Bioorganic Chemistry p. 45 - 52 (2005)
Update date:2022-08-17
Topics:
Sievers, Annette
Wolfenden, Richard
The second order rate constant (kcat/Km) 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 (Ki = 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.
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