Dataset Information

Mechanism of the orotidine 5'-monophosphate decarboxylase-catalyzed reaction: evidence for substrate destabilization.

ABSTRACT: The reaction catalyzed by orotidine 5'-monophosphate decarboxylase (OMPDC) involves a stabilized anionic intermediate, although the structural basis for the rate acceleration (k(cat)/k(non), 7.1 x 10(16)) and proficiency [(k(cat)/K(M))/k(non), 4.8 x 10(22) M(-1)] is uncertain. That the OMPDCs from Methanothermobacter thermautotrophicus (MtOMPDC) and Saccharomyces cerevisiae (ScOMPDC) catalyze the exchange of H6 of the UMP product with solvent deuterium allows an estimate of a lower limit on the rate acceleration associated with stabilization of the intermediate and its flanking transition states (>or=10(10)). The origin of the "missing" contribution, or=10(10)), is of interest. Based on structures of liganded complexes, unfavorable electrostatic interactions between the substrate carboxylate group and a proximal Asp (Asp 70 in MtOMPDC and Asp 91 in ScOMPDC) have been proposed to contribute to the catalytic efficiency [Wu, N., Mo, Y., Gao, J., and Pai, E. F. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 2017-2022]. We investigated that hypothesis by structural and functional characterization of the D70N and D70G mutants of MtOMPDC and the D91N mutant of ScOMPDC. The substitutions for Asp 70 in MtOMPDC significantly decrease the value of k(cat) for decarboxylation of FOMP (a more reactive substrate analogue) but have little effect on the value of k(ex) for exchange of H6 of FUMP with solvent deuterium; the structures of wild-type MtOMPDC and its mutants are superimposable when complexed with 6-azaUMP. In contrast, the D91N mutant of ScOMPDC does not catalyze exchange of H6 of FUMP; the structures of wild-type ScOMPDC and its D91N mutant are not superimposable when complexed with 6-azaUMP, with differences in both the conformation of the active site loop and the orientation of the ligand vis a vis the active site residues. We propose that the differential effects of substitutions for Asp 70 of MtOMPDC on decarboxylation and exchange provide additional evidence for a carbanionic intermediate as well as the involvement of Asp 70 in substrate destabilization.

SUBMITTER: Chan KK

PROVIDER: S-EPMC2697262 | biostudies-literature | 2009 Jun

REPOSITORIES: biostudies-literature

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Mechanism of the orotidine 5'-monophosphate decarboxylase-catalyzed reaction: evidence for substrate destabilization.

Chan Kui K KK Wood B McKay BM Fedorov Alexander A AA Fedorov Elena V EV Imker Heidi J HJ Amyes Tina L TL Richard John P JP Almo Steven C SC Gerlt John A JA

Biochemistry 20090601 24

The reaction catalyzed by orotidine 5'-monophosphate decarboxylase (OMPDC) involves a stabilized anionic intermediate, although the structural basis for the rate acceleration (k(cat)/k(non), 7.1 x 10(16)) and proficiency [(k(cat)/K(M))/k(non), 4.8 x 10(22) M(-1)] is uncertain. That the OMPDCs from Methanothermobacter thermautotrophicus (MtOMPDC) and Saccharomyces cerevisiae (ScOMPDC) catalyze the exchange of H6 of the UMP product with solvent deuterium allows an estimate of a lower limit on the ...[more]

PMID: 19435314

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Project description:The reaction catalyzed by orotidine 5'-monophosphate decarboxylase (OMPDC) is accompanied by exceptional values for rate enhancement (k(cat)/k(non) = 7.1 × 10(16)) and catalytic proficiency [(k(cat)/K(M))/k(non) = 4.8 × 10(22) M(-1)]. Although a stabilized vinyl carbanion/carbene intermediate is located on the reaction coordinate, the structural strategies by which the reduction in the activation energy barrier is realized remain incompletely understood. This laboratory recently reported that "substrate destabilization" by Asp 70 in the OMPDC from Methanothermobacter thermoautotrophicus (MtOMPDC) lowers the activation energy barrier by ?5 kcal/mol (contributing ~2.7 × 10(3) to the rate enhancement) [Chan, K. K., Wood, B. M., Fedorov, A. A., Fedorov, E. V., Imker, H. J., Amyes, T. L., Richard, J. P., Almo, S. C., and Gerlt, J. A. (2009) Biochemistry 48, 5518-5531]. We now report that substitutions of hydrophobic residues in a pocket proximal to the carboxylate group of the substrate (Ile 96, Leu 123, and Val 155) with neutral hydrophilic residues decrease the value of k(cat) by as much as 400-fold but have a minimal effect on the value of k(ex) for exchange of H6 of the FUMP product analogue with solvent deuterium; we hypothesize that this pocket destabilizes the substrate by preventing hydration of the substrate carboxylate group. We also report that substitutions of Ser 127 that is proximal to O4 of the orotate ring decrease the value of k(cat)/K(M), with the S127P substitution that eliminates hydrogen bonding interactions with O4 producing a 2.5 × 10(6)-fold reduction; this effect is consistent with delocalization of the negative charge of the carbanionic intermediate on O4 that produces an anionic carbene intermediate and thereby provides a structural strategy for stabilization of the intermediate. These observations provide additional information about the identities of the active site residues that contribute to the rate enhancement and, therefore, insights into the structural strategies for catalysis.

Dataset Information

Mechanism of the orotidine 5'-monophosphate decarboxylase-catalyzed reaction: evidence for substrate destabilization.

Publications

Mechanism of the orotidine 5'-monophosphate decarboxylase-catalyzed reaction: evidence for substrate destabilization.

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