1016 CHAPTER TWENTY-SIX Lipids 26.1 ACETYL COENZYME A The form in which acetate is used in most of its important biochemical reactions is acetyl coenzyme A(Figure 26.la). Acetyl coenzyme A is a thioester(Section 20.12). Its for mation from pyruvate involves several steps and is summarized in the overall equation CH3CCOH+ CoASH NAD-CH3 CSCoA NADH CO2 t H Oxidized Acetyl Reduced Carbon Proton form of dioxide nicotinamide dinucleotide dinucleotide Coenzyme A was isolated All the individual steps are catalyzed by enzymes. NAD(Section 15.11)is required as an oxidizing agent, and coenzyme A(Figure 26 1b) is the acetyl group acceptor. Coen- hemist Lipmann shared the zyme A is a thiol; its chain terminates in a sulfhydryl(--SH) group. Acetylation of the mann, an American bio- 953 Nobel Prize in physiol- sulthydryl group of coenzyme A gives acetyl coenzyme A ogy or medicine for this As we saw in Chapter 20, thioesters are more reactive than ordinary esters toward nucleophilic acyl substitution. They also contain a greater proportion of enol at equilib- rium. Both properties are apparent in the properties of acetyl coenzyme A. In some reac tions it is the carbonyl group of acetyl coenzyme a that reacts; in others it is the a carbon atom O CH3CSCOA CHa=CSCOA cetyl coenzyme A Enol form nucleophilic reaction at substitution HY: CH3C-Y:+ HSCoA E-CHCSCoA H HO OQ OH OH H Ho-P-0tooVon O NH, FIGURE 26.1 Structures of (a)R=CCH, Acetyl coenzyme A(abbreviation: CH, CSCoA (a)acetyl coenzyme a and R=H Coenzyme A(abbreviation: CoASH) (b)coenzyme A Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1016 CHAPTER TWENTY-SIX Lipids 26.1 ACETYL COENZYME A The form in which acetate is used in most of its important biochemical reactions is acetyl coenzyme A (Figure 26.1a). Acetyl coenzyme A is a thioester (Section 20.12). Its for￾mation from pyruvate involves several steps and is summarized in the overall equation: All the individual steps are catalyzed by enzymes. NAD (Section 15.11) is required as an oxidizing agent, and coenzyme A (Figure 26.1b) is the acetyl group acceptor. Coen￾zyme A is a thiol; its chain terminates in a sulfhydryl (±SH) group. Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A. As we saw in Chapter 20, thioesters are more reactive than ordinary esters toward nucleophilic acyl substitution. They also contain a greater proportion of enol at equilib￾rium. Both properties are apparent in the properties of acetyl coenzyme A. In some reac￾tions it is the carbonyl group of acetyl coenzyme A that reacts; in others it is the - carbon atom. O CH3CSCoA Acetyl coenzyme A CH2 OH CSCoA Enol form reaction at carbon nucleophilic acyl substitution HY E E O CH2CSCoA H Y O CH3C HSCoA OO CH3CCOH Pyruvic acid O CH3CSCoA Acetyl coenzyme A CoASH Coenzyme A NAD Oxidized form of nicotinamide adenine dinucleotide NADH Reduced form of nicotinamide adenine dinucleotide CO2 Carbon dioxide H Proton HO P O HO SR CH3 NH2 O N N N N O N O N H H OH O O O P P HO OH O O CH3 O HO (a) (b) Acetyl coenzyme A (abbreviation: CH 3 O R  H Coenzyme A (abbreviation: CoASH) R  CCH O CSCoA) 3 Coenzyme A was isolated and identified by Fritz Lip￾mann, an American bio￾chemist. Lipmann shared the 1953 Nobel Prize in physiol￾ogy or medicine for this work. FIGURE 26.1 Structures of (a) acetyl coenzyme A and (b) coenzyme A. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website