Activation of R235A mutant orotidine 5′-monophosphate decarboxylase by the guanidinium cation: Effective molarity of the cationic side chain of Arg-235

The R235A mutation at yeast orotidine 5′-monophosphate decarboxylase (OMPDC) results in a 1300-fold increase in K_m and a 14-fold decrease in k_cat for decarboxylation of orotidine 5′-monophosphate, corresponding to a 5.8 kcal/mol destabilization of the transition state. There is strong activation of this mutant enzyme by added guanidinium cation (Gua ⁺): 1 M Gua⁺ stabilizes the transition state by ca. 3 kcal/mol. This stabilization is due to the binding of Gua⁺ to the binary E_mut·OMP complex, with a K_d of 50 mM, to form the 9-fold more reactive ternary E_mut·OMP·Gua ⁺ complex. The "effectivemolarity" of the cationic side chain of Arg-235 at the wild-type enzyme is calculated to be 160 M.