Add time:08/17/2019         Source:sciencedirect.com

The effects of the inhibitors trimethylacetyl phosphate and cAMP have been determined in reactions catalyzed by d-glyceraldehyde-3-phosphate dehydrogenase. These inhibitors must influence the oxidation of aldehydes through substrate dependent co-operative conformational changes. Both trimethylacetyl phosphate and cAMP give sigmoidal 1V vs (I) plots in oxidation of glyceraldehyde 3-phosphate, but exert linear competitive effects on the acyl phosphatase site in acylation reactions of β-(2-furyl) acryloyl phosphate. The linear inhibition in the latter reactions indicates that one inhibitor molecule is bound per active site. Hydride transfer to NAD+ is the ratedetermining step in oxidation of benzaldehyde to an acylenzyme, as shown by the threefold decrease in Vmax without change in Km when 1-deuterobenzaldehyde is the substrate; it is very likely this step that is affected by acyl phosphate inhibitors. Plots of 1V vs cAMP concentration for oxidation of benzaldehyde at a series of trimethylacetyl phosphate concentrations are parallel at concentrations of acyl phosphate less than 0.00625 m, which demonstrates that binding of the inhibitors is mutually exclusive. However, at higher concentrations of trimethylacetyl phosphate, the slopes are affected, which shows that both inhibitors are then binding. Thus, the binding of high concentrations of acyl phosphate must result in a conformational change of the enzyme that permits binding of both inhibitors. A number of conformations with different kinetic properties are formed with the various substrate and inhibitor combinations. In reactions of muscle d-glyceraldehyde-3-phosphate dehydrogenase, binding of these inhibitors is best explained in terms of induced fit and a sequential model of conformational changes.

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