Biosynthesis of Lipids: A Study Guide PDF

Biosynthesis of lipids Learning objectives 1. Explain the significances of lipid biosynthesis 2. Understand the reactions steps for fatty acid biosynthesis by acetyl-CoA carboxylase and fatty acid synthase (metabolites, chemical changes, utilization of ATP) 3. Briefly understand the pr...

Biosynthesis of lipids Learning objectives 1. Explain the significances of lipid biosynthesis 2. Understand the reactions steps for fatty acid biosynthesis by acetyl-CoA carboxylase and fatty acid synthase (metabolites, chemical changes, utilization of ATP) 3. Briefly understand the processes of fatty acid elongation and desaturation 4. Describe how fatty acids can be synthesized from carbohydrates and amino acids 5. Recognize the regulation of lipid metabolism during high and low blood glucose levels 6. Briefly understand the biosynthesis of triacylglycerols, phospholipids, eicosanoids, and cholesterol  Subcellular Locations of Lipid Metabolism Glyoxysomes Glyoxysomes * *malic enzyme Malate Pyruvate + CO2 2 NADP+ NADPH + H+ Fatty Acid Biosynthesis  Fatty acids are synthesized in the cytosol  Not simply the reverse of fatty acid -oxidation Biosynthesis of palmitic acid (16:0) Overall reaction: 8 Acetyl-CoA + 7 ATP + 14 NADPH + 14 H+ Palmitic acid + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O + 8 Co-enzyme A Involvement of two enzymes: Formation of 7 malonyl-CoA by Acetyl-CoA Carboxylase: 7 acetyl-CoA + 7 CO2 + 7ATP → 7 malonyl-CoA + 7 ADP + 7 Pi Seven cycles of reactions by Fatty Acid Synthase: + Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH + 14 H Palmitic acid + 7 CO2 + 14 NADP+ + 6 H2O + 8 Co-enzyme A 3 (1) Acetyl-CoA carboxylase (ACC) - Catalyzes the carboxylation of acetyl-CoA to produce malonyl-CoA - In animals, malonyl-CoA is a committed metabolite for fatty acid biosynthesis - Biotin is a prosthetic group in ACC 4 (2) Fatty Acid Synthase (FAS) complex - A protein complex with three polypeptides (subunits) - One subunit is a small polypeptide called Acyl Carrier Protein (ACP) which contains a phosphopantetheine group for attachment of an acyl chain. - The other two subunits are identical polypeptides with 7 enzymatic activities - Cycles of “condensation, reduction, dehydration, reduction” reactions occur in which an acyl chain is growing from ACP. - At the end of each cycle, 2 carbons are added to the growing fatty acyl chain - When the fatty acyl chain has reached 16 carbons in length after 7 cycles, it is cleaved from the ACP. - There are two attachment points for acyl chains in the FAS complex: Cysteine residue Cys – SH Acyl carrier ACP – SH 5 protein  β-Ketoacyl-ACP synthase (condensation) β-Hydroxyacyl-ACP dehydratase β-Ketoacyl-ACP reductase (dehydration) (reduction) Enoyl-ACP reductase (reduction) condensation 6  condensation reduction dehydration reduction Palmityol thioesterase 7 Summary of palmitic acid synthesis by the Fatty Acid Synthase complex Acetyl-CoA - + HCO3 + ATP Acetyl-CoA Malonyl-CoA condensation Acetyl-CoA reduction (NADPH) dehydration - + HCO3 + ATP reduction (NADPH) C4 acyl-ACP Malonyl-CoA condensation reduction (NADPH) dehydration reduction (NADPH) C6 acyl-ACP 5 more cycles - Each cycle: 1 malonyl-CoA (1 ATP) + 2 NADPH C16 acyl-ACP Palmitic acid (16:0) 8 Triclosan - An antibacterial agent found in cosmetics, toothpastes, antiseptic soaps, etc. - An inhibitor of fatty acid synthesis - Specifically inhibit enoyl-ACP reductase (step 7) in bacteria 9 Transport of acetyl-CoA from mitochondrion to cytosol for fatty acid synthesis: Tricarboxylate and Pyruvate Transporters *Sources of acetyl-CoA: - Pyruvate (derived Fatty acid from glucose by Acetyl CoA biosynthesis glycolysis in cytosol ) is converted to acetyl- CoA by pyruvate * dehydrogenase in the matrix - Amino acid catabolism in matrix - Hence, carbohydrates and amino acids can be used to make fatty acids 10 Elongation and desaturation of fatty acids (synthesized by fatty acid synthase) 11  [Animals and human do not have enzymes to introduce double bonds beyond C-9] * *Essential fatty acids * (ARA) 12  Linoleoyl CoA Linoleate + CoA-SH AMP + PPi ATP Desaturation [Both the acyl chain and NADH are oxidized by O2] condensation Elongation Desaturation thioesterase ARA + CoA-SH 13  Biosynthesis of Triacylglycerols (in cytosol)  Fatty acids are stored in adipose tissues as triacylglycerols  ~15 kg triacylglycerol in a 70-kg man  Excess carbohydrate is converted to triacylglycerols Formation of glycerol backbone: (from glycolysis) 14 Acyl-CoA Acyl-CoA Acyl-CoA 15 Sites of regulation of fatty acid metabolism Triacylglycerols - Fuel for energy-requiring processes - Storage for future use - Synthesis and degradation involve different tissues (e.g. liver and adipocytes) Action of hormones (1) Glucagon (low blood sugar) - increases cAMP level which causes protein phosphorylation - Activates lipase in adipocyte - Inhibits acetyl-CoA carboxylase (2) Insulin (high blood sugar) - decreases cAMP level which causes protein dephosphorylation - Activates acetyl-CoA carboxylase Malonyl-CoA - Inhibits entry of fatty acyl-CoA into mitochondria Long term regulation - Changes in enzyme quantities 16 Biosynthesis of Membrane Lipids  Synthesis of new membrane requires production of membrane lipids  Occurs on smooth ER surfaces  Two types of membrane lipids - Glycerophospholipids - Sphingolipids 17 Biosynthesis of Glycerophospholipids - Glycerophospholipids contain a glycerol backbone - Two alternative biosynthesis pathways for glycerophospholipids (1) Activation of polar head group (e.g. ethanolamine or choline) by CTP 18 (X = H) (X = CH3) 19 (2) Activation of phosphatidate by CTP 20 Biosynthesis of Sphingolipids - Sphingolipids contain a C-18 amino alcohol backbone (Sphinganine) 21  (Acyl-CoA) For polar head group attachment 22 - Precursor for different sphingolipids Examples of sphingolipids with different polar head groups Ganglioside 23 Biosynthesis of Eicosanoids Eicosanoids - Potent signaling molecules - e.g. Prostaglandins and thromboxanes Arachidonic acid (ARA) is a precursor for Eicosanoids ARA Prostaglandins - Trigger pain and inflammation - Regulate the release of mucin for stomach protection Thromboxanes - Induce constriction of blood vessels, platelet aggregation, blood clotting 24 Prostaglandin H2 synthase (COX) - The first enzyme in eicosanoid biosynthesis - Two catalytic activities: (1) cyclooxygenase and (2) peroxidase - 2 isozymes: COX-1 and COX-2, very similar in structure - COX-1: making prostaglandins for regulation of gastric mucin secretion - COX-2: making prostaglandins for induction of inflammation, pain, fever Thromboxanes (induce blood clotting) Other prostaglandins 25 Prostaglandin H2 Different pain killers/anti-inflammatory drugs targeting COX - Inhibition of cyclooxygenase activity of COX enzymes - Side effects: stomach ulcers (due to reduced mucin secretion) - Low dosage of aspirin: reduces risks of heart attack and stroke due to lowered thromboxane production - Vioxx and Celebrex: COX-2 specific with no side effects on stomach - Vioxx was withdrawn from the market due to unanticipated cardiac side effect (increased risks of cardiovascular disease) 26 Salicylic acid - Natural pain-killer found in bark of willow trees used by ancient Greek - Structurally similar to aspirin - Bitter tasting and unpleasant side effects including severe stomach irritation - Naturally a plant hormone to suppress diseases and wilting Salicylic acid Roses stay fresh longer in aspirin? 27 Cholesterol Biosynthesis 28  Formation of mevalonate Cholesterol-lowering drugs - e.g. Lovastatin, Atorvastatin - Structural analogs of HMG-CoA - Competitive inhibitor of HMG-CoA reductase Same as the first 2 steps in ketone body formation 29 Formation of isoprene units Isoprene units (C5) 30 Formation of squalene - Condensation of 6 isoprene units 4x 2x Condensation (multiple steps) 31 Formation of cholesterol from squalene - More than 20 steps - Ring closure reactions - Oxidation reactions - Loss of 3 methyl groups (C-30) 32 (C-27) Fates of newly synthesized cholesterol - Incorporation into cell - storage membrane - Packaging in lipoproteins - Precursor for steroid (in liver) for circulation hormones (e.g. testosterone and estrogen in appropriate tissues) 33  Summary of lipid metabolism Membrane lipids (phospholipids and Eicosanoids sphingolipids) (e.g. prostaglandins) Carbohydrates 34