Chapter 21 – Lipid Biosynthesis- Specialized lipids can be:o Pigmentso Cofactorso Detergentso Transporterso Hormoneso Extracellular and intracellular messengerso Anchors for membrane proteins21.1 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- Malonyl-CoA – a three-carbon intermediate that is not involved in fatty acid breakdown but is involved in biosynthesisMalonyl-CoA Is Formed from Acetyl-CoA and Bicarbonate- Acetyl-CoA carboxylase – catalyzes the formation of malonyl-CoA from acetyl-CoA in an irreversible processo The bacterial version has three subunitso In animals, all three activities are part of a single multifunctional polypeptideo Plants contain both kindso 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 reactiono This serves as a temporary carrier of CO2, transferring it to acetyl-CoA to yield malonyl-CoAFatty Acid Synthesis Proceeds in a Repeating Reaction Sequence- Fatty acid synthase – a system that catalyzes the formation of long carbon chains of fatty acids in four stepso 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 domainso 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 pathwayso Produces a variety of fatty acidso Found in vertebrate mitochondriaThe Mammalian Fatty Acid Synthase Has Multiple Active Sites- In FAS I, the intermediates remain covalently attached as thioesterso Hydrolysis of these bonds releases energy to make two different stepsthermodynamically favorable- Acyl carrier protein (ACP) – the shuttle that holds the system togethero 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 complexFatty 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 groupso 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 synthaseo 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 (KR) Electron donor is NADPHo Dehydration Water is removed from C-2 and C-3 of D--hydroxybutyryl-ACPto yield a double bond in trans - 2 -butenoyl-ACP -Hydroxyacyl-ACP dehydratase – catalyzes this dehydrationo 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 donorThe 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 theacetyl 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 CO2o Product is a six-carbon acyl group covalently bonded to the phosphopantethiene –SH groupo 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-CoAo 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 mitochondriaFatty Acid Synthesis Occurs in the Cytosol of Many Organisms but in the Chloroplasts ofPlants- 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 membraneo Intermitochondrial acetyl-CoA reacts with oxaloacetate to form citratein the citric acid cycle Catalyzed by citrate synthaseo Citrate passes through the inner membrane on the citrate transportero Citrate is then cleaved by citrate lyase, regenerating acetyl-CoA and oxaloacetate ATP-dependento Malate dehydrogenase reduces the oxaloacetate to malate, which can return to the mitochondrial matrix on the malate--ketoglutarate transporter in exchange for citrateo Malate is reoxidized to oxaloacetateo Most of the malate is used to generate cytosolic NADPH via malic enzymeo Pyruvate is transported to the mitochondria via the pyruvate transporter and converted to oxaloacetate
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