Identification of Cys139 and Glu207 as catalytically important groups in the active site of isopentenyl diphosphate:dimethylallyl diphosphate isomerase.

Isopentenyl diphosphate:dimethylallyl diphosphate isomerase (EC 5.3.3.2) catalyzes the antarafacial [1.3] allylic rearrangement of isopentenyl diphosphate (IPP) to its electrophilic allylic isomer dimethylallyl diphosphate (DMAPP). Active-site thiols at C138 and C139 were recently identified by covalent modification using active-site-directed irreversible inhibitors [Street, I. P., & Poulter, C. D. (1990) Biochemistry 29, 7531-7538; Lu, X. J., Christensen, D. J., & Poulter, C. D. (1992) Biochemistry 31, 9955-9960]. Kinetic studies were conducted with site-directed mutants of IPP isomerase (IPPIase) to evaluate the roles of these amino acids. C138S and C138V mutants were active catalysts with V/K values only 10-fold lower than that of wild-type IPPIase. In contrast, the C139S mutant was a poor catalyst, and the C139A and C139V mutants were inactive. Treatment of the C139S mutant with 3-(fluoromethyl)-3-butenyl diphosphate, an electrophilic active-site-directed irreversible inhibitor, resulted in inactivation of the enzyme by covalent modification of E207. The E207Q and E207V mutants were inactive, suggesting a role for the E207 carboxylate moiety in catalysis.