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) x OMP complex, with a K(d) of 50 mM, to form the 9-fold more reactive ternary E(mut) x OMP x Gua(+) complex. The "effective molarity" of the cationic side chain of Arg-235 at the wild-type enzyme is calculated to be 160 M.