The effective molarity of the substrate phosphoryl group in the transition state for yeast OMP decarboxylase

Abstract: The second order rate constant (k cat/K m) for decarboxylation of orotidine by yeast OMP decarboxylase (ODCase), measured by trapping 14CO2 released during the reaction, is 2×10−4 M−1 s−1. This very low activity may be compared with a value of 3×107 M−1 s−1 for the action of yeast OMP decarboxylase on the normal substrate OMP. Both activities are strongly inhibited by 6-hydroxy UMP (BMP), and abrogated by mutation of Asp-96 to alanine. These results, in conjunction with the binding affinity of inorganic phosphate as a competitive inhibitor (K i =7×10−4 M), imply an effective concentration of 1.1×109 M for the substrate phosphoryl group in stabilizing the transition state for enzymatic decarboxylation of OMP. The observed difference in rate (1.5×1011-fold) is the largest effect of a simple substituent that appears to have been reported for an enzyme reaction. [Copyright &y& Elsevier]