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

Article abstract of DOI:10.1016/j.bioorg.2004.08.005

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.