title: Aspects of the chemistry of D-glyceraldehyde 3-phosphate dehydrogenase.
abstract: Crystalline d-glyceraldehyde 3-phosphate dehydrogenase from lobster tail contains 4 moles of NAD(+) bound and reacts specifically with 4 moles of iodoacetic acid/mole of tetramer. The essential thiol group of d-glyceraldehyde 3-phosphate dehydrogenase appears to react with iodoacetic acid with a rate constant for the overall process that is independent of the extent of carboxymethylation. The d-glyceraldehyde 3-phosphate dehydrogenase-NAD(+) absorption band has a variable molar extinction coefficient in the presence of phosphate that may be correlated with a proton dissociation of pK 6.86. The binding of NAD(+) to d-glyceraldehyde 3-phosphate dehydrogenase weakens as alkylating agents react with the enzyme, and NAD(+) promotes the reactivity of the essential thiol group. It is suggested that, on binding to d-glyceraldehyde 3-phosphate dehydrogenase, NAD(+) lowers the pK of the essential thiol group, resulting in a catalytic role of NAD(+) in the reaction catalysed by d-glyceraldehyde 3-phosphate dehydrogenase. If this theory is correct, then it is likely that a proton will be liberated during the phosphorolysis of the acyl-enzyme rather than in the redox step.
mesh_terms: D000477:Alkylating Agents; D000818:Animals; D002384:Catalysis; D055598:Chemical Phenomena; D002621:Chemistry; D003445:Crustacea; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D007461:Iodoacetates; D009243:NAD
chemical_list: D000477:Alkylating Agents; D007461:Iodoacetates; D009243:NAD; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases