Uronate Isomerase: A Nonhydrolytic Member of the Amidohydrolase Superfamily with an Ambivalent Requirement for a Divalent Metal Ion

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Abstract
Uronate isomerase, a member of the amidohydrolase superfamily, catalyzes the isomerization of d-glucuronate and d-fructuronate. During the interconversion of substrate and product the hydrogen at C2 of d-glucuronate is transferred to the pro-R position at C1 of the product, d-fructuronate. The exchange of the transferred hydrogen with solvent deuterium occurs at a rate that is 4 orders of magnitude slower than the interconversion of substrate and product. The enzyme catalyzes the elimination of fluoride from 3-deoxy-3-fluoro-d-glucuronate. These results have been interpreted to suggest a chemical reaction mechanism in which an active site base abstracts the proton from C2 of d-glucuronate to form a cis-enediol intermediate. The conjugate acid then transfers this proton to C1 of the cis-enediol intermediate to form d-fructuronate. The loss of fluoride from 3-deoxy-3-fluoro-d-glucuronate is consistent with a stabilized carbanion at C2 of the substrate during substrate turnover. The slow exchange of the transferred hydrogen with solvent water is consistent with a shielded conjugate acid after abstraction of the proton from either d-glucuronate or d-fructuronate during the isomerization reaction. This conclusion is supported by the competitive inhibition of the enzymatic reaction by d-arabinaric acid and the monohydroxamate derivative with Ki values of 13 and 670 nM, respectively. There is no evidence to support a hydride transfer mechanism for uronate isomerase. The wild type enzyme was found to contain 1 equiv of zinc per subunit. The divalent cation could be removed by dialysis against the metal chelator, dipicolinate. However, the apoenzyme has the same catalytic activity as the Zn-substituted enzyme and thus the divalent metal ion is not required for enzymatic activity. This is the only documented example of a member in the amidohydrolase superfamily that does not require one or two divalent cations for enzymatic activity.

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