Glyceraldehyde-3-phosphate dehydrogenase EC

Reaction evidently involves formation of the S-thiohemiacetal with the super-reactive Cys-149. His-176 is thought to activate Cys-149 and to facilitate proton removal (the oxidation step being transfer of hydride to the coenzyme, with formation of the thioacyl intermediate). Thr-179 and residue 181 (e.g., Thr or Asn) probably interact with the 3-phosphate, while Ser-148 can possibly form hydrogen bonds with the C-2 hydroxyl and with the inorganic phosphate (Fig. 19B) [52]. Thr-208 or Arg-231 may also bind the inorganic phosphate. Nucleophilic attack by the phosphate on C-l, with rupture of the C-S bond, gives 1,3-diphosphoglycerate. 

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A cartoon representation of the oxidation path used in Equation 9.5 is shown in Scheme 9.5. The process is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.12). A large body of evidence has been accumulated about the role of the enzyme and, in particular, that the catalytic site on the enzyme that is used to help the oxidation occur bears a thiol (-SH, Chapter 8) group (from the amino acid cysteine. Chapter 12). Further, it has been shown that hydride (H ) transfer to nicotinamide dinucleotide (NAD ") (see Chapter 12), a required cofactor that accounts for the transfer of the hydride anion and, thus, the oxidation itself, is also involved. 

Scheme 9.6. A cartoon representation of the enzyme (named glyceraldehyde-3-phosphate dehydrogenase, EC 1.2.1.12) catalyzed oxidation of the aldehyde glyceraldehyde-3-phosphate to the carboxylic acid-phosphoric acid mixed anhydride, 1,3-bisphosphoglycerate (which can be hydrolyzed to the corresponding carboxyhc acid and phosphoric acid). Start in a clockwise direction from the npper left.

Glyceraldehyde-3-phosphate dehydrogenase (NADP+) (nonphosphorylating) [EC 1.2.1.9], also referred to as triose-phosphate dehydrogenase, catalyzes the reaction of D-glyceraldehyde 3-phosphate with NADP+ and water to produce 3-phospho-D-glycerate and NADPH. 

Conversion of acetyl-CoA to triose phosphate (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate) requires that two of the reactions of the catabolic pathway be replaced by alternative reactions catalysed by different enzymes these are (i) the pyruvate dehydrogenase complex-catalysed oxidative decarboxylation of pyruvate to acetyl-CoA, which is replac by the pyruvate synthase (EC 1.2.7.1)-catalysed reductive carboxylation of acetyl-CoA (eq. 3 I, Fig.), in which the reductant is reduced ferredoxin (Fd,ed). and (ii) the pyruvate kinase (EC 2.7.1.40)-catalysed, ATP-generating conversion of phosphoeno/pyruvate to pyruvate, which is replaced by the pyruvate, orthophosphate dikinase (EC 2.7.9.1)-catalysed, ATP-dtiven conversion of pyruvate to phosphoeno/pyruvate (eq. 4 F, Fig.),... 

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