Histidine residues glyceraldehyde-3-phosphate dehydrogenase

Let us consider the mechanism of glyceraldehyde 3-phosphate dehydrogenase in detail (Figure 16.8). In step 1, the aldehyde substrate reacts with the sulfhydryl group of cysteine 149 on the enzyme to form a hemithioacetal. Step 2 is the transfer of a hydride ion to a molecule of NAD + that is tightly bound to the enzyme and is adjacent to the cysteine residue. This reaction is favored by the deprotonation of the hemithioacetal by histidine 176. The products of this reaction are the reduced coenzyme NADH and a thioester intermediate. This thioester intermediate has a free energy close to that of the reactants. In step 3, orthophosphate attacks the thioester to form 1,3-BPG and free the cysteine residue. This displacement occurs only after the NADH formed from the aldehyde oxidation has left the enzyme and been replaced by a second NAD+. The positive charge on the NAD+ may help polarize the thioester intermediate to facilitate the attack by orthophosphate. 

Figure 16.7. Structure of Glyceraldehyde 3-Phosphate Dehydrogenase. The active site includes a cysteine residue and a histidine residue adjacent to a hound NAD+. 

A. Soukri, A. Mougin, C. Corbier, A. Wonacott, C. Branlant, and G. Branlant. 1989. Role of the histidine 176 residue in glyceraldehyde-3-phosphate dehydrogenase as probed by site-directed mutagenesis Biochemistry 28 2586-2592. (PubMed)... 

Figure 16.6 Structure of glyceraldehyde 3-phosphate dehydrogenase. Notice that the active site includes a cysteine residue and a histidine residue adjacent to a bound NAD molecule. The sulfur atom of cysteine will link with the substrate to form a transitory thioester intermediate. [Drawn from IGAD.pdb,]...

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