Lecture 27 FAS PDF

Fatty Acid Synthesis Fatty acids: built in several “passes”, one acetate unit at a time. acetate is derived from activated malonate in the form of malonyl- CoA. Overall mechanism: attach acetate...

Fatty Acid Synthesis Fatty acids: built in several “passes”, one acetate unit at a time. acetate is derived from activated malonate in the form of malonyl- CoA. Overall mechanism: attach acetate unit (2-C) derived from malonyl- CoA to a growing FA chain via an Acyl Carrier Protein (ACP) and then reduce it Stage 1: Malonyl-CoA Is Formed from Acetyl-CoA and HCO3- by Acetyl-CoA carboxylase (ACC) – HCO3− (bicarbonate) = soluble source of CO2. – ACC has three subunits: One unit has biotin (B7) covalently linked to Lys of biotin carrier protein. Biotin carries CO2. All three subunits are on one polypeptide chain. Acetyl CoA carboxylase (ACC) E1 – biotin carboxylase Acetyl CoA carboxylase (ACC) E2 – transcarboxylase Biotin cofactor plays similar role in other carboxylases eg pyruvate carboxylase  Fatty Acid Synthesis Is Tightly Regulated via ACC Acetyl CoA carboxylase (ACC): – feedback-inhibited by palmitoyl-CoA. – activated by citrate. acetyl-CoAmt and ATP  Citrate signals excess energy to be converted to fat. citrate is exported to cytosol Activates ACC  Acetyl CoA carboxylase (ACC) - Active enzyme forms long filaments in cytosol - Inactivated by protein phosphorylation (PKA) - less sensitive to citrate stimulation - Phosphorylation > dimerisation  Stage 2: Fatty Acid Synthase (FAS) Acyl Carrier Protein (ACP) Serves as a Shuttle in Fatty Acid Synthesis prosthetic group 4’-phosphopantetheine – flexible arm - tethers acyl chain while carrying intermediates from one enzyme subunit to the next Delivers acetate (in the first step) or malonate (in all the next steps) to the fatty acid synthase Shuttles the growing chain from one active site to another during the four-step reaction The Fatty Acid Cycle Fatty Acid Synthase is a dimer: 7 enzymes per monomer + ACP  Growing chain initially attached to FAS via a thioester linkage. - malonyl/acetyl-CoA ACP transacylase (MAT) FAS reaction involves cycles of four enzyme-catalyzed steps: 1. condensation with activated acetate -ketoacyl-ACP synthase (KS) 2. reduction of carbonyl to hydroxyl -ketoacyl-ACP reductase (KR) 3. dehydration of alcohol to trans-alkene -hydroxyacyl-ACP dehydratase (DH) 4. reduction of alkene to alkane enoyl-ACP reductase (ER) Repeat 1 – 4, x7 Release of product – thioesterase (TE) Overall Palmitate (16C) Synthesis  Stoichiometry of Synthesis of Palmitate (16:0) 1. 7 acetyl-CoAs are carboxylated to make 7 malonyl-CoAs… using ATP. 7 acetyl-CoA + 7 CO2 + 7 ATP → 7 malonyl-CoA + 7 ADP + 7 Pi 2. Seven cycles of condensation, reduction, dehydration, and reduction… using NADPH to reduce the -keto group and trans- double bond acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H+ → palmitate (16-carbons) + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O  Palmitate Can Be Lengthened to Longer-Chain Fatty Acids Elongation systems in the endoplasmic reticulum and mitochondria create longer fatty acids. As in palmitate synthesis, each step adds units of 2 C. Stearate (18:0) is the most common product.  Double bonds are introduced by desaturation of a Fatty Acid Fatty Acyl-CoA Desaturase (mixed function oxidase) O2 accepts 4 e- from two substrates. 2 e- from saturated fatty acid. 2 e- from ferrous state of cytochrome b5.  Fatty Acid Metabolism is Tightly Regulated High blood glucose, insulin Protein phosphatase activates Acetyl-CoA carboxylase (1st step in FA synthesis) Malonyl-CoA inhibits CAT 1, prevents FA entry into mitochondria - FA oxid’n, FA synthesis Low blood glucose: glucagon release - PKA and AMPK inactivate ACC Decrease in malonyl-CoA removes inhibition of FA oxidation Cytoplasm  In response to glucagon/cAMP, PKA regulates multiple pathways glycogen metabolism fatty acid mobilization - glycogen synthase (inhibits) - hormone-sensitive lipase - glycogen phosphorylase kinase (activates) (activates) PKA fatty acid synthesis glycolysis - ACC (inhibits) - PFK-2 (inhibits)/fructose 2,6-bisphosphatase (activates) - pyruvate kinase (inhibits) Synthesis of storage lipids Triglycerides Cholesterol (next class)  Synthesis of Triacyl Glycerols (TAGs) Begins by synthesizing phosphatidic acid from G3P = precursor to TAGs and phospholipids. – Fatty acids converted to acyl- CoA (synthetase) – attached to glycerol backbone by acyl transferases – releases CoA Phosphatidic Acid - forms Phospholipids or TAGs Phosphatidic acid phosphatase (lipin) removes the 3-phosphate from the phosphatidic acid. – yields 1,2-diacylglycerol The third carbon can be acylated with a third fatty acid. – yields triacylglycerol Or - attachment of polar head group – membrane lipids