The enediolate analogue 5-phosphoarabinonate as a mechanistic probe for phosphoglucose isomerase

A stable analogue has been prepared of the enediolate anion believed to occur transiently in the reaction of phosphoglucose isomerase. This compound, 5-phosphoarabinonate, is the strongest known competitive inhibitor of the enzyme (Ki = 3 times 10(-7) M below pH 7). A distinctive pH dependence of binding, also found for two other aldonic acid omega-phosphates, 6-phosphogluconate and 4-phosphoerythronate, involves pertubation of a pKa from 7.0 in the free enzyme to 9.0 in the enzyme-inhibitor complex. This perturbation may reflect a catalytically advantageous increase in basicity which occurs around the transition state of the normal enzymatic reaction.