry-red spot.”

4. Essential Fatty Acid Deficiency

• Essential FAs: Linoleic acid (ω-6), α-linolenic acid (ω-3).

• Deficiency: Scaly dermatitis, alopecia, growth retardation, impaired wound healing.

5. Respiratory Distress Syndrome (RDS)

• Preterm neonates have low dipalmitoyl lecithin (phosphatidylcholine, a lung surfactant).

• Leads to alveolar collapse (atelectasis) → hypoxia.

• Management: Maternal corticosteroids antenatally, exogenous surfactant therapy.

6. Lipids in Nutrition and Disease

• High saturated fat & trans-fat → ↑ LDL → risk of CAD.

• Omega-3 fatty acids (EPA, DHA) → cardioprotective, anti-inflammatory.

• Lipid peroxidation by free radicals damages membranes → contributes to aging, neurodegeneration, atherosclerosis.

                                                                                          ? Introduction

• Sphingolipids are a specialized class of complex lipids that contain sphingosine as the backbone instead of glycerol.

• They are important components of cell membranes, especially in the brain and nervous tissue (myelin sheath).

• Apart from structural roles, they also function in cell signaling, cell–cell recognition, and act as receptors for toxins and viruses.
  

                                                                       ? Chemistry of Sphingolipids

Basic structure

• Backbone: Sphingosine (an amino alcohol with a long-chain hydrocarbon tail).

• Amide linkage: Fatty acid attached to sphingosine via an amide bond → forms ceramide (the parent compound).

• Different substituents at the C1 hydroxyl group of sphingosine define the type of sphingolipid.

Ceramide = Sphingosine + Fatty acid

• It acts as the basic building block for all sphingolipids.

? Classification of Sphingolipids

Sphingolipids are broadly classified into three groups depending on the head group attached to ceramide:

1. Sphingophospholipids

• Contain phosphate + choline or ethanolamine at C1.

• Main example: Sphingomyelin.

  • Ceramide + phosphocholine (or phosphoethanolamine).

  • Found in plasma membranes and abundantly in the myelin sheath of nerve fibers.

  • Function: Structural integrity, insulation of axons, signal transduction.

• Clinical correlation:

  • Niemann–Pick disease → deficiency of sphingomyelinase → accumulation of sphingomyelin → hepatosplenomegaly, neurodegeneration, cherry-red spot on retina.

2. Glycosphingolipids (Glycolipids)

• Contain carbohydrate group at C1 (instead of phosphate).

• Do not contain phosphate.

• Subdivided into:

a) Cerebrosides

• Ceramide + single sugar (glucose or galactose).

• Found in brain white matter and myelin sheath.

• Function: Cell recognition and insulation.

• Clinical: Gaucher disease (glucocerebrosidase deficiency), Krabbe disease (galactocerebrosidase deficiency).

b) Sulfatides

• Cerebroside + sulfate group.

• Found in myelin and kidney.

• Clinical: Metachromatic leukodystrophy (arylsulfatase A deficiency → sulfatide accumulation).

c) Globosides

• Ceramide + oligosaccharide (2–4 neutral sugars like glucose, galactose, N-acetylgalactosamine).

• Found in RBC membranes, kidneys, spleen.

• Involved in blood group antigens (P blood group system).

d) Gangliosides

• Ceramide + complex oligosaccharide + sialic acid (N-acetylneuraminic acid).

• Located mainly in gray matter of brain.

• Functions:

  • Cell-to-cell communication.

  • Act as receptors for toxins (e.g., cholera toxin, tetanus toxin, influenza virus).

• Clinical: Tay–Sachs disease (hexosaminidase A deficiency → GM2 ganglioside accumulation).

3. Other Derived Sphingolipids

• Ceramide itself has signaling functions → induces apoptosis and cell differentiation.

• Sphingosine-1-phosphate → regulates cell growth, survival, and immune responses.

                                                               ⚙️ Functions of Sphingolipids

• Structural role:

  • Integral components of cell membranes.

  • Provide stability and rigidity.

  • Form lipid rafts involved in signaling.

• Nervous system:

  • Myelin sheath contains sphingomyelin, cerebrosides, and sulfatides → necessary for nerve impulse conduction.

• Cell recognition & signaling:

  • Glycosphingolipids act as cell surface antigens (e.g., ABO blood group antigens are glycosphingolipids).

  • Receptors for toxins and viruses.

• Second messengers:

  • Ceramide and sphingosine derivatives regulate apoptosis, cell growth, immune function.

                                                    ? Clinical Aspects — Sphingolipidoses

Sphingolipidoses are a group of lysosomal storage disorders due to defective degradation of sphingolipids.

• Tay–Sachs disease:

  • Deficiency: Hexosaminidase A.

  • Accumulation: GM2 ganglioside.

  • Features: Neurodegeneration, developmental delay, cherry-red spot on macula, no hepatosplenomegaly.

• Gaucher disease:

  • Deficiency: Glucocerebrosidase.

  • Accumulation: Glucocerebroside.

  • Features: Hepatosplenomegaly, bone crises, “crumpled tissue paper” macrophages.

• Niemann–Pick disease:

  • Deficiency: Sphingomyelinase.

  • Accumulation: Sphingomyelin.

  • Features: Hepatosplenomegaly, neurodegeneration, cherry-red spot.

• Krabbe disease:

  • Deficiency: Galactocerebrosidase.

  • Accumulation: Galactocerebroside + psychosine.

  • Features: Peripheral neuropathy, developmental delay, optic atrophy.

• Metachromatic leukodystrophy:

  • Deficiency: Arylsulfatase A.

  • Accumulation: Sulfatides.

  • Features: Progressive demyelination, ataxia, dementia.

? High-Yield Summary

• Sphingolipids = sphingosine + fatty acid (ceramide backbone).

• Three groups:

  • Sphingophospholipids → sphingomyelin (myelin sheath).

  • Glycosphingolipids → cerebrosides, sulfatides, globosides, gangliosides.

  • Derivatives → ceramide, sphingosine-1-phosphate (cell signaling).

• Clinical significance: Sphingolipidoses (Tay–Sachs, Gaucher, Niemann–Pick, Krabbe, Metachromatic leukodystrophy).

• Functions: Structural integrity of membranes, nervous system insulation, receptors for pathogens, role in apoptosis and signaling.

                                                                                             ? Introduction

• Sphingolipidoses are a group of inherited lysosomal storage disorders caused by deficiency of enzymes required for the degradation of sphingolipids (glycolipids, sphingomyelin).

• Result → accumulation of partially degraded sphingolipids inside lysosomes of neurons, macrophages, and other tissues → progressive neurological and visceral dysfunction.

• Mode of inheritance: Mostly autosomal recessive, except Fabry disease (X-linked).
  

                                                                                        ? Biochemical Basis

• Sphingolipids normally undergo stepwise degradation in lysosomes by specific hydrolases.

• Deficiency of any of these enzymes → storage of undegraded product.

• This explains why each disease is associated with specific substrate accumulation and distinct clinical features.

                                                                                       ? Major Sphingolipidoses

1. Tay–Sachs Disease

• Defective enzyme: Hexosaminidase A.

• Accumulated substrate: GM₂ ganglioside.

• Organs affected: Neurons of brain and spinal cord, retina.

• Clinical features:

  • Onset in infancy.

  • Progressive neurodegeneration, developmental delay, hypotonia.

  • Exaggerated startle response.

  • Cherry-red spot on macula (classic).

  • Blindness, seizures, paralysis.

  • No hepatosplenomegaly (differentiates from Niemann–Pick).

• Fatal by 2–4 years.

• Mnemonic: “Tay–Sachs = hexosaminidase A, GM₂ ganglioside, cherry-red spot, no HSM.”

2. Gaucher Disease (most common GSD)

• Defective enzyme: Glucocerebrosidase (β-glucosidase).

• Accumulated substrate: Glucocerebroside.

• Organs affected: Macrophages in spleen, liver, bone marrow, CNS (in neuronopathic forms).

• Clinical features:

  • Hepatosplenomegaly.

  • Pancytopenia (due to bone marrow infiltration).

  • Bone crises, osteoporosis, avascular necrosis of femur.

  • Neurological involvement in Type II (acute infantile) and Type III (chronic juvenile).

• Histology: Gaucher cells — macrophages with lipid-laden cytoplasm, “crumpled tissue paper” appearance.

• Treatment: Enzyme replacement therapy.

3. Niemann–Pick Disease (Types A & B)

• Defective enzyme: Sphingomyelinase.

• Accumulated substrate: Sphingomyelin.

• Organs affected: Neurons, macrophages of liver, spleen, bone marrow, lungs.

• Clinical features:

  • Hepatosplenomegaly.

  • Neurodegeneration, developmental regression.

  • Cherry-red spot on retina (like Tay–Sachs, but with HSM).

  • Foam cells (lipid-laden macrophages).

• Type A: Severe, infantile onset, fatal by age 2–3.

• Type B: Milder, visceral involvement without CNS disease.

4. Krabbe Disease (Globoid Cell Leukodystrophy)

• Defective enzyme: Galactocerebrosidase (galactosylceramidase).

• Accumulated substrate: Galactocerebroside and psychosine.

• Organs affected: CNS white matter (myelin).

• Clinical features:

  • Onset in infancy.

  • Developmental delay, irritability.

  • Peripheral neuropathy (areflexia).

  • Optic atrophy → blindness.

  • Globoid cells (multinucleated macrophages filled with lipids).

5. Metachromatic Leukodystrophy

• Defective enzyme: Arylsulfatase A.

• Accumulated substrate: Sulfatides.

• Organs affected: CNS and peripheral nerves.

• Clinical features:

  • Progressive demyelination → motor regression, ataxia, hypotonia, dementia.

  • Peripheral neuropathy.

  • Histology: Sulfatide accumulation causes metachromatic staining in white matter.

6. Fabry Disease (X-linked recessive)

• Defective enzyme: α-galactosidase A.

• Accumulated substrate: Ceramide trihexoside (globotriaosylceramide).

• Clinical features:

  • Early: Angiokeratomas (skin lesions), acroparesthesia (burning pain in hands/feet), hypohidrosis (reduced sweating).

  • Late: Progressive renal failure, cardiovascular disease (LV hypertrophy, stroke).

• Treatment: Enzyme replacement therapy.

7. Farber Disease (rare)

• Defective enzyme: Ceramidase.

• Accumulated substrate: Ceramide.

• Clinical features:

  • Painful joint swelling, hoarseness.

  • Subcutaneous nodules.

  • Progressive neurodegeneration.
    

⚙️ Summary of Sphingolipidoses — Pattern Recognition

• Tay–Sachs → GM₂ ganglioside, Hexosaminidase A, cherry-red spot, no HSM.

• Niemann–Pick → Sphingomyelin, sphingomyelinase, cherry-red spot + HSM.

• Gaucher → Glucocerebroside, glucocerebrosidase, hepatosplenomegaly, bone pain, crumpled macrophages.

• Krabbe → Galactocerebroside, galactocerebrosidase, peripheral neuropathy, optic atrophy, globoid cells.

• Metachromatic leukodystrophy → Sulfatides, arylsulfatase A, demyelination, ataxia, dementia.

• Fabry → Ceramide trihexoside, α-galactosidase A, angiokeratomas, renal & cardiac failure (X-linked)

               ? Mobilization of Fatty Acids from Adipose Tissue, Carnitine Shuttle & Clinical Aspects

? Introduction

• Fatty acids are the major fuel reserve in the human body (stored as triacylglycerols, TAGs, in adipose tissue).

• Mobilization of fatty acids refers to the hydrolysis of TAGs into free fatty acids (FFAs) and glycerol, followed by their release into the blood.

• These FFAs then undergo transport into mitochondria (via the carnitine shuttle) for β-oxidation and ATP production.
  

                                               ? Mobilization of Fatty Acids from Adipose Tissue

Step 1 — Lipolysis of Triglycerides

• Stored form: TAGs in adipocytes.

• Hormone-Sensitive Lipase (HSL): Key enzyme that hydrolyzes TAGs → diacylglycerol → monoacylglycerol → glycerol + 3 FFAs.

• Monoacylglycerol lipase completes the last step.

Step 2 — Hormonal Regulation

• Stimulators of lipolysis:

  • Catecholamines (epinephrine, norepinephrine) → via β-adrenergic receptors → ↑ cAMP → activates Protein Kinase A (PKA) → phosphorylates and activates HSL.

  • Glucagon (fasting state).

  • Cortisol (chronic stress → increases lipolysis indirectly).

• Inhibitors of lipolysis:

  • Insulin: Activates phosphodiesterase → decreases cAMP → inactivates HSL. Promotes storage, not mobilization.

Step 3 — Transport in Blood

• FFAs are released from adipocytes into circulation.

• They bind albumin for transport (since FFAs are hydrophobic).

• Glycerol cannot be metabolized in adipose (lacks glycerol kinase) → transported to liver for gluconeogenesis or TAG synthesis.
  

                                                         ? Entry of Fatty Acids into Mitochondria

• β-oxidation occurs in the mitochondrial matrix.

• Long-chain fatty acids (LCFA, >12 carbons) cannot cross the inner mitochondrial membrane directly.

• They require a special transport system → Carnitine shuttle.

? Carnitine Shuttle — Stepwise

Step 1 — Activation of Fatty Acid

• Occurs in the cytosol/outer mitochondrial membrane.

• Enzyme: Fatty acyl-CoA synthetase (thiokinase).

• Reaction: Fatty acid + CoA + ATP → Fatty acyl-CoA + AMP + PPi.

• Requires 2 high-energy bonds (equivalent to 2 ATP).

Step 2 — Carnitine Palmitoyltransferase I (CPT-I)

• Located on the outer mitochondrial membrane.

• Transfers fatty acyl group from CoA → carnitine, forming fatty acylcarnitine.

• Inhibited by malonyl-CoA (first committed intermediate of fatty acid synthesis).

  • This prevents simultaneous fatty acid synthesis and degradation → reciprocal regulation.

Step 3 — Translocation across Inner Membrane

• Enzyme: Carnitine–acylcarnitine translocase.

• Transports fatty acylcarnitine into the mitochondrial matrix while exporting free carnitine back to cytosol (antiport system).

Step 4 — Carnitine Palmitoyltransferase II (CPT-II)

• Located on the inner mitochondrial membrane (matrix side).

• Transfers fatty acyl group back to CoA, regenerating fatty acyl-CoA.

• Carnitine is released and recycled back to cytosol via the translocase.

Step 5 — β-Oxidation

• Once inside the matrix, fatty acyl-CoA undergoes β-oxidation → sequential removal of 2-carbon units as acetyl-CoA.

• Acetyl-CoA then enters the TCA cycle or ketogenesis.

⚙️ Regulation of Fatty Acid Entry

• Rate-limiting step: Carnitine palmitoyltransferase I (CPT-I).

• Inhibited by malonyl-CoA (product of acetyl-CoA carboxylase in fatty acid synthesis).

• This ensures fatty acid oxidation is suppressed during active lipogenesis (fed state).

                                                                                      ? Clinical Aspects

1. Carnitine Deficiency

• Causes: Genetic defect, malnutrition, hemodialysis, liver disease, or increased loss in urine.

• Effects:

  • Impaired transport of long-chain fatty acids into mitochondria.

  • Reduced β-oxidation → ↓ ATP, ↓ ketone bodies.

  • Leads to hypoketotic hypoglycemia (since both gluconeogenesis and ketogenesis are impaired).

• Symptoms: Muscle weakness, cardiomyopathy, encephalopathy.

• Management: Carnitine supplementation, high-carbohydrate diet.

2. CPT-I Deficiency (liver type)

• Defect in CPT-I enzyme (outer mitochondrial membrane).

• Symptoms: Hypoketotic hypoglycemia during fasting (liver can’t oxidize fatty acids for gluconeogenesis/ketogenesis).

3. CPT-II Deficiency (muscle type)

• Defect in CPT-II enzyme (inner mitochondrial membrane).

• Symptoms: Muscle weakness, myoglobinuria following exercise, lipid accumulation in muscle.

• Precipitated by prolonged exercise, fasting, infections.

4. Medium-Chain Acyl-CoA Dehydrogenase (MCAD) Deficiency (β-oxidation defect, not shuttle)

• Most common inherited defect in fatty acid oxidation.

• Can mimic carnitine deficiency because β-oxidation cannot proceed.

• Symptoms: Hypoketotic hypoglycemia, vomiting, lethargy, sudden infant death.

? High-Yield Exam Summary

• Mobilization: TAG → FA + glycerol via hormone-sensitive lipase (stimulated by glucagon, epinephrine; inhibited by insulin).

• Transport: FA travel in plasma bound to albumin.

• Carnitine shuttle:

  • Activation (FA → acyl-CoA).

  • CPT-I (inhibited by malonyl-CoA).

  • Translocase.

  • CPT-II.

• Clinical: Carnitine deficiency, CPT-I and II deficiency, MCAD deficiency → all cause hypoketotic hypoglycemia.

                                                                                        ? Introduction

• Lipids in diet are mainly triacylglycerols (TAGs) (90–95%), with smaller amounts of phospholipids, cholesterol esters, and fat-soluble vitamins (A, D, E, K).

• Unlike carbohydrates and proteins, lipids are hydrophobic and insoluble in water.

• Their digestion and absorption require emulsification by bile salts and specialized enzymes (lipases) to overcome their insolubility.

                                                                           ? Sources of Dietary Lipids

• Triglycerides → major energy source.

• Phospholipids → structural.

• Cholesterol & cholesterol esters → membrane and steroid precursor.

• Fat-soluble vitamins → absorbed along with dietary fats.

                                                                                      ? Digestion of Lipids

1. Digestion in the Mouth & Stomach (Minimal)

• Lingual lipase (secreted by glands of tongue) and gastric lipase (chief cells of stomach).

• Hydrolyze TAGs (mainly short- and medium-chain fatty acids, e.g., in milk).

• Important in infants (who consume milk fat).

• Contribution: ~10–20% of lipid digestion.

2. Digestion in the Small Intestine (Major Site)

a) Emulsification

• Bile salts (secreted from liver, stored in gallbladder) are released into duodenum in response to cholecystokinin (CCK).

• They emulsify large fat globules into small droplets → increases surface area for enzyme action.

b) Pancreatic Lipase

• Main enzyme for triglyceride digestion.

• Requires a cofactor colipase (secreted as pro-colipase, activated by trypsin).

• Action: Hydrolyzes TAGs → 2 free fatty acids + 2-monoacylglycerol.

• Inhibited by bile salts, but colipase prevents this inhibition.

c) Phospholipase A₂

• Secreted as proenzyme, activated by trypsin.

• Hydrolyzes phospholipids → lysophospholipids + free fatty acid.

d) Cholesteryl Esterase (Nonspecific Lipase)

• Hydrolyzes cholesterol esters → free cholesterol + fatty acid.

• Also acts on TAGs and phospholipids to some extent.

? Products of Lipid Digestion

• Free fatty acids (long-, medium-, and short-chain).

• 2-monoacylglycerols.

• Free cholesterol.

• Lysophospholipids.

• Fat-soluble vitamins.

These products are hydrophobic and must be packaged into micelles for absorption.

                                                                                 ? Absorption of Lipids

1. Micelle Formation

• Bile salts surround the products of digestion → form mixed micelles (tiny water-soluble aggregates).

• Micelles carry fatty acids, cholesterol, and monoglycerides through the unstirred water layer of intestinal mucosa.

• Essential for absorption of long-chain fatty acids and fat-soluble vitamins.

• Note: Short- and medium-chain fatty acids (≤12 carbons) do not need micelles; they are absorbed directly into portal blood.

2. Uptake into Enterocytes

• Micelles deliver lipid components to the brush border membrane of enterocytes (mainly jejunum).

• Lipid components diffuse into the enterocytes, while bile salts remain in the lumen and are reabsorbed later in the ileum (enterohepatic circulation).

3. Re-Esterification in Enterocytes

• Inside the enterocyte, absorbed products are reassembled:

  • Long-chain fatty acids + glycerol → TAGs.

  • Cholesterol + fatty acids → cholesteryl esters.

  • Lysophospholipids + fatty acids → phospholipids.

4. Chylomicron Formation

• TAGs, cholesteryl esters, and phospholipids are packaged with apolipoprotein B-48 into chylomicrons.

• Chylomicrons are exocytosed into lacteals (lymphatic vessels) → thoracic duct → systemic circulation.

• Function: Deliver dietary TAGs to adipose and muscle (via lipoprotein lipase).

5. Transport of Medium- and Short-Chain Fatty Acids

• Absorbed directly into portal blood bound to albumin.

• Bypasses chylomicron pathway.

⚙️ Hormonal Regulation of Lipid Digestion

• Cholecystokinin (CCK):

  • Released from duodenal mucosa in response to fat and protein.

  • Stimulates gallbladder contraction → bile secretion.

  • Stimulates pancreas to release digestive enzymes.

  • Slows gastric emptying.

• Secretin:

  • Released in response to acidic chyme.

  • Stimulates pancreas to release bicarbonate → neutralizes acid, providing optimal pH for lipases.

                                                                                   ? Clinical Aspects

1. Steatorrhea

• Excess fat in stool → bulky, greasy, foul-smelling stool.

• Causes:

  • Pancreatic insufficiency (chronic pancreatitis, cystic fibrosis) → ↓ lipase/colipase.

  • Bile salt deficiency (cholestasis, liver disease, gallstones, ileal resection).

  • Intestinal mucosal damage (celiac disease, tropical sprue).

• Consequences: Loss of fat-soluble vitamins (A, D, E, K).

2. Bile Salt Deficiency

• Seen in cholestasis (bile duct obstruction), ileal disease (Crohn’s, ileal resection).

• Leads to poor micelle formation → impaired absorption of fats and fat-soluble vitamins.

3. Cystic Fibrosis

• Mutation in CFTR → thick secretions → blockage of pancreatic ducts → ↓ delivery of pancreatic enzymes and bicarbonate → lipid malabsorption.

• Treatment: Pancreatic enzyme replacement, fat-soluble vitamin supplementation.

4. Abetalipoproteinemia

• Rare genetic disorder.

• Defect in ApoB-48 → failure to form chylomicrons → fat malabsorption, steatorrhea, failure to thrive, neurological symptoms (due to vitamin E deficiency).

                                                                                           ? Introduction

• Triacylglycerols (TAGs) = storage form of fat, made of glycerol + 3 fatty acids.

• TAGs are the major energy reserve in humans:

  • Highly reduced hydrocarbon chains → yield more ATP per gram than carbohydrates.

  • Stored in anhydrous form → compact energy storage.

• Sites of TAG synthesis:

  • Liver (major site).

  • Adipose tissue (storage depot).

  • Intestinal mucosa (during absorption, TAGs are resynthesized for chylomicrons).
    

                                                                                      ? Precursors Needed

1. Glycerol-3-phosphate

   • Backbone for TAG synthesis.

   • Sources:

     • From dihydroxyacetone phosphate (DHAP) in glycolysis (via glycerol-3-phosphate dehydrogenase).

     • From glycerol (via glycerol kinase, only in liver and kidney).

     • Adipose tissue cannot use free glycerol (lacks glycerol kinase) → depends on glycolysis for DHAP → G3P.

2. Activated fatty acids (Fatty acyl-CoA)

   • Formed by acyl-CoA synthetase (thiokinase).

   • Requires ATP → AMP + PPi.
     

                                     ? Steps in TAG Synthesis (via Glycerol-3-Phosphate Pathway)

Step 1 — Formation of Lysophosphatidic Acid (Monoacylglycerol-3-phosphate)

• Enzyme: Glycerol-3-phosphate acyltransferase.

• Reaction: G3P + fatty acyl-CoA → Lysophosphatidic acid.

• First fatty acid is usually added at sn-1 position of glycerol.

Step 2 — Formation of Phosphatidic Acid (Diacylglycerol-3-phosphate)

• Enzyme: 1-acylglycerol-3-phosphate acyltransferase.

• Reaction: Lysophosphatidic acid + fatty acyl-CoA → Phosphatidic acid.

• Phosphatidic acid = common intermediate for both TAG synthesis and glycerophospholipid synthesis.

Step 3 — Formation of Diacylglycerol (DAG)

• Enzyme: Phosphatidic acid phosphatase (lipin).

• Removes phosphate group from phosphatidic acid → DAG.

Step 4 — Formation of Triacylglycerol (TAG)

• Enzyme: Diacylglycerol acyltransferase (DGAT).

• Reaction: DAG + fatty acyl-CoA → Triacylglycerol.

                                                                         ? Fate of Newly Synthesized TAGs

• In Liver:

  • TAGs are packaged into very-low-density lipoproteins (VLDL) with apolipoproteins, cholesterol, and phospholipids.

  • Secreted into blood → deliver TAGs to peripheral tissues (muscle, adipose).

• In Adipose tissue:

  • TAGs are stored as large lipid droplets.

  • Storage regulated by insulin (promotes synthesis, inhibits lipolysis).

• In Intestinal mucosa:

  • Absorbed dietary TAGs are resynthesized from 2-monoacylglycerol + fatty acids → packaged into chylomicrons → lymph → blood.

                                                                     ⚙️ Regulation of TAG Synthesis

• Hormonal regulation:

  • Insulin: Promotes TAG synthesis in both liver and adipose tissue.

    • Increases glucose uptake (via GLUT-4 in adipose).

    • Activates acetyl-CoA carboxylase → ↑ malonyl-CoA → ↑ fatty acid synthesis.

    • Provides glycerol-3-phosphate from glycolysis.

  • Glucagon & Epinephrine: Inhibit TAG synthesis (favor lipolysis and fatty acid oxidation).

• Nutritional state:

  • Fed state: High TAG synthesis (excess carbohydrate converted to fat).

  • Fasting state: TAG synthesis suppressed; lipolysis predominates.

                                                                                ? Clinical Aspects

1. Fatty Liver (Hepatic Steatosis)

• Excessive TAG accumulation in liver.

• Causes:

  • ↑ Fatty acid delivery (e.g., uncontrolled diabetes, starvation, alcoholism).

  • ↓ VLDL secretion (e.g., protein malnutrition, Kwashiorkor, abetalipoproteinemia).

• Alcohol → ↑ NADH/NAD⁺ ratio → impairs fatty acid oxidation → TAG accumulation.

2. Obesity & Insulin Resistance

• Chronic positive energy balance → increased TAG storage in adipose tissue.

• Insulin resistance → dysregulated TAG metabolism → hypertriglyceridemia, metabolic syndrome.

3. Abetalipoproteinemia

• Deficiency of ApoB-48 & ApoB-100.

• Chylomicrons and VLDL not formed → fat malabsorption, acanthocytosis, failure to thrive, neurological problems.

4. Tangier Disease

• Rare genetic disorder → defective HDL formation, indirectly affecting TAG mobilization and cholesterol transport.

5. Lipodystrophies

• Abnormal distribution of TAGs in body due to defects in TAG synthesis, storage, or mobilization.

• Can cause insulin resistance, diabetes, and fatty liver.

? High-Yield Summary

• TAG synthesis requires glycerol-3-phosphate + fatty acyl-CoA.

• Pathway: G3P → lysophosphatidic acid → phosphatidic acid → DAG → TAG.

• Phosphatidic acid is a common intermediate for TAG and phospholipid synthesis.

• Liver exports TAGs in VLDL; adipose stores TAGs; intestine re-forms TAGs for chylomicrons.

• Insulin promotes TAG synthesis; glucagon/epinephrine inhibit it.

• Clinical relevance: Fatty liver, obesity, abetalipoproteinemia, lipodystrophies.

                                                                                       ? Introduction

• Fatty acid synthesis is the anabolic pathway in which fatty acids are formed from acetyl-CoA.

• Occurs mainly in the cytosol (contrast: β-oxidation in mitochondria).

• Site of synthesis: Liver (major), adipose tissue, lactating mammary gland, kidney cortex.

• Provides palmitic acid (16:0) as the primary product, which can be further elongated and desaturated.

                                                          ? Requirements for Fatty Acid Synthesis

1. Acetyl-CoA (main building block).

   • Generated in mitochondria (from pyruvate via PDH, or β-oxidation).

   • Transported to cytosol as citrate shuttle.

2. NADPH (reducing power).

   • Mainly from the pentose phosphate pathway (PPP).

   • Also from the malic enzyme reaction: Malate → pyruvate + CO₂ + NADPH.

3. ATP (energy requirement).
   

                                            ? Transport of Acetyl-CoA from Mitochondria to Cytosol

• Acetyl-CoA cannot cross the mitochondrial membrane directly.

• Transported as citrate:

  • In mitochondria: Acetyl-CoA + OAA → Citrate (citrate synthase).

  • Citrate crosses to cytosol.

  • In cytosol: Citrate → Acetyl-CoA + OAA (via ATP-citrate lyase, requires ATP).

• OAA is recycled back to pyruvate (via malate), producing NADPH via malic enzyme.

                                                        ? Stepwise Reactions of Fatty Acid Synthesis

Step 1 — Carboxylation of Acetyl-CoA → Malonyl-CoA

• Enzyme: Acetyl-CoA carboxylase (ACC).

• Cofactor: Biotin (vitamin B₇).

• Reaction: Acetyl-CoA + CO₂ + ATP → Malonyl-CoA.

• Committed / Rate-limiting step of fatty acid synthesis.

• Regulation:

  • Activated by citrate, insulin.

  • Inhibited by palmitoyl-CoA, glucagon, AMP-activated kinase.

Step 2 — Fatty Acid Synthase (FAS) Complex

• A large multienzyme complex with acyl carrier protein (ACP).

• Catalyzes elongation of fatty acid chain by sequential addition of 2-carbon units from malonyl-CoA.

• Uses NADPH for reduction steps.

Cycle of Reactions (Repeated until 16 carbons = Palmitate):

1. Priming: Acetyl group attached to FAS.

2. Condensation: Acetyl group condenses with malonyl-CoA → β-ketoacyl intermediate.

3. Reduction: NADPH reduces keto group → hydroxyacyl.

4. Dehydration: Removal of H₂O → double bond.

5. Second reduction: NADPH reduces double bond → saturated acyl group.

6. Chain elongation: Cycle repeats, each time adding 2 carbons from malonyl-CoA.

• Final product: Palmitate (C16:0).

? Further Modifications of Palmitate

• Elongation: In endoplasmic reticulum & mitochondria, adding 2 carbons at a time (e.g., palmitate → stearate).

• Desaturation: By desaturases in ER (insert double bonds at specific positions: Δ⁴, Δ⁵, Δ⁶, Δ⁹).

  • Humans cannot introduce double bonds beyond C-9 → essential fatty acids (linoleic, linolenic acids) must be obtained from diet.
    

                                                             ⚙️ Regulation of Fatty Acid Synthesis

1. Enzyme Level (ACC)

• Allosteric regulation:

  • Citrate activates ACC (indicates high energy & substrate availability).

  • Long-chain acyl-CoA (palmitoyl-CoA) inhibits.

• Covalent modification:

  • Insulin → activates phosphatase → dephosphorylates & activates ACC.

  • Glucagon/epinephrine → activate AMPK → phosphorylates & inactivates ACC.

2. Substrate Availability

• Cytosolic acetyl-CoA & NADPH levels control synthesis rate.

3. Hormonal Regulation

• Insulin stimulates synthesis (fed state).

• Glucagon & epinephrine inhibit (fasting/exercise).

                                                                                     ? Clinical Aspects

1. Obesity and Insulin Resistance

• Chronic high insulin → ↑ fatty acid synthesis → fat accumulation.

• Dysregulated ACC activity contributes to fatty liver and metabolic syndrome.

2. Fatty Liver (Hepatic Steatosis)

• Alcohol metabolism → ↑ NADH/NAD⁺ ratio → blocks β-oxidation → promotes fatty acid synthesis.

• Excess TAGs accumulate in hepatocytes.

3. Essential Fatty Acid Deficiency

• Linoleic (ω-6) and α-linolenic (ω-3) acids are essential precursors for arachidonic acid (prostaglandins).

• Deficiency → scaly dermatitis, alopecia, poor wound healing, growth retardation.

4. Cancer Metabolism (Lipogenesis Upregulation)

• Rapidly dividing tumor cells upregulate fatty acid synthase (FAS) to support membrane biosynthesis.

• FAS inhibitors are being studied as anticancer drugs.
  

                                                                     ? High-Yield Exam Summary

• Site: Cytosol of liver, adipose, lactating mammary gland.

• Key precursors: Acetyl-CoA (from citrate shuttle) + NADPH (from PPP & malic enzyme).

• Rate-limiting enzyme: Acetyl-CoA carboxylase (ACC).

• Main product: Palmitate (C16:0).

• Regulation:

  • ↑ by citrate, insulin.

  • ↓ by palmitoyl-CoA, glucagon, epinephrine, AMPK.

• Clinical: Fatty liver, obesity, EFA deficiency, cancer lipogenesis.

                                                                                         ? Introduction

• De novo fatty acid synthesis in humans produces palmitic acid (C16:0) as the primary end product.

• To generate the wide variety of fatty acids found in membranes and lipids, palmitate can undergo:

  1. Chain elongation (addition of carbons → longer fatty acids).

  2. Desaturation (introduction of double bonds → unsaturated fatty acids).

These processes occur mainly in the endoplasmic reticulum (ER), with some elongation also in the mitochondria.

                                                                            ? Fatty Acid Chain Elongation

Sites

• Endoplasmic Reticulum (ER) → major site.

• Mitochondria → limited elongation.

Mechanism in ER

• Uses malonyl-CoA as the 2-carbon donor.

• Steps are similar to fatty acid synthase reactions:

  • Condensation of fatty acyl-CoA with malonyl-CoA.

  • Reduction (NADPH-dependent).

  • Dehydration.

  • Reduction again.

• Repeats until fatty acid reaches desired length.

Mitochondrial Elongation

• Uses acetyl-CoA instead of malonyl-CoA as 2-carbon donor.

• Operates mainly for maintaining proper chain length during energy metabolism.

Products

• Stearic acid (C18:0) → common elongation product.

• Very long-chain fatty acids (VLCFAs, >22 carbons) → important in brain lipids, myelin sheath, and sphingolipids.
  

                                                                         ? Fatty Acid Desaturation

Definition

• Process of introducing cis double bonds into saturated fatty acyl chains.

• Makes fatty acids more fluid and biologically versatile.

Site

• Endoplasmic Reticulum.

Enzyme system: Fatty acyl-CoA desaturase

• Requires:

  • Oxygen (O₂).

  • NADH.

  • Cytochrome b₅ and cytochrome b₅ reductase.

• Reaction:
  Fatty acyl-CoA + NADH + O₂ → Monounsaturated fatty acyl-CoA + NAD⁺ + H₂O.

Double bond positions

• Humans can introduce double bonds only at the Δ⁴, Δ⁵, Δ⁶, and Δ⁹ positions (counting from the carboxyl end).

• Cannot introduce double bonds beyond carbon 9 (i.e., no Δ¹² or Δ¹⁵).
  

                                                                         ⚠️ Essential Fatty Acids

• Because humans cannot introduce double bonds beyond C-9, we cannot synthesize linoleic acid (18:2, Δ9,12, ω-6) and α-linolenic acid (18:3, Δ9,12,15, ω-3).

• These are essential fatty acids (EFAs) that must be obtained from diet.

• EFAs are precursors of arachidonic acid (20:4, ω-6) and eicosanoids (prostaglandins, thromboxanes, leukotrienes).

? Biological Importance

• Chain elongation: Produces VLCFAs for myelin and sphingolipids.

• Desaturation: Provides fluidity to membranes, generates monounsaturated fatty acids (like oleic acid).

• Essential fatty acids: Critical for cell membranes, growth, neural development, inflammatory mediators.

                                                                                 ? Clinical Aspects

1. Essential Fatty Acid Deficiency

• Rare, but seen in premature infants, parenteral nutrition without EFAs, malnutrition.

• Features: Scaly dermatitis, alopecia, poor wound healing, impaired growth, thrombocytopenia.

2. Disorders of VLCFA Metabolism

• Adrenoleukodystrophy (X-linked): Defective peroxisomal β-oxidation → accumulation of VLCFAs in adrenal glands, white matter of brain → neurodegeneration, adrenal insufficiency.

3. Neurological Disorders

• VLCFA elongation defects affect myelination → developmental delay, neuropathy.

4. Cardiovascular Health

• ω-3 fatty acids (EPA, DHA) → anti-inflammatory, reduce triglycerides, cardioprotective.

• ω-6 fatty acids → pro-inflammatory (arachidonic acid-derived eicosanoids).

• Balance of ω-3/ω-6 is important for cardiovascular risk.

? High-Yield Summary

• Elongation: Occurs in ER (malonyl-CoA donor) and mitochondria (acetyl-CoA donor). Produces stearate and VLCFAs.

• Desaturation: Occurs in ER via fatty acyl-CoA desaturase, requires O₂ and NADH.

• Double bond limits: Humans cannot desaturate beyond C-9.

• Essential fatty acids: Linoleic and α-linolenic acids must be obtained from diet.

• Clinical links: EFA deficiency → dermatitis, growth failure; VLCFA metabolism defects → adrenoleukodystrophy; ω-3 fatty acids cardioprotective.

                                                                                    ? Introduction

• β-Oxidation = the catabolic pathway by which fatty acids are degraded in mitochondria to generate acetyl-CoA, NADH, and FADH₂.

• Called “β-oxidation” because oxidation occurs at the β-carbon (C-3) of the fatty acyl chain.

• Main site: Mitochondrial matrix (in most tissues, especially liver, heart, skeletal muscle).

• End product: Acetyl-CoA, which enters TCA cycle or is used for ketone body synthesis.

                                                                    ? Prerequisites for β-Oxidation

1. Activation of Fatty Acid

• Occurs in cytosol (outer mitochondrial membrane).

• Enzyme: Acyl-CoA synthetase (thiokinase).

• Reaction: FA + CoA + ATP → Fatty acyl-CoA + AMP + PPi.

• Cost: 2 high-energy phosphate bonds (equivalent to 2 ATP).

1. Transport into Mitochondria (for long-chain FA)

• Via carnitine shuttle:

  • CPT-I (outer membrane) → acyl-carnitine.

  • Translocase (inner membrane) → transport.

  • CPT-II (matrix side) → regenerates acyl-CoA.

• Rate-limiting step: CPT-I (inhibited by malonyl-CoA).

                                                           ? Steps of β-Oxidation (One Cycle = 2C removal)

Each cycle shortens the fatty acyl-CoA by two carbons and generates:

• 1 NADH

• 1 FADH₂

• 1 Acetyl-CoA

Step 1 — Oxidation (Dehydrogenation)

• Enzyme: Acyl-CoA dehydrogenase (specific isoforms: VLCAD, MCAD, SCAD).

• Reaction: Fatty acyl-CoA → trans-Δ²-enoyl-CoA.

• Coenzyme: FAD → FADH₂ (goes to ETC → 2 ATP).

Step 2 — Hydration

• Enzyme: Enoyl-CoA hydratase.

• Reaction: trans-Δ²-enoyl-CoA → L-β-hydroxyacyl-CoA.

• Adds water across the double bond.

Step 3 — Oxidation (Dehydrogenation)

• Enzyme: β-hydroxyacyl-CoA dehydrogenase.

• Reaction: L-β-hydroxyacyl-CoA → β-ketoacyl-CoA.

• Coenzyme: NAD⁺ → NADH + H⁺ (goes to ETC → 3 ATP).

Step 4 — Thiolysis

• Enzyme: β-ketothiolase.

• Reaction: β-ketoacyl-CoA + CoA → Acetyl-CoA + Fatty acyl-CoA (shortened by 2 carbons).

• The shortened fatty acyl-CoA re-enters the cycle.
  

                                               ? Overall Reaction (Palmitic Acid Example: C16:0)

Palmitoyl-CoA + 7 FAD + 7 NAD⁺ + 7 CoA + 7 H₂O → 8 Acetyl-CoA + 7 FADH₂ + 7 NADH + 7 H⁺

• For palmitic acid (16C):

  • 7 cycles of β-oxidation.

  • Yields 8 acetyl-CoA.

  • Produces 7 FADH₂ and 7 NADH.

⚡ Energetics of β-Oxidation (Palmitate, 16C)

• 7 FADH₂ → 14 ATP (2 ATP each).

• 7 NADH → 21 ATP (3 ATP each).

• 8 Acetyl-CoA → 8 × 12 = 96 ATP (via TCA cycle).

• Total = 131 ATP.

• Minus 2 ATP (activation cost) → 129 ATP net from one palmitate.

? Thus, fatty acids are very energy-rich compared to glucose.

                                                                            ⚙️ Regulation of β-Oxidation

• Substrate availability: Fatty acids mobilized from adipose (HSL activity).

• Transport regulation: CPT-I inhibited by malonyl-CoA (ensures FA oxidation doesn’t occur simultaneously with FA synthesis).

• Hormonal regulation:

  • Insulin → inhibits lipolysis & β-oxidation.

  • Glucagon/epinephrine → promote lipolysis → increase β-oxidation.

                                                                                  ? Clinical Aspects

1. Carnitine Deficiency

• Defective shuttle → impaired long-chain FA oxidation.

• Symptoms: Hypoketotic hypoglycemia, muscle weakness, cardiomyopathy.

• Treatment: Carnitine supplementation, avoid fasting.

2. CPT-I Deficiency (liver)

• Hypoglycemia, hypoketonemia (liver cannot oxidize FA to support gluconeogenesis).

3. CPT-II Deficiency (muscle)

• Muscle pain, weakness, myoglobinuria after exercise.

4. MCAD Deficiency (Medium-chain acyl-CoA dehydrogenase)

• Most common inherited FA oxidation defect.

• Inability to oxidize medium-chain FAs.

• Symptoms: Severe fasting hypoketotic hypoglycemia, vomiting, lethargy, seizures, sudden infant death.

• Treatment: Avoid fasting, give frequent carbohydrates.

5. Reye’s Syndrome (linked with β-oxidation impairment)

• Rare but severe pediatric illness (often after aspirin in viral illness).

• Involves mitochondrial dysfunction → impaired β-oxidation → hypoglycemia + encephalopathy.

? High-Yield Exam Summary

• Location: Mitochondrial matrix.

• Rate-limiting transport step: CPT-I (inhibited by malonyl-CoA).

• Each β-oxidation cycle → 1 FADH₂ (2 ATP), 1 NADH (3 ATP), 1 acetyl-CoA (12 ATP via TCA).

• Palmitate (C16) → 129 ATP net.

• Clinical: Carnitine deficiency, CPT-I/CPT-II deficiency, MCAD deficiency → all cause hypoketotic hypoglycemia.

                                                                                    ? Introduction

• The main pathway of fatty acid degradation is β-oxidation in the mitochondria.

• But when fatty acids are branched, very long, or when β-oxidation is defective, alternative oxidation pathways are used:

  1. α-Oxidation → for branched-chain fatty acids.

  2. ω-Oxidation → microsomal (ER) oxidation producing dicarboxylic acids.

These are clinically important because defects cause neurological disease, metabolic acidosis, and dicarboxylic aciduria.

                                                           ? Oxidation of Branched-Chain Fatty Acids

Why needed?

• Certain dietary fatty acids (e.g., phytanic acid) are highly branched at the β-carbon.

• Normal β-oxidation cannot proceed because the β-carbon is substituted.

• Instead, α-oxidation (removal of one carbon from the carboxyl end) makes the chain suitable for subsequent β-oxidation.

Phytanic Acid Metabolism (α-Oxidation)

• Source: Phytol side chain of chlorophyll (dairy products, ruminant fats, some fish).

• Step 1: Phytanic acid → phytanoyl-CoA (activation).

• Step 2: Phytanoyl-CoA → pristanic acid (via phytanoyl-CoA hydroxylase, introduces hydroxyl group at α-carbon → decarboxylation).

• Step 3: Pristanic acid undergoes β-oxidation in peroxisomes, releasing acetyl-CoA and propionyl-CoA.

? Clinical Aspect — Refsum Disease

• Defect: Autosomal recessive deficiency of phytanoyl-CoA hydroxylase.

• Result: Accumulation of phytanic acid in plasma and tissues.

• Features:

  • Progressive neurological dysfunction.

  • Retinitis pigmentosa (night blindness → progressive vision loss).

  • Peripheral neuropathy.

  • Ataxia, hearing loss.

  • Ichthyosis (scaly skin).

• Treatment: Restrict dietary phytanic acid (avoid dairy, beef, green vegetables).
  

                                                           ? ω (Omega)-Oxidation of Fatty Acids

Definition

• An alternate minor pathway of fatty acid oxidation.

• Occurs in the endoplasmic reticulum (ER) of liver and kidney.

• Involves oxidation of the ω-carbon (the terminal methyl group, far end of fatty acid).

Steps

1. ω-hydroxylation:

   • Enzyme: Mixed-function oxidase (uses NADPH, O₂, cytochrome P450).

   • Adds hydroxyl group (–OH) to ω-carbon → forms ω-hydroxy fatty acid.

2. Dehydrogenation:

   • ω-hydroxy group → oxidized to aldehyde → further oxidized to carboxylic acid.

3. End product: Dicarboxylic acid (–COOH at both ends).

4. Fate: These dicarboxylic acids undergo β-oxidation in mitochondria or peroxisomes, producing shorter-chain dicarboxylic acids → excreted in urine.

Physiological Role

• Normally minor.

• Becomes important when β-oxidation is impaired (e.g., carnitine deficiency, CPT deficiency, MCAD deficiency).

• Acts as a rescue pathway for fatty acid metabolism.

? Clinical Aspect — Dicarboxylic Aciduria

• Seen in disorders of β-oxidation (e.g., MCAD deficiency).

• Because fatty acids are diverted to ω-oxidation, leading to accumulation of medium-chain dicarboxylic acids in blood and urine.

• Clinical picture: Hypoketotic hypoglycemia + dicarboxylic aciduria.

? High-Yield Summary

• α-Oxidation: Used for branched-chain fatty acids (phytanic acid).

  • Defect → Refsum disease (accumulation of phytanic acid, neuro + eye symptoms).

• ω-Oxidation: Microsomal ER pathway, produces dicarboxylic acids.

  • Increased in β-oxidation defects → dicarboxylic aciduria.

• Clinical clues:

  • Refsum disease → retinitis pigmentosa, neuropathy, ichthyosis.

  • MCAD deficiency → hypoketotic hypoglycemia + dicarboxylic aciduria.

                                                                                        ? Introduction

• Even-chain fatty acids undergo β-oxidation → produce only acetyl-CoA.

• Odd-chain fatty acids (found in dairy fat, ruminant fats, some plants, marine lipids) yield acetyl-CoA + propionyl-CoA.

• Very long-chain fatty acids (VLCFAs, >22 carbons) are oxidized in peroxisomes, because mitochondria cannot handle such long chains directly.

Both pathways have unique clinical importance.

? Oxidation of Odd-Numbered Fatty Acids

Steps (same as β-oxidation until the last cycle)

1. β-oxidation proceeds as usual → sequential removal of 2C units as acetyl-CoA.

2. Final 3C fragment left = propionyl-CoA.
   

                                  Conversion of Propionyl-CoA to Succinyl-CoA (Anaplerotic Pathway)

Step 1: Propionyl-CoA → D-methylmalonyl-CoA

• Enzyme: Propionyl-CoA carboxylase

• Cofactor: Biotin (Vit B7)

• Requires ATP.

Step 2: D-methylmalonyl-CoA → L-methylmalonyl-CoA

• Enzyme: Racemase.

Step 3: L-methylmalonyl-CoA → Succinyl-CoA

• Enzyme: Methylmalonyl-CoA mutase

• Cofactor: Vitamin B₁₂ (adenosylcobalamin).

Final fate: Succinyl-CoA enters TCA cycle (gluconeogenic potential).

⚙️ Significance

• Odd-chain fatty acids are glucogenic (via succinyl-CoA) + ketogenic (via acetyl-CoA).

• This is in contrast to even-chain fatty acids, which are only ketogenic.

? Clinical Aspect — Vitamin B₁₂ Deficiency

• Methylmalonyl-CoA → accumulates → converted to methylmalonic acid.

• Leads to methylmalonic aciduria and metabolic acidosis.

• Neurological manifestations (demyelination, neuropathy) due to abnormal fatty acid incorporation into neuronal lipids.
  

                                       ? Oxidation of Very Long Chain Fatty Acids (VLCFAs, >22C)

Site

• Occurs mainly in peroxisomes.

• Shortened products (medium-chain acyl-CoAs) are then transported to mitochondria for further β-oxidation.

Steps

1. VLCFAs transported into peroxisomes via ABC transporters.

2. Oxidation is similar to β-oxidation but with some differences:

   • First oxidation: Catalyzed by acyl-CoA oxidase, transfers electrons to O₂ → forms H₂O₂ (not FADH₂).

   • H₂O₂ is detoxified by catalase.

   • No ATP generated directly.

3. Shortened fatty acyl-CoA chains are exported to mitochondria for complete oxidation.

⚙️ Special Case: Branched-Chain and Unusual Fatty Acids

• Also oxidized in peroxisomes.

• Example: Phytanic acid (α-oxidation, Refsum disease).

                                                                ? Clinical Aspects of VLCFA Oxidation

1. Zellweger Syndrome (Peroxisome Biogenesis Disorder)

• Absence or malfunction of peroxisomes.

• VLCFAs and branched-chain fatty acids accumulate.

• Features: Hypotonia, seizures, craniofacial dysmorphism, hepatomegaly, early death in infancy.

2. X-linked Adrenoleukodystrophy (ALD)

• Defective peroxisomal ABC transporter → impaired VLCFA import into peroxisomes.

• VLCFA accumulation in adrenal cortex and white matter of brain.

• Features: Progressive neurodegeneration, adrenal insufficiency, behavioral changes.

• Often presents in boys, adolescence to adulthood.

3. Refsum Disease

• Deficiency of phytanoyl-CoA hydroxylase (α-oxidation defect).

• Accumulation of phytanic acid → retinitis pigmentosa, neuropathy, ichthyosis.

                                                                                  ? Introduction

• Fatty acid oxidation (mainly β-oxidation in mitochondria) is a major source of ATP during fasting, exercise, and stress.

• It must be tightly regulated to:

  1. Prevent futile cycling (oxidation & synthesis occurring together).

  2. Match energy supply with demand.

  3. Coordinate with carbohydrate metabolism.

Regulation occurs at multiple levels: Substrate availability, transport into mitochondria, allosteric control, hormonal regulation, and genetic control.

                                                                              ? Levels of Regulation

1. Regulation at the Level of Lipolysis (Substrate Supply)

• Hormone-sensitive lipase (HSL) in adipose tissue controls release of free fatty acids (FFA).

• Stimulated by:

  • Glucagon, epinephrine, norepinephrine → via cAMP/PKA phosphorylation.

  • Cortisol (long-term lipolysis).

• Inhibited by:

  • Insulin → activates phosphodiesterase → ↓ cAMP → inactivates HSL.

? Thus, fasting/stress → ↑ lipolysis → ↑ FFA → ↑ oxidation.
? Fed state → insulin dominance → lipolysis suppressed.

2. Regulation at Transport Step (Carnitine Shuttle Control)

• Rate-limiting step of fatty acid oxidation = Entry of long-chain fatty acids into mitochondria via CPT-I (Carnitine Palmitoyltransferase I).

• Inhibited by malonyl-CoA (product of acetyl-CoA carboxylase in fatty acid synthesis).

  • When synthesis is active (fed state, insulin present) → ↑ malonyl-CoA → CPT-I inhibited → oxidation blocked.

  • During fasting (low insulin, high glucagon) → ↓ acetyl-CoA carboxylase activity → ↓ malonyl-CoA → CPT-I active → oxidation proceeds.

? This ensures reciprocal regulation between fatty acid synthesis and oxidation.

3. Allosteric Regulation within Mitochondria

• High NADH/NAD⁺ ratio (produced during oxidation itself) inhibits:

  • β-hydroxyacyl-CoA dehydrogenase.

  • Entry of acetyl-CoA into TCA cycle.

• High acetyl-CoA inhibits thiolase (β-ketothiolase) but activates pyruvate carboxylase (shunting pyruvate → OAA for gluconeogenesis).

? This links FA oxidation with gluconeogenesis in fasting.

4. Hormonal Regulation

• Insulin:

  • Inhibits lipolysis (↓ HSL).

  • Activates acetyl-CoA carboxylase (↑ malonyl-CoA → inhibits CPT-I).

  • Net: ↓ FA oxidation, ↑ FA synthesis & storage.

• Glucagon & Epinephrine:

  • Stimulate lipolysis (↑ HSL).

  • Inhibit acetyl-CoA carboxylase (↓ malonyl-CoA → CPT-I active).

  • Net: ↑ FA oxidation.

5. Genetic/Adaptive Regulation

• Prolonged fasting, high-fat diet, and endurance training ↑ gene expression of β-oxidation enzymes.

• PPAR-α (peroxisome proliferator–activated receptor alpha) regulates expression of FA oxidation enzymes in liver.

? Integration with Carbohydrate Metabolism

• Fed state (high insulin):

  • Glycolysis and FA synthesis predominate.

  • Malonyl-CoA levels high → CPT-I blocked → oxidation suppressed.

• Fasting state (high glucagon):

  • Glycogenolysis, gluconeogenesis, and FA oxidation predominate.

  • Malonyl-CoA levels low → CPT-I active → oxidation stimulated.

• Exercise:

  • Catecholamines stimulate lipolysis.

  • Muscle oxidizes FFAs to spare glucose.
    

                                                                            ? Clinical Correlations

1. Carnitine Deficiency

• Impaired transport of long-chain FAs → ↓ β-oxidation.

• Features: Hypoketotic hypoglycemia, muscle weakness, cardiomyopathy.

2. CPT-I Deficiency (Liver)

• Fasting hypoglycemia, hypoketonemia.

3. CPT-II Deficiency (Muscle)

• Muscle pain, weakness, myoglobinuria after exercise.

4. MCAD Deficiency

• Medium-chain acyl-CoA dehydrogenase defect.

• Severe fasting hypoglycemia, dicarboxylic aciduria (ω-oxidation backup).

• Sudden infant death (undiagnosed cases).

5. Alcohol Metabolism

• Increases NADH/NAD⁺ ratio → inhibits β-oxidation → promotes fatty liver (steatosis).

                                                                                 ? Introduction

• Ketone bodies are water-soluble fuel molecules derived from acetyl-CoA (mainly from fatty acid oxidation).

• They provide an alternate energy source during prolonged fasting, starvation, low-carbohydrate diets, uncontrolled diabetes, and in neonatal life.

• They are produced in the liver mitochondria but are not used by the liver itself (liver lacks the key enzyme thiophorase).

• Ketone bodies:

  1. Acetoacetate

  2. β-Hydroxybutyrate

  3. Acetone (non-metabolizable, excreted via breath/urine).
     

                                                    ? Steps in Ketone Body Synthesis (Ketogenesis)

Occurs in liver mitochondria during excess fatty acid oxidation (fasting/diabetes).

1. Condensation of 2 Acetyl-CoA → Acetoacetyl-CoA

• Enzyme: Thiolase.

1. Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA

• Enzyme: HMG-CoA synthase (rate-limiting enzyme of ketogenesis).

1. HMG-CoA → Acetoacetate + Acetyl-CoA

• Enzyme: HMG-CoA lyase.

1. Fates of Acetoacetate:

   • Reduced to β-hydroxybutyrate (by β-hydroxybutyrate dehydrogenase, depends on NADH/NAD⁺ ratio).

   • Spontaneously decarboxylates to acetone (exhaled as “fruity odor” in ketoacidosis).
     

                                                          ? Ketone Body Utilization (Ketolysis)

Occurs in extrahepatic tissues (skeletal muscle, cardiac muscle, renal cortex, brain in starvation).

1. β-Hydroxybutyrate → Acetoacetate

• Enzyme: β-hydroxybutyrate dehydrogenase.

1. Acetoacetate → Acetoacetyl-CoA

• Enzyme: Succinyl-CoA:acetoacetate CoA transferase (thiophorase).

• Uses succinyl-CoA as CoA donor.

• Absent in liver → prevents liver from using ketones.

1. Acetoacetyl-CoA → 2 Acetyl-CoA

• Enzyme: Thiolase.

• Acetyl-CoA enters TCA cycle → ATP generation.

⚙️ Regulation of Ketone Body Metabolism

• High fatty acid oxidation → ↑ acetyl-CoA → exceeds TCA cycle capacity (due to low OAA during fasting/gluconeogenesis).

• Excess acetyl-CoA diverted into ketone body synthesis.

• NADH/NAD⁺ ratio:

  • High NADH favors conversion of acetoacetate → β-hydroxybutyrate.

  • In starvation/diabetes → β-hydroxybutyrate predominates.

• Hormonal regulation:

  • Insulin suppresses ketogenesis (promotes lipogenesis, inhibits lipolysis).

  • Glucagon & catecholamines stimulate lipolysis → ↑ FA oxidation → ↑ ketogenesis.

? Physiological Role of Ketone Bodies

• Provide energy during glucose scarcity.

• Brain: Normally glucose-dependent, but in prolonged fasting can derive up to two-thirds of energy from ketones, sparing muscle protein.

• Heart & skeletal muscle: Prefer ketones over glucose during fasting.

• Newborns: High reliance on ketones due to high-fat milk diet.

? Clinical Aspects

1. Ketoacidosis

• Excessive ketone body production → acidosis (low pH).

• Causes:

  • Uncontrolled type 1 diabetes mellitus (diabetic ketoacidosis, DKA).

  • Prolonged fasting/starvation.

  • Alcoholic ketoacidosis (excess NADH from ethanol metabolism → favors β-hydroxybutyrate).

• Features: Kussmaul breathing (deep, rapid), dehydration, vomiting, confusion, fruity odor breath (acetone), coma if severe.

2. Ketosis (Nutritional / Physiological)

• Moderate rise in ketone levels (but without severe acidosis).

• Occurs in fasting, low-carb diets, intense exercise.

3. Ketone Body Detection

• Rothera’s test: Nitroprusside test for acetoacetate in urine.

• Ketone meters: Measure β-hydroxybutyrate in blood (more accurate in ketoacidosis).

4. Inborn Errors of Metabolism

• HMG-CoA lyase deficiency: Impaired ketogenesis → hypoketotic hypoglycemia.

• Succinyl-CoA:acetoacetate transferase deficiency: Inability to utilize ketone bodies → severe ketoacidosis.

                                                                                      ? Introduction

• Cholesterol is a 27-carbon sterol synthesized in virtually all nucleated cells, but mainly in:

  • Liver (major site).

  • Intestine, adrenal cortex, gonads, skin.

• It is the precursor for:

  • Steroid hormones (cortisol, aldosterone, sex hormones).

  • Vitamin D.

  • Bile acids and bile salts.

• Although dietary cholesterol is absorbed, endogenous synthesis is the primary source for the body.

• Biosynthesis occurs in the cytosol and smooth endoplasmic reticulum (SER).

? Building Blocks & Requirements

• Carbon source: Acetyl-CoA (all 27 carbons of cholesterol come from acetyl-CoA).

• Reducing power: NADPH (mainly from Pentose Phosphate Pathway).

• Energy: ATP.

• Oxygen: Needed for hydroxylation steps.
  

                                                                  ? Steps of Cholesterol Biosynthesis

Stage 1 — Formation of Mevalonate (Committed Step)

1. 2 Acetyl-CoA → Acetoacetyl-CoA

   • Enzyme: Thiolase.

2. Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA (3-hydroxy-3-methylglutaryl-CoA)

   • Enzyme: HMG-CoA synthase (cytosolic isoform).

3. HMG-CoA → Mevalonate

   • Enzyme: HMG-CoA reductase (rate-limiting enzyme).

   • Cofactors: 2 NADPH.

   • Location: Smooth ER.

   • Key regulatory step in cholesterol synthesis.

Stage 2 — Formation of Activated Isoprenes

1. Mevalonate → 5-Phosphomevalonate → Isopentenyl pyrophosphate (IPP, C5)

   • Requires 3 ATP.

   • IPP is the basic 5-carbon building block.

2. IPP ↔ Dimethylallyl pyrophosphate (DMAPP, C5)

   • Isomerization reaction.

Stage 3 — Formation of Squalene (C30)

1. IPP + DMAPP → Geranyl pyrophosphate (GPP, C10)

   • Enzyme: Prenyltransferase.

2. GPP + IPP → Farnesyl pyrophosphate (FPP, C15).

3. 2 FPP molecules → Squalene (C30)

   • Enzyme: Squalene synthase (requires NADPH).

Stage 4 — Formation of Lanosterol and Cholesterol

1. Squalene → Squalene epoxide (requires oxygen, NADPH, squalene monooxygenase).

2. Squalene epoxide → Lanosterol (C30 sterol) via cyclization.

3. Lanosterol → Cholesterol (C27)

• Through multiple steps (demethylation, reduction, rearrangement).
  

                                                        ⚙️ Regulation of Cholesterol Biosynthesis

1. HMG-CoA Reductase (Rate-limiting Enzyme)

• Transcriptional regulation:

  • Controlled by SREBP (Sterol Regulatory Element Binding Protein).

  • When cholesterol is low → SREBP activated → ↑ HMG-CoA reductase synthesis.

• Covalent modification:

  • Active when dephosphorylated (insulin favors).

  • Inactive when phosphorylated (glucagon, AMP-activated kinase).

• Feedback inhibition:

  • High intracellular cholesterol → inhibits enzyme.

2. Hormonal Control

• Insulin & thyroxine: Upregulate HMG-CoA reductase (stimulate synthesis).

• Glucagon & glucocorticoids: Downregulate enzyme (inhibit synthesis).

3. Drug Regulation

• Statins (e.g., lovastatin, simvastatin): Competitive inhibitors of HMG-CoA reductase → ↓ endogenous cholesterol synthesis.
  

                                                                            ? Clinical Importance

1. Hypercholesterolemia & Atherosclerosis

• High cholesterol → deposition in arterial walls → plaque formation → ischemic heart disease, stroke.

2. Statins

• Mainstay therapy for hypercholesterolemia.

• Inhibit HMG-CoA reductase → lower LDL cholesterol.

3. Smith–Lemli–Opitz Syndrome (SLOS)

• Autosomal recessive disorder.

• Deficiency of 7-dehydrocholesterol reductase → impaired cholesterol synthesis.

• Features: Microcephaly, intellectual disability, ambiguous genitalia.

4. Gallstones

• Cholesterol is the main component of bile stones when solubility balance is disturbed.

? Fate of Cholesterol

• Membranes: Major structural lipid.

• Bile acids/salts: Synthesized in liver for fat digestion.

• Steroid hormones: Cortisol, aldosterone, estrogens, androgens, progesterone.

• Vitamin D: Precursor is 7-dehydrocholesterol.

• Pathology: Atherosclerotic plaques, gallstones when excessive.

? Introduction

• Bile acids are amphipathic molecules synthesized in the liver from cholesterol.

• They are the major route of cholesterol catabolism (elimination).

• Secreted into bile, where they play critical roles in:

  • Digestion and absorption of lipids (emulsification, micelle formation).

  • Excretion of excess cholesterol.

? Chemistry

• Primary bile acids: Synthesized in liver.

  • Cholic acid.

  • Chenodeoxycholic acid.

• Secondary bile acids: Formed in the intestine by bacterial modification of primary bile acids.

  • Deoxycholic acid (from cholic acid).

  • Lithocholic acid (from chenodeoxycholic acid).

• Bile salts: Conjugated forms (with glycine or taurine) that are more water-soluble and effective detergents.

  • Glycocholic acid, Taurocholic acid.

  • Glycochenodeoxycholic acid, Taurochenodeoxycholic acid.

? Pathway of Bile Acid Synthesis

Starting material: Cholesterol (C27).

Step 1 — Hydroxylation at 7α position (Rate-limiting Step)

• Enzyme: Cholesterol 7α-hydroxylase (CYP7A1).

• Requires: NADPH, O₂, cytochrome P450.

• Product: 7α-hydroxycholesterol.

• Regulation: Inhibited by bile acids (feedback inhibition).

Step 2 — Further hydroxylation and side chain oxidation

• Modifications at C12 and C24 → produce cholic acid and chenodeoxycholic acid.

• Side chain shortened to C24 with a carboxyl group.

Step 3 — Conjugation

• In liver, bile acids conjugated with glycine or taurine → conjugated bile acids (bile salts).

• This lowers their pKa, making them ionized and more effective detergents at intestinal pH.

Step 4 — Secretion into bile

• Conjugated bile acids secreted into bile canaliculi with cholesterol and phospholipids (mainly phosphatidylcholine).

• Stored in gallbladder, released into duodenum in response to cholecystokinin (CCK).

? Enterohepatic Circulation

• About 95% of bile salts are reabsorbed in ileum and returned to liver via portal circulation.

• Only ~5% lost in feces (major pathway for cholesterol excretion).

• Efficient recycling reduces need for de novo bile acid synthesis.

⚙️ Regulation

• Rate-limiting enzyme: Cholesterol 7α-hydroxylase.

• Inhibited by: Bile acids (negative feedback).

• Stimulated by: Cholesterol (substrate availability), insulin, thyroid hormones.

? Functions of Bile Acids

1. Digestion & Absorption of Lipids:

   • Emulsify dietary fats → increase surface area for pancreatic lipase.

   • Form mixed micelles with monoglycerides, free fatty acids, cholesterol, and fat-soluble vitamins → absorption.

2. Cholesterol Homeostasis:

   • Bile acid synthesis is the only significant route for cholesterol elimination from the body.

3. Antimicrobial Role:

   • Disrupt bacterial membranes in intestine.

? Clinical Aspects

1. Gallstones (Cholelithiasis)

• Caused by imbalance in composition of bile:

  • Excess cholesterol (supersaturation).

  • Deficiency of bile salts or phosphatidylcholine.

• Risk factors: Obesity, female sex, multiple pregnancies, rapid weight loss.

2. Bile Acid Sequestrants (Drugs)

• e.g., Cholestyramine.

• Bind bile acids in intestine → prevent reabsorption → ↑ bile acid synthesis from cholesterol → ↓ plasma cholesterol.

3. Bile Acid Malabsorption

• Ileal resection (e.g., Crohn’s disease) → ↓ bile salt reabsorption → fat malabsorption, steatorrhea, fat-soluble vitamin deficiency.

4. Progressive Familial Intrahepatic Cholestasis (PFIC)

• Genetic defects in bile acid transporters → accumulation of bile acids in liver → cholestasis, liver damage.

? Introduction

• Cholesterol synthesis is a highly energy- and NADPH-consuming process.

• Since cholesterol cannot be degraded to CO₂ and water in humans, its synthesis must be tightly regulated to prevent harmful accumulation (→ atherosclerosis, gallstones).

• Regulation occurs mainly at the rate-limiting step catalyzed by HMG-CoA reductase.

? Key Control Point — HMG-CoA Reductase

• Reaction: HMG-CoA → Mevalonate (in smooth ER).

• Requires 2 NADPH.

• Rate-limiting enzyme of cholesterol biosynthesis.

• Target of statin drugs (competitive inhibitors).

⚙️ Levels of Regulation

1. Transcriptional Regulation (Gene Expression Level)

• Sterol Regulatory Element-Binding Protein (SREBP):

  • A transcription factor that binds DNA at Sterol Regulatory Elements (SREs).

  • When cholesterol is low:

    • SREBP is activated and translocates to nucleus → ↑ HMG-CoA reductase mRNA → ↑ cholesterol synthesis.

  • When cholesterol is high:

    • SREBP activation is suppressed → ↓ HMG-CoA reductase synthesis.

? Feedback regulation: Cholesterol itself regulates its own synthesis via SREBP.

2. Proteolytic Degradation of HMG-CoA Reductase

• The enzyme is an integral ER membrane protein.

• When sterol levels are high, the enzyme undergoes ubiquitination and proteasomal degradation.

• This ensures enzyme levels fall when cholesterol is abundant.

3. Covalent Modification (Phosphorylation/Dephosphorylation)

• Enzyme activity is regulated by phosphorylation state:

  • Dephosphorylated form = active.

  • Phosphorylated form = inactive.

• AMP-activated protein kinase (AMPK): Phosphorylates and inactivates HMG-CoA reductase (active in energy deficit).

• Protein phosphatase: Dephosphorylates and activates reductase.

4. Hormonal Regulation

• Insulin: Activates protein phosphatase → dephosphorylates and activates HMG-CoA reductase → ↑ cholesterol synthesis (fed state).

• Thyroxine: Also upregulates enzyme activity.

• Glucagon & Epinephrine: Activate AMPK → phosphorylate and inactivate enzyme → ↓ cholesterol synthesis (fasting/stress).

• Glucocorticoids: Suppress enzyme expression.

5. Feedback Inhibition by End Products

• High cholesterol and bile acids inhibit HMG-CoA reductase expression and activity.

• This is why bile acid sequestrants (e.g., cholestyramine) lower cholesterol:

  • Prevent reabsorption of bile acids → ↑ bile acid synthesis from cholesterol → ↓ plasma cholesterol.

6. Drug Regulation

• Statins: Competitive inhibitors of HMG-CoA reductase → potent cholesterol-lowering drugs.

• Examples: Lovastatin, Simvastatin, Atorvastatin.

? Integration with Lipid Metabolism

• When dietary cholesterol is high → de novo synthesis decreases.

• When dietary cholesterol is low → de novo synthesis increases.

• Insulin (fed state) stimulates cholesterol synthesis.

• Glucagon (fasting state) inhibits synthesis and promotes fatty acid oxidation.

? Clinical Aspects

1. Familial Hypercholesterolemia (FH)

• LDL receptor defect → decreased LDL clearance.

• Despite reduced intracellular cholesterol, HMG-CoA reductase remains active → further cholesterol overproduction → premature atherosclerosis.

2. Statin Therapy

• Statins inhibit HMG-CoA reductase → lower hepatic cholesterol.

• This ↑ LDL receptor expression → ↑ LDL clearance from blood → ↓ plasma cholesterol.

3. Smith–Lemli–Opitz Syndrome (SLOS)

• Deficiency of 7-dehydrocholesterol reductase → reduced cholesterol synthesis.

• Clinical: Intellectual disability, dysmorphic features, ambiguous genitalia.

? High-Yield Summary

• Rate-limiting enzyme: HMG-CoA reductase.

• Regulation:

  • Gene expression: SREBP (up when cholesterol low, down when high).

  • Proteolysis: High cholesterol accelerates degradation.

  • Covalent modification: Active (dephosphorylated, insulin), Inactive (phosphorylated, glucagon/AMPK).

  • Hormonal: ↑ by insulin/thyroxine; ↓ by glucagon/glucocorticoids.

  • Feedback: Inhibited by cholesterol & bile acids.

• Statins = pharmacological inhibitors.

• Clinical links: Hypercholesterolemia, statin therapy, SLOS.

? Introduction

• Cholesterol is essential for cell membranes, steroid hormones, bile acids, and vitamin D.

• But when its metabolism or transport is deranged, it leads to major clinical disorders:

  • Atherosclerosis and cardiovascular disease.

  • Gallstone disease.

  • Inherited lipid disorders.

  • Rare congenital enzyme deficiencies.

Let’s integrate the applied aspects one by one.

? Plasma Cholesterol Levels

• Normal total plasma cholesterol: < 200 mg/dL.

• Borderline high: 200–239 mg/dL.

• High: ≥ 240 mg/dL → ↑ risk of coronary artery disease (CAD).

? Clinical Aspects of Cholesterol

1. Atherosclerosis & Cardiovascular Disease

• Pathogenesis:

  • LDL cholesterol deposits in intima of arteries.

  • Oxidized LDL taken up by macrophages → foam cells → fatty streaks.

  • Smooth muscle proliferation + fibrous cap → atherosclerotic plaque.

  • Plaque rupture → thrombosis → myocardial infarction/stroke.

• Risk factors:

  • High LDL, low HDL.

  • Diabetes mellitus, smoking, hypertension, obesity, sedentary lifestyle.

• Preventive/therapeutic strategies:

  • Statins (HMG-CoA reductase inhibitors).

  • Bile acid sequestrants.

  • Ezetimibe (inhibits cholesterol absorption).

  • Lifestyle: diet, exercise.

2. Gallstone Disease (Cholelithiasis)

• Composition: Most gallstones are cholesterol stones.

• Pathogenesis:

  • Supersaturation of bile with cholesterol.

  • Deficiency of bile salts or phospholipids (lecithin).

  • Gallbladder hypomotility → stasis.

• Risk factors (4 F’s):

  • Female, Fat, Fertile, Forty.

• Clinical features: Biliary colic, cholecystitis, obstructive jaundice.

• Management: Cholecystectomy, ursodeoxycholic acid (dissolves cholesterol stones).

3. Familial Hypercholesterolemia (FH)

• Cause: LDL receptor mutation → defective clearance of LDL.

• Biochemistry: Very high LDL cholesterol, normal triglycerides.

• Clinical features:

  • Xanthomas (tendon, tuberous).

  • Xanthelasma (eyelids).

  • Corneal arcus (lipid ring at corneal margin).

  • Premature CAD (MI in <20 years in homozygotes).

• Inheritance: Autosomal dominant.

• Treatment: Statins, PCSK9 inhibitors, LDL apheresis in severe cases.

4. Smith–Lemli–Opitz Syndrome (SLOS)

• Cause: Deficiency of 7-dehydrocholesterol reductase → impaired cholesterol synthesis.

• Clinical features:

  • Microcephaly, intellectual disability.

  • Dysmorphic facies, ambiguous genitalia.

  • Failure to thrive.

• Inheritance: Autosomal recessive.

• Management: Cholesterol supplementation.

5. Abetalipoproteinemia (linked with cholesterol transport)

• Cause: ApoB-48 and ApoB-100 deficiency → no chylomicrons/VLDL formation.

• Effect: Fat malabsorption, acanthocytosis, hypocholesterolemia.

• Clinical features: Steatorrhea, failure to thrive, neurological problems (due to vitamin E deficiency).

6. Hypothyroidism & Cholesterol

• Hypothyroidism → ↓ LDL receptor activity → hypercholesterolemia.

• Hyperthyroidism → ↑ LDL receptor activity → hypocholesterolemia.

7. Cholesterol & Steroid Hormone Disorders

• Since cholesterol is precursor for cortisol, aldosterone, and sex hormones →

  • Defects in adrenal steroidogenesis (e.g., congenital adrenal hyperplasias) link back to cholesterol utilization.

8. Pharmacological Manipulation of Cholesterol

• Statins: Competitive inhibitors of HMG-CoA reductase → ↓ synthesis.

• Ezetimibe: Inhibits intestinal cholesterol absorption.

• Bile acid sequestrants (Cholestyramine): Prevent bile salt reabsorption → ↑ cholesterol catabolism to bile acids.

• PCSK9 inhibitors (Alirocumab, Evolocumab): ↑ LDL receptor recycling → ↓ plasma LDL.

? Introduction

• Plasma lipids (cholesterol, triglycerides, phospholipids) are transported as lipoproteins because they are insoluble in water.

• Each lipoprotein = lipid core (TAG + cholesteryl esters) + amphipathic shell (phospholipids, free cholesterol, apolipoproteins).

• Among them, LDL and Lp(a) are especially important in atherosclerosis and cardiovascular risk.

? Low-Density Lipoprotein (LDL)

Origin

• Derived from VLDL via lipolysis by lipoprotein lipase → VLDL remnants (IDL) → LDL.

• Contains ApoB-100 as the only apoprotein.

Function

• Main carrier of cholesterol in blood (delivers cholesterol to peripheral tissues).

• Binds LDL receptor (ApoB-100 receptor) → receptor-mediated endocytosis.

? Applied Clinical Aspects of LDL

1. LDL and Atherosclerosis

• High plasma LDL → deposition in arterial intima.

• LDL undergoes oxidation by ROS → taken up by macrophages via scavenger receptors (SR-A) → foam cells.

• Foam cell accumulation → fatty streaks → atherosclerotic plaques → ischemic heart disease, stroke.

? LDL = “bad cholesterol”.

2. Familial Hypercholesterolemia (FH)

• Cause: LDL receptor deficiency or ApoB-100 mutations.

• Biochemistry: Very high LDL levels, normal triglycerides.

• Clinical features:

  • Xanthomas (tendinous, tuberous).

  • Corneal arcus.

  • Premature CAD (MI in 20s/30s in heterozygotes, childhood in homozygotes).

• Inheritance: Autosomal dominant.

• Treatment: Statins (HMG-CoA reductase inhibitors), ezetimibe, PCSK9 inhibitors, LDL apheresis in severe cases.

3. Statin Therapy

• Statins lower LDL by inhibiting HMG-CoA reductase → ↓ hepatic cholesterol → ↑ LDL receptor expression → ↑ clearance of LDL from plasma.

4. Secondary Hypercholesterolemia

• Causes: Hypothyroidism, nephrotic syndrome, cholestatic liver disease, diabetes mellitus.

? Lipoprotein(a) [Lp(a)]

Structure

• Lp(a) is an LDL-like particle with an additional apoprotein — Apo(a) covalently bound to ApoB-100.

• Apo(a) is structurally homologous to plasminogen (fibrinolytic protein).

Function

• Not fully understood.

• Competes with plasminogen → interferes with fibrinolysis → prothrombotic effect.

• Also delivers cholesterol to arterial walls → accelerates atherosclerosis.

? Applied Clinical Aspects of Lp(a)

1. Cardiovascular Disease

• Elevated Lp(a) is an independent risk factor for:

  • Myocardial infarction (MI).

  • Stroke.

  • Aortic stenosis.

• Risk persists even when LDL levels are normal.

2. Genetic Control

• Plasma Lp(a) levels are highly heritable.

• Not significantly affected by diet or lifestyle.

3. Laboratory Measurement

• Lp(a) levels are measured by immunoassays.

• Elevated if > 30 mg/dL (cutoffs vary).

4. Treatment

• Currently, no highly effective drugs for lowering Lp(a).

• Niacin (nicotinic acid) may modestly reduce Lp(a), but use is limited by side effects.

• PCSK9 inhibitors may reduce Lp(a) to some extent.

• Genetic therapies (antisense oligonucleotides against Apo(a)) are under investigation.

? Clinical Pearls

• LDL:

  • Primary cholesterol carrier, strongly linked to atherosclerosis.

  • Target of lipid-lowering therapy (statins, ezetimibe, PCSK9 inhibitors).

• Lp(a):

  • LDL-like but with Apo(a).

  • Independent, genetic risk factor for cardiovascular disease.

  • Not significantly modified by diet/exercise.

? High-Yield Summary

• LDL = “bad cholesterol”: delivers cholesterol to tissues, excess leads to atherosclerosis.

• Familial hypercholesterolemia = LDL receptor defect → very high LDL, premature CAD.

• Lp(a) = LDL + Apo(a), resembles plasminogen, interferes with fibrinolysis → thrombosis + atherosclerosis.

• Elevated Lp(a) = independent genetic risk factor for MI, stroke, aortic stenosis.

• Therapy: Statins for LDL, limited options for Lp(a) (PCSK9 inhibitors, new gene therapies).

)

? Introduction

• Lipoproteins transport insoluble lipids (TAGs, cholesterol, fat-soluble vitamins) in plasma.

• Among them:

  • Chylomicrons: Transport dietary lipids from intestine → blood → tissues.

  • Very Low-Density Lipoproteins (VLDL): Transport endogenously synthesized TAGs (mainly from liver) → peripheral tissues.

Both are TAG-rich lipoproteins but differ in origin and clinical associations.

? Metabolism of Chylomicrons

Formation

• Site: Intestinal mucosa (enterocytes) after fat digestion/absorption.

• Contain: High TAG, some cholesterol, ApoB-48 (essential structural apoprotein).

• Secreted into lymph (lacteals) → thoracic duct → systemic circulation.

Maturation in Blood

• Acquire ApoC-II and ApoE from HDL.

• ApoC-II = cofactor for lipoprotein lipase (LPL).

• ApoE = ligand for hepatic receptor uptake.

Catabolism (TAG Delivery)

• Enzyme: Lipoprotein lipase (LPL) (on vascular endothelium of adipose tissue, muscle, heart).

• Requires ApoC-II.

• Hydrolyzes TAGs → free fatty acids (FFA) + glycerol.

  • FFA → oxidized (muscle) or stored (adipose).

  • Glycerol → liver (gluconeogenesis or TAG synthesis).

Chylomicron Remnants

• After TAG removal, remnants rich in cholesterol, ApoB-48, ApoE.

• Taken up by liver via ApoE-mediated receptor endocytosis.

? Metabolism of VLDL

Formation

• Site: Liver hepatocytes.

• Contain: Endogenous TAGs + cholesterol + ApoB-100.

• Secreted into blood.

Maturation

• Acquire ApoC-II and ApoE from HDL.

TAG Hydrolysis

• LPL (with ApoC-II cofactor) hydrolyzes TAGs in VLDL.

• Produces FFA + glycerol (utilized like in chylomicrons).

Conversion to IDL and LDL

• After TAG depletion → IDL (intermediate density lipoprotein).

• IDL fate:

  • Taken up by liver (via ApoE).

  • Further metabolized to LDL (cholesterol-rich, ApoB-100 only).

⚙️ Key Differences

• Chylomicrons: Exogenous (dietary) TAG transport, ApoB-48, secreted via lymphatics.

• VLDL: Endogenous TAG transport, ApoB-100, secreted directly into blood.

? Applied Clinical Aspects

1. Type I Hyperlipoproteinemia (Familial Chylomicronemia)

• Defect: Lipoprotein lipase deficiency or ApoC-II deficiency.

• Biochemistry: ↑↑ Chylomicrons, severe hypertriglyceridemia (>1000 mg/dL).

• Clinical:

  • Eruptive xanthomas.

  • Pancreatitis (due to very high TAGs).

  • Lipemia retinalis (milky appearance of retinal vessels).

• Inheritance: Autosomal recessive.

• Treatment: Strict low-fat diet.

2. Type III Hyperlipoproteinemia (Familial Dysbetalipoproteinemia)

• Defect: ApoE2 variant (poor binding to hepatic remnant receptors).

• Biochemistry: ↑ Chylomicron remnants + IDL (broad β-band in electrophoresis).

• Clinical:

  • Palmar xanthomas.

  • Premature atherosclerosis.

• Inheritance: Autosomal recessive.

3. Type IV Hyperlipoproteinemia (Familial Hypertriglyceridemia)

• Defect: Overproduction of VLDL.

• Biochemistry: ↑ VLDL, hypertriglyceridemia.

• Clinical: Pancreatitis risk, obesity, diabetes link.

4. Metabolic Syndrome / Secondary Causes

• VLDL levels rise in obesity, diabetes mellitus, alcohol use, nephrotic syndrome.

• Leads to atherogenic dyslipidemia:

  • ↑ VLDL and triglycerides.

  • Low HDL.

  • Small, dense LDL.

5. Drug Targets

• Fibrates (e.g., gemfibrozil, fenofibrate): Activate PPAR-α → ↑ LPL activity → ↓ VLDL → ↓ TAGs.

• Niacin: Decreases VLDL secretion → lowers TAGs and LDL, raises HDL.

• Statins: Lower LDL, but also modest effect on VLDL remnants.

? High-Yield Summary

• Chylomicrons: Carry dietary TAGs. ApoB-48, ApoC-II, ApoE. → Catabolized by LPL → remnants taken by liver.

• VLDL: Carry endogenous TAGs. ApoB-100, ApoC-II, ApoE. → LPL action → IDL → LDL.

• Clinical correlations:

  • Type I hyperlipoproteinemia = LPL/ApoC-II deficiency → ↑ chylomicrons.

  • Type III = ApoE2 defect → ↑ chylomicron remnants + IDL.

  • Type IV = VLDL overproduction → hypertriglyceridemia.

• Complications: Pancreatitis, atherosclerosis.

• Therapy: Fibrates (↑ LPL), Niacin (↓ VLDL secretion), Statins (↓ LDL).

? Introduction

• High-Density Lipoprotein (HDL) is often called “good cholesterol”.

• It plays a critical role in reverse cholesterol transport — carrying excess cholesterol from peripheral tissues back to the liver for excretion.

• HDL also donates apoproteins to other lipoproteins and exerts anti-atherogenic effects.

? Chemistry & Structure of HDL

• Smallest, densest lipoprotein (high protein, low TAG content).

• Main apoproteins: ApoA-I (structural, activates LCAT), ApoA-II, ApoC, ApoE.

• Synthesized in: Liver and intestine.

• Secreted as nascent HDL (discoidal shape), then matures by acquiring cholesterol and phospholipids.

? Metabolism of HDL

1. Nascent HDL Formation

• Liver & intestine secrete ApoA-I–rich HDL in a discoidal shape (empty “scavenger” particle).

2. Acquisition of Cholesterol from Tissues

• HDL accepts cholesterol from peripheral cells via:

  • ABCA1 transporter (ATP-binding cassette protein).

  • Scavenger receptor class B1 (SR-B1).

3. Esterification of Cholesterol

• Enzyme: LCAT (Lecithin–Cholesterol Acyltransferase).

• Activated by ApoA-I.

• Reaction: Free cholesterol → cholesterol ester (hydrophobic) → migrates to HDL core.

• This transforms discoidal HDL → spherical mature HDL.

4. Cholesterol Delivery to Liver

Two pathways:

a) Direct Transfer

• HDL binds to SR-B1 receptor on hepatocytes → cholesterol esters selectively taken up.

b) Indirect Transfer (via CETP)

• CETP (Cholesteryl Ester Transfer Protein) exchanges cholesterol esters from HDL for TAGs from VLDL/IDL.

• Cholesterol esters end up in LDL and remnants, which are cleared by liver.

⚙️ Functions of HDL

1. Reverse cholesterol transport (periphery → liver).

2. Reservoir of apoproteins: Supplies ApoC-II (for LPL activation) and ApoE (for remnant uptake) to chylomicrons and VLDL.

3. Anti-oxidant and anti-inflammatory effects: Protects LDL from oxidation.

4. Inhibits platelet aggregation and promotes endothelial repair.

? Applied Clinical Aspects

1. HDL & Cardiovascular Disease

• High HDL (>60 mg/dL) → protective against atherosclerosis.

• Low HDL (<40 mg/dL) → independent risk factor for CAD, stroke, metabolic syndrome.

• Mechanism: Low HDL impairs reverse cholesterol transport → cholesterol deposition in arteries.

2. Tangier Disease (Familial HDL Deficiency)

• Cause: Defect in ABCA1 transporter.

• Biochemistry: Very low/absent HDL, very low ApoA-I.

• Clinical features:

  • Cholesterol ester deposition in tissues.

  • Enlarged, orange tonsils (classic sign).

  • Peripheral neuropathy.

  • Premature atherosclerosis.

3. LCAT Deficiency

• Cause: Mutation in LCAT enzyme.

• Effect: HDL cannot esterify cholesterol → remains discoidal, dysfunctional.

• Clinical features:

  • Corneal clouding.

  • Anemia.

  • Proteinuria, progressive renal failure.

  • Premature CAD.

4. Secondary Low HDL Causes

• Diabetes mellitus, metabolic syndrome, smoking, obesity, chronic kidney disease, hypertriglyceridemia.

5. Pharmacology

• Niacin (Vitamin B3): Raises HDL significantly (but use limited by flushing, hepatotoxicity).

• Fibrates: Modest HDL increase.

• Newer agents targeting CETP (torcetrapib, anacetrapib): CETP inhibition raises HDL, but cardiovascular benefit still debated.

? Clinical Pearls

• HDL is cardioprotective not just by cholesterol removal but also by anti-inflammatory and antioxidant functions.

• Tangier disease = orange tonsils, absent HDL.

• LCAT deficiency = corneal clouding + renal disease + low HDL.

• Secondary low HDL should always be evaluated in metabolic syndrome patients.