Chapter 21 Lipid Biosynthesis Specialized lipids can be o Pigments o Cofactors o Detergents o Transporters o Hormones o Extracellular and intracellular messengers o Anchors for membrane proteins 21 1 Biosynthesis of Fatty Acids and Eicosanoids Fatty acid biosynthesis and breakdown occur by different pathways are catalyzed by different enzymes and take place in different parts of the cell breakdown but is involved in biosynthesis a three carbon intermediate that is not involved in fatty acid Malonyl CoA Malonyl CoA Is Formed from Acetyl CoA and Bicarbonate Acetyl CoA carboxylase acetyl CoA in an irreversible process catalyzes the formation of malonyl CoA from o The bacterial version has three subunits o In animals all three activities are part of a single multifunctional polypeptide o Plants contain both kinds o In all cases contains a biotin prosthetic group covalently bound in amide linkage to the amino group of a Lys residue one of three polypeptides or domains A carboxyl group derived from bicarbonate is transferred to biotin in an ATP dependent reaction to yield malonyl CoA o This serves as a temporary carrier of CO2 transferring it to acetyl CoA Fatty Acid Synthesis Proceeds in a Repeating Reaction Sequence Fatty acid synthase chains of fatty acids in four steps a system that catalyzes the formation of long carbon o With each passage through the cycle the chain grows by two The reducing agent is NADPH and the activating groups are two different enzyme bound SH groups Two major variants of fatty acid synthase o Fatty acid synthase I FAS I found in vertebrates and fungi A single multifunctional polypeptide chain Seven active sites in separate domains o Fatty acid synthase II FAS II found in plants and bacteria With FAS I systems fatty acid synthesis leads to a single product with no intermediates being released Intermediates of the FAS II system can be diverted into other pathways o Produces a variety of fatty acids o Found in vertebrate mitochondria The Mammalian Fatty Acid Synthase Has Multiple Active Sites In FAS I the intermediates remain covalently attached as thioesters o Hydrolysis of these bonds releases energy to make two different steps thermodynamically favorable Acyl carrier protein ACP the shuttle that holds the system together o In E coli a small protein containing the prosthetic group 4 phosphopantetheine which is believed to serve as a flexible arm tethering the growing fatty acyl chain to the surface of the complex Fatty Acid Synthase Receives the Acetyl and Malonyl Groups Before the condensation reactions that build up the fatty acid chain can begin the two thiol groups on the enzyme complex must be charged with the correct acyl groups o The acetyl group of acetyl CoA is transferred to ACP in a reaction catalyzed by the malonyl acetyl CoA ACP transferase MAT domain of the multifunctional polypeptide o The acetyl group is then transferred to the Cys SH group of the ketoacyl ACP synthase KS Activation of acetyl and malonyl groups o Condensation A formal Claisen condensation involving activated acetyl and malonyl groups to form acetoacetyl ACP Also produces CO2 Catalyzed by ketoacyl ACP synthase o Reduction of the Carbonyl Group The acetoacetyl ACP formed in the previous step now undergoes reduction of the carbonyl group at C 3 to form D hydroxybutyryl ACP Catalyzed by ketocyl ACP reductase Electron donor is NADPH KR o Dehydration Water is removed from C 2 and C 3 of D hydroxybutyryl ACP to yield a double bond in trans Hydroxyacyl ACP dehydratase 2 butenoyl ACP catalyzes this dehydration o Reduction of the Double Bond The double bond of trans 2 butenoyl ACP is reduced to form butyryl ACP Catalyzed by enoyl ACP reductase NADPH is the electron donor The Fatty Acid Synthase Reactions Are Repeated to Form Palmitate The butyryl group is transferred from the phosphopantethiene SH group of ACP to the Cys SH group of ketoacyl ACP synthase which initially bore the acetyl group To start the next cycle of four reactions that lengthens the chain by two carbons another malonyl group is linked to the now unoccupied phosphopantethiene SH group of ACP Condensation occurs as the butyryl group is linked to two carbons of the malonyl ACP group with concurrent loss of CO2 o Product is a six carbon acyl group covalently bonded to the phosphopantethiene SH group o Its keto group is reduced in the next three steps of the synthase cycle to yield the saturated acyl group Palmitate is released from the ACP by hydrolytic activity in the multifunctional protein The biosynthesis of fatty acids requires acetyl CoA and energy in two forms o The group transfer potential of ATP required to attach CO2 to acetyl CoA to make malonyl CoA o The reducing power of NADPH required to reduce the double bonds In nonphotosynthetic eukaryotes this requires an extra ATP because acetyl CoA is generated in the mitochondria Fatty Acid Synthesis Occurs in the Cytosol of Many Organisms but in the Chloroplasts of Plants Because fatty acid synthesis occurs in the cytosol it is segregated from degradative reactions which take place in the mitochondria In hepatocytes and adipocytes cytosolic NADPH is largely generated by the pentose phosphate pathway and by malic enzyme In photosynthetic organisms fatty acid synthesis occurs in the chloroplast stroma Acetate Is Shuttled out of Mitochondria as Citrate Acetyl CoA from the oxidation of fatty acids is not a significant source of acetyl CoA for fatty acid biosynthesis in animals because the two pathways are reciprocally regulated Acetyl CoA is shuttled across the mitochondrial inner membrane o Intermitochondrial acetyl CoA reacts with oxaloacetate to form citrate in the citric acid cycle Catalyzed by citrate synthase o Citrate passes through the inner membrane on the citrate transporter o Citrate is then cleaved by citrate lyase regenerating acetyl CoA and oxaloacetate ATP dependent o Malate dehydrogenase reduces the oxaloacetate to malate which can return to the mitochondrial matrix on the malate ketoglutarate transporter in exchange for citrate o Malate is reoxidized to oxaloacetate o Most of the malate is used to generate cytosolic NADPH via malic enzyme o Pyruvate is transported to the mitochondria via the pyruvate transporter and converted to oxaloacetate by pyruvate carboxylase in the matrix o Consumes 2 ATP citrate lyase and pyruvate carboxylase for every molecule of acetyl CoA Fatty Acid Biosynthesis Is Tightly Regulated The reaction catalyzed by acetyl CoA
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