How Do Enzymes Work Citrate Synthase

Enzymes work by bringing reactant molecules together, bolding them in the orientation necessary for reaction, and providing any necessary acidic or basic sites to catalyze specific steps. As an example, let s look at citrate synthase, an enzyme that catalyzes the aldol-like addition of acetyl CoA to oxaloacetate to give citrate. The reaction is the first step in the citric acid cycle, in which acetyl 

FIGURE 19.9 X-ray crystal structure of citrate synthase. Part (a) is a spacefilling model and part (b) is a ribbon model, which emphasizes the a-helical segments of the protein chain and indicates that the enzyme is dimeric that is, it consists of two identical chains held together by hydrogen bonds and other inter-molecular attractions. 

Part (c) is a close-up of the active site, in which oxaloacetate and an unreactive acetyl CoA mimic are bound. 

CHAPTER 19 biomolecules amino acids, peptides, and proteins 

O The side-chain carboxylate group of an aspartic acid acts as a base and removes an acidic a proton from acetyl CoA, while the N-H group on the side chain of a histidine acts as an acid and donates a proton to the carbonyl oxygen, giving an enol. 

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How Do Enzymes Work Citrate Synthase 1043 Focus On... The Protein Data Bank 1048... 

How Do Enzymes Work Citrate Synthase 1045 Table 26.3 Structures of Some Common Coenzymes (continued)... 

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