Enzyme Architecture: Erection of Active Orotidine 5′-Monophosphate Decarboxylase by Substrate-Induced Conformational Changes

Orotidine 5′-monophosphate decarboxylase (OMPDC) catalyzes the decarboxylation of 5-fluoroorotate (FO) with k_cat/K_m = 1.4 × 10^-7 M^-1 s^-1. Combining this and related kinetic parameters shows that the 31 kcal/mol stabilization of the transition state for decarboxylation of OMP provided by OMPDC represents the sum of 11.8 and 10.6 kcal/mol stabilization by the substrate phosphodianion and the ribosyl ring, respectively, and an 8.6 kcal/mol stabilization from the orotate ring. The transition state for OMPDC-catalyzed decarboxylation of FO is stabilized by 5.2, 7.2, and 9.0 kcal/mol, respectively, by 1.0 M phosphite dianion, d-glycerol 3-phosphate and d-erythritol 4-phosphate. The stabilization is due to the utilization of binding interactions of the substrate fragments to drive an enzyme conformational change, which locks the orotate ring of the whole substrate, or the substrate pieces in a caged complex. We propose that enzyme-activation is a possible, and perhaps probable, consequence of any substrate-induced enzyme conformational change.