Phosphoenol-3-bromopyruvate. A mechanism-based inhibitor of phosphoenolpyruvate carboxylase from maize.

Phosphoenol-3-bromopyruvate is an excellent competitive inhibitor (versus phosphoenolpyruvate) of maize phosphoenolpyruvate carboxylase (Ki = 30 microM), provided that preincubation of enzyme with inhibitor is avoided. If the enzyme is preincubated with inhibitor in the presence of Mn2+ and HCO3(-), complete inactivation occurs over the course of about 1 h. The inactivation is first order in enzyme and is saturable with respect to inhibitor, with an apparent Michaelis constant of 67.5 microM and a half-life at high inhibitor concentration of 13.4 min. The inactivation is inhibited by phospholactate and is much slower at low HCO3(-) concentration. Incubation of the enzyme with phosphoenol-3-bromopyruvate in the presence of H14CO3(-) and Mn2+ followed by treatment with NaBH4 leads to incorporation of 14C into the inactive enzyme. If treatment with NaBH4 is omitted, the enzyme is inactivated, but no 14C is incorporated into the inactive product. It appears that phosphoenol-3-bromopyruvate is a mechanism-based inactivator of phosphoenolpyruvate carboxylase, probably because of enzyme-catalyzed conversion to 3-bromooxalacetate, which alkylates the enzyme.