BIOCHEMISTRY: Who's the Most Proficient of All?

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Science  05 Aug 2005:
Vol. 309, Issue 5736, pp. 852d
DOI: 10.1126/science.309.5736.852d

It might seem a straight-forward matter to determine how much the enzyme urease accelerates the rate of decomposition of urea, which, after all, contains only four nonhydrogen atoms. The problem, however, is that in the nonenzymatic pathway, the elimination of ammonia precedes the addition of water, whereas the enzyme promotes the nucleophilic attack of water, yielding a tetrahedral intermediate and a different reaction pathway to the same products.

Previously, Estiu and Merz carried out a computational analysis of the uncatalyzed reaction, relying both on earlier studies with small dinickel molecules and on structural analysis of the dinickel cluster at the urease active site. They found that the catalytic proficiency of the enzyme, calculated by dividing the biochemical quantity kcat/Km by the rate constant of the noncatalyzed reaction (knon), is many orders of magnitude greater than that of the reigning champion, ornithine 5'-monophosphate decarboxylase. In contrast, Callahan et al. have measured the hydrolysis of substituted ureas, which cannot undergo elimination, and extrapolated from these data to arrive at a much faster non-catalyzed hydrolysis rate—one that confirms urease as a proficient, but not record-setting, catalyst. — GJC

J. Am. Chem. Soc. 126, 6932; 10.1021/ja0525399 (2005).

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