قناة ثالثة بشري (دفعه 52)

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Requirements for TAG Synthesis • Glycerol-3-phosphate • Fatty acyl-CoA 1. Synthesis of Glycerol-3-Phosphate From Glucose (Liver & Adipose) • Glucose → DHAP → glycerol-3-P (glycerol-3-P dehydrogenase) From Glycerol (Liver only) • Glycerol → glycerol-3-P (glycerol kinase) ⚠️ Adipose tissue lacks glycerol kinase → depends on glucose ⚠️ Insulin is essential for TAG synthesis in adipose tissue 2. Activation of Fatty Acids • Fatty acids → fatty acyl-CoA • Enzyme: Fatty acyl-CoA synthetase (thiokinase) 3. TAG Formation • Sequential addition of: • Two fatty acids → phosphatidic acid • Removal of phosphate • Addition of third fatty acid → TAG Fate of TAG • Adipose tissue: stored as energy • Liver: packaged into VLDL and transported to tissues

🛑🛑 De-novo Synthesis of Fatty Acids & I. De-novo Fatty Acid Synthesis General Features • Occurs mainly in the cytosol • Major tissues: • Liver • Lactating mammary glands • Adipose tissue (to a lesser extent) • Endergonic process (requires energy) • Active in the fed state • Stimulated by insulin (lipogenic hormone) • Acetyl-CoA for FA synthesis is derived from glucose, not fatty acids Requirements for Fatty Acid Synthesis 1. Acetyl-CoA 2. Acetyl-CoA carboxylase (ACC) 3. NADPH 4. Fatty acid synthase complex (FAS) Sources of NADPH • Pentose phosphate pathway (PPP) • Cytosolic isocitrate dehydrogenase • Malic enzyme (malate → pyruvate) A. Transport of Acetyl-CoA to the Cytosol B. Carboxylation of Acetyl-CoA (Rate-Limiting Step) • Enzyme: Acetyl-CoA carboxylase (ACC) • Converts acetyl-CoA → malonyl-CoA • Requires: • ATP • CO₂ • Biotin (covalently bound to lysine) • This is the regulated and rate-limiting step of FA synthesis Regulation of Acetyl-CoA Carboxylase 1. Short-Term Regulation Allosteric Regulation • Activated by citrate • Inhibited by fatty acyl-CoA (end product) Hormonal Regulation • Low blood glucose → high glucagon • ↑ cAMP → protein kinase activation • ACC phosphorylated → inactive • High blood glucose → high insulin • Activates protein phosphatase • ACC dephosphorylated → active Insulin stimulates FA synthesis by 1. Activating ACC and pyruvate dehydrogenase 2. Increasing glucose uptake → ↑ acetyl-CoA ⸻ 2. Long-Term Regulation • High carbohydrate / fat-free diet: • ↑ synthesis of ACC → ↑ FA synthesis • Fasting or high-fat diet: • ↓ ACC synthesis → ↓ FA synthesis C. Fatty Acid Synthase (FAS) Complex

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10. Glycolipids (Glycosphingolipids) General Features • Contain: • Sphingosine • One fatty acid • Carbohydrate • Do not contain phosphate 11. Synthesis of Cerebrosides • Ceramide monosaccharides • Types: • Galactocerebroside (most common in myelin) • Glucocerebroside Location • Brain and peripheral nerves • High concentration in myelin sheath Steps 1. Sphingosine synthesis 2. Ceramide formation 3. Activation of galactose 4. UDP-glucose → UDP-galactose (epimerase) 5. Galactosyl transferase reaction: • UDP-galactose + Ceramide → Galactocerebroside + UDP 12. Synthesis of Gangliosides • Complex glycolipids containing sialic acid (NANA) • Upon hydrolysis yield: • Ceramide • Hexoses (glucose, galactose) • Hexosamines (NANA, N-acetylgalactosamine) Major Brain Gangliosides • GM1 • GM2 • GM3 Location • CNS ganglion cells • High concentration at nerve endings

9. Degradation of Phospholipids Glycerophospholipids • Degraded by: • Phospholipase A₁, A₂, C, and D • Found in tissues and pancreatic juice Sphingomyelin • Degraded by sphingomyelinase • Deficiency → Niemann–Pick disease • Accumulation of sphingomyelin in the brain

6. Sphingolipids Types 1. Phosphosphingolipids • Main example: Sphingomyelin 2. Glycosphingolipids • Cerebrosides • Gangliosides Importance • Essential components of: • Cell membranes • Nervous tissue and brain 7. Synthesis of Ceramide (Key Objective) Ceramide synthesis begins in the endoplasmic reticulum: 1. Palmitoyl-CoA + Serine • Coenzyme: Pyridoxal phosphate (Vitamin B6) • Forms sphingosine 2. Sphingosine + Acyl-CoA • Forms ceramide 8. Synthesis of Sphingomyelin • Ceramide reacts with CDP-choline • Product: Sphingomyelin • Found in: • Cell membranes • Myelin sheath

4. Role of Phosphatidylcholine in Lung Surfactant • Dipalmitoyl phosphatidylcholine (DPPC) is the major lipid of lung surfactant • Functions: • Reduces alveolar surface tension • Prevents alveolar collapse • Deficiency causes respiratory distress syndrome in premature infants 5. Hepatic Synthesis of Phosphatidylcholine • Liver requires large amounts of PC for: • Bile secretion • Plasma lipoproteins • Pathway: • PS → PE (via PS decarboxylase) • PE → PC via three methylation steps • S-adenosyl methionine (SAM) is the methyl donor

3. Glycerophospholipids Synthesis of Phospholipids Phosphatidic Acid (PA) • Key precursor of glycerophospholipids • Formed from: • Glycerol-3-phosphate • Dihydroxyacetone phosphate (DHAP) Synthesis of Major Phospholipids • Choline and ethanolamine • Obtained from diet or phospholipid turnover • Activated by CDP to form: • CDP-choline • CDP-ethanolamine Reactions • CDP-choline + DAG → Phosphatidylcholine (PC) • CDP-ethanolamine + DAG → Phosphatidylethanolamine (PE) • Phosphatidylserine (PS) → PE (by decarboxylation)