The effective molarity of the substrate phosphoryl group in the transition state for yeast OMP decarboxylase.
The second order rate constant (k(cat)/K(m)) for decarboxylation of orotidine by yeast OMP decarboxylase (ODCase), measured by trapping (14)CO(2) released during the reaction, is 2 x 10(-4)M(-1)s(-1). This very low activity may be compared with a value of 3 x 10(7)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 x 10(-4)M), imply an effective concentration of 1.1 x 10(9)M for the substrate phosphoryl group in stabilizing the transition state for enzymatic decarboxylation of OMP. The observed difference in rate (1.5 x 10(11)-fold) is the largest effect of a simple substituent that appears to have been reported for an enzyme reaction.
Sievers
Annette
A
Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599-7260, USA.
Wolfenden
Richard
R
D000409:Alanine / Q000737:chemistry; D001224:Aspartic Acid / Q000737:chemistry; D001667:Binding, Competitive; D003653:Decarboxylation; D015394:Molecular Structure; D009154:Mutation; D009964:Orotidine-5'-Phosphate Decarboxylase / Q000737:chemistry* / Q000378:metabolism*; D010710:Phosphates / Q000737:chemistry / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D013379:Substrate Specificity; D013997:Time Factors; D014529:Uridine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism
D010710:Phosphates; D001224:Aspartic Acid; C008714:orotidine; D009964:Orotidine-5'-Phosphate Decarboxylase; D000409:Alanine; D014529:Uridine