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ch2opo3

1,3-Bisphosphoglycerate aG'° = 6.3 kJ/mol

This is the first of the two energy-conserving reactions of glycolysis that eventually lead to the formation of ATP. The aldehyde group of glyceraldehyde 3-phosphate is oxidized, not to a free carboxyl group but to a carboxylic acid anhydride with phosphoric acid. This type of anhydride, called an acyl phosphate, has a very high standard free energy of hydrolysis (AG'° = —49.3 kJ/mol; see Fig. 13-4, Table 13-6). Much of the free energy of oxidation of the aldehyde group of glyceraldehyde 3-phosphate is conserved by formation of the acyl phosphate group at C-1 of 1,3-bisphosphoglycerate.

The acceptor of hydrogen in the glyceraldehyde 3-phosphate dehydrogenase reaction is NAD+ (see Fig. 13-15), bound to a Rossmann fold as shown in Figure 13-16. The reduction of NAD+ proceeds by the enzymatic transfer of a hydride ion (:H—) from the aldehyde group of glyceraldehyde 3-phosphate to the nicoti namide ring of NAD+, yielding the reduced coenzyme NADH. The other hydrogen atom of the substrate molecule is released to the solution as H+.

Glyceraldehyde 3-phosphate is covalently bound to the dehydrogenase during the reaction (Fig. 14-7). The aldehyde group of glyceraldehyde 3-phosphate reacts with the —SH group of an essential Cys residue in the active site, in a reaction analogous to the formation of a hemiacetal (see Fig. 7-5), in this case producing a thio-hemiacetal. Reaction of the essential Cys residue with a heavy metal such as Hg2 + irreversibly inhibits the enzyme.

Because cells maintain only limited amounts of NAD + , glycolysis would soon come to a halt if the NADH formed in this step of glycolysis were not continuously reoxidized. The reactions in which NAD+ is regenerated anaerobically are described in detail in Section 14.3, in our discussion of the alternative fates of pyruvate.

Glyceraldehyde

3-phosphate dehydrogenase

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