The R235A mutation at yeast orotidine 5′-monophosphate decarboxylase (OMPDC) results in a 1300-fold increase in Km and a 14-fold decrease in kcat 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 Emut·OMP complex, with a Kd of 50 mM, to form the 9-fold more reactive ternary Emut·OMP·Gua + complex. The "effectivemolarity" of the cationic side chain of Arg-235 at the wild-type enzyme is calculated to be 160 M.
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