Reactions of d-glyceraldehyde 3-phosphate dehydrogenase facilitated by oxidized nicotinamide–adenine dinucleotide

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Abstract
Transient kinetic methods have been used to study the influence of NAD+ on the rate of elementary processes of the reversible oxidative phosphorylation of d-glyceraldehyde 3-phosphate catalysed by d-glyceraldehyde 3-phosphate dehydrogenase. In the pH range 5–8 NAD+ is bound to the enzyme during the following elementary processes of the mechanism: phosphorolysis of the acyl-enzyme, its formation from 1,3-diphosphoglycerate and the enzyme and the formation and breakdown of the glyceraldehyde 3-phosphate–enzyme complex. The rates of these four elementary processes only equal or exceed the turnover rate of the enzyme when NAD+ is bound and are as much as 104 times the rates in the absence of NAD+. Autocatalysis of the reductive dephosphorylation of 1,3-diphosphoglycerate occurs when glyceraldehyde 3-phosphate release is rate determining because NAD+ is a reaction product. An important feature of the enzyme mechanism is that the negative-free-energy change of a chemical reaction, acyl-enzyme formation, is linked in a simple way to the positive-free-energy change of a dissociation reaction, NAD+ release.

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