The simultaneous oxidation and phosphorylation of glyceraldehyde 3-phosphate forms a highly reactive covalent thioester bond between a cysteine side chain (reactive group ?SH) on the enzyme (glyceraldehyde 3-phosphate dehydrogenase) and the oxidized intermediate (see the arrow in Figure 13-23A).If the enzyme had a serine (reactive group ?OH) instead of a cysteine at this position, which could form only a much lower energy bond to the oxidized substrate (see the arrow in Figure 13-23B), how might this new enzyme act? (A)

(B)

Figure 13-23

Question

The simultaneous oxidation and phosphorylation of glyceraldehyde 3-phosphate forms a highly reactive covalent thioester bond between a cysteine side chain (reactive group ?SH) on the enzyme (glyceraldehyde 3-phosphate dehydrogenase) and the oxidized intermediate (see the arrow in Figure 13-23A) .If the enzyme had a serine (reactive group ?OH) instead of a cysteine at this position, which could form only a much lower energy bond to the oxidized substrate (see the arrow in Figure 13-23B) , how might this new enzyme act? (A)

(B)

Figure 13-23

(B)

Figure 13-23

Quizplus · Accepted Answer

The covalent bond between the cysteine side chain and the oxidized intermediate is crucial to the release of the phosphorylated product. Without the high-energy bond provided by the thioester bond, the enzyme would not be able to release the product, so the new enzyme with a serine at this position would oxidize the substrate but not release it. Therefore, Choice B is the best answer.

The Simultaneous Oxidation and Phosphorylation of Glyceraldehyde 3-Phosphate Forms a Highly