Succinyl phosphate. Its nonenzymatic hydrolysis and reaction with coenzyme A.

Succinyl phosphate reacts rapidly and nonenzymatically with coenzyme A in the pH range 3 to 8 to yield succinyl coenzyme A. This hitherto unsuspected reaction of an acyl phosphate with a thiol depends critically on the presence within the molecule of a free carboxyl group, which bears a “succinyl” relationship-and is spatially accessible-to the phosphorylated carboxyl. The free carboxyl group interacts with the anhydride portion of the molecule at neutral pH and below. A variety of kinetic experiments point to the cyclic form of succinyl phosphate as the immediate reactant with coenzyme A in the synthesis of succinyl coenzyme A. The somewhat unorthodox mechanism which is proposed is not to be taken as proved. Succinyl phosphate and its congeners exhibit characteristic pH rate profiles of hydrolysis, which are unmistakably distinct in their contour from those of acyl phosphates which are unreactive with coenzyme A. During hydrolysis at neutral pH and below, succinyl phosphate may also react in a cyclic form, possibly as succinic anhydride following elimination of orthophosphate.