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Anatomy of a proficient enzyme: the structure of orotidine 5'-monophosphate decarboxylase in the presence and absence of a potential transition state analog.

ABSTRACT: Orotidine 5'-phosphate decarboxylase produces the largest rate enhancement that has been reported for any enzyme. The crystal structure of the recombinant Saccharomyces cerevisiae enzyme has been determined in the absence and presence of the proposed transition state analog 6-hydroxyuridine 5'-phosphate, at a resolution of 2.1 A and 2.4 A, respectively. Orotidine 5'-phosphate decarboxylase folds as a TIM-barrel with the ligand binding site near the open end of the barrel. The binding of 6-hydroxyuridine 5'-phosphate is accompanied by protein loop movements that envelop the ligand almost completely, forming numerous favorable interactions with the phosphoryl group, the ribofuranosyl group, and the pyrimidine ring. Lysine-93 appears to be anchored in such a way as to optimize electrostatic interactions with developing negative charge at C-6 of the pyrimidine ring, and to donate the proton that replaces the carboxylate group at C-6 of the product. In addition, H-bonds from the active site to O-2 and O-4 help to delocalize negative charge in the transition state. Interactions between the enzyme and the phosphoribosyl group anchor the pyrimidine within the active site, helping to explain the phosphoribosyl group's remarkably large contribution to catalysis despite its distance from the site of decarboxylation.

SUBMITTER: Miller BG

PROVIDER: S-EPMC15745 | biostudies-literature | 2000 Feb

REPOSITORIES: biostudies-literature

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Anatomy of a proficient enzyme: the structure of orotidine 5'-monophosphate decarboxylase in the presence and absence of a potential transition state analog.

Miller B G BG Hassell A M AM Wolfenden R R Milburn M V MV Short S A SA

Proceedings of the National Academy of Sciences of the United States of America 20000201 5

Orotidine 5'-phosphate decarboxylase produces the largest rate enhancement that has been reported for any enzyme. The crystal structure of the recombinant Saccharomyces cerevisiae enzyme has been determined in the absence and presence of the proposed transition state analog 6-hydroxyuridine 5'-phosphate, at a resolution of 2.1 A and 2.4 A, respectively. Orotidine 5'-phosphate decarboxylase folds as a TIM-barrel with the ligand binding site near the open end of the barrel. The binding of 6-hydrox ...[more]

PMID: 10681417

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Project description:The mystery associated with catalysis by what were once regarded as protein black boxes, diminished with the X-ray crystallographic determination of the three-dimensional structures of enzyme-substrate complexes. The report that several high-resolution X-ray crystal structures of orotidine 5'-monophosphate decarboxylase (OMPDC) failed to provide a consensus mechanism for enzyme-catalyzed decarboxylation of OMP to form uridine 5'-monophosphate, therefore, provoked a flurry of controversy. This controversy was fueled by the enormous 1023-fold rate acceleration for this enzyme, which had " jolted many biochemists' assumptions about the catalytic potential of enzymes." Our studies on the mechanism of action of OMPDC provide strong evidence that catalysis by this enzyme is not fundamentally different from less proficient catalysts, while highlighting important architectural elements that enable a peak level of performance. Many enzymes undergo substrate-induced protein conformational changes that trap their substrates in solvent occluded protein cages, but the conformational change induced by ligand binding to OMPDC is incredibly complex, as required to enable the development of 22 kcal/mol of stabilizing binding interactions with the phosphodianion and ribosyl substrate fragments of OMP. The binding energy from these fragments is utilized to activate OMPDC for catalysis of decarboxylation at the orotate fragment of OMP, through the creation of a tight, catalytically active, protein cage from the floppy, open, unliganded form of OMPDC. Such utilization of binding energy for ligand-driven conformational changes provides a general mechanism to obtain specificity in transition state binding. The rate enhancement that results from the binding of carbon acid substrates to enzymes is partly due to a reduction in the carbon acid p Ka that is associated with ligand binding. The binding of UMP to OMPDC results in an unusually large >12 unit decrease in the p Ka = 29 for abstraction of the C-6 substrate hydrogen, due to stabilization of an enzyme-bound vinyl carbanion, which is also an intermediate of OMPDC-catalyzed decarboxylation. The protein-ligand interactions operate to stabilize the vinyl carbanion at the enzyme active site compared to aqueous solution, rather than to stabilize the transition state for the concerted electrophilic displacement of CO2 by H+ that avoids formation of this reaction intermediate. There is evidence that OMPDC induces strain into the bound substrate. The interaction between the amide side chain of Gln-215 from the phosphodianion gripper loop and the hydroxymethylene side chain of Ser-154 from the pyrimidine umbrella of ScOMPDC position the amide side chain to interact with the phosphodianion of OMP. There are no direct stabilizing interactions between dianion gripper protein side chains Gln-215, Tyr-217, and Arg-235 and the pyrimidine ring at the decarboxylation transition state. Rather these side chains function solely to hold OMPDC in the catalytically active closed conformation. The hydrophobic side chains that line the active site of OMPDC in the region of the departing CO2 product may function to stabilize the decarboxylation transition state by providing hydrophobic solvation of this product.

Project description:The caged complex between orotidine 5'-monophosphate decarboxylase (ScOMPDC) and 5-fluoroorotidine 5'-monophosphate (FOMP) undergoes decarboxylation ?300 times faster than the caged complex between ScOMPDC and the physiological substrate, orotidine 5'-monophosphate (OMP). Consequently, the enzyme conformational changes required to lock FOMP at a protein cage and release product 5-fluorouridine 5'-monophosphate (FUMP) are kinetically significant steps. The caged form of ScOMPDC is stabilized by interactions between the side chains from Gln215, Tyr217, and Arg235 and the substrate phosphodianion. The control of these interactions over the barrier to the binding of FOMP and the release of FUMP was probed by determining the effect of all combinations of single, double, and triple Q215A, Y217F, and R235A mutations on kcat/Km and kcat for turnover of FOMP by wild-type ScOMPDC; its values are limited by the rates of substrate binding and product release, respectively. The Q215A and Y217F mutations each result in an increase in kcat and a decrease in kcat/Km, due to a weakening of the protein-phosphodianion interactions that favor fast product release and slow substrate binding. The Q215A/R235A mutation causes a large decrease in the kinetic parameters for ScOMPDC-catalyzed decarboxylation of OMP, which are limited by the rate of the decarboxylation step, but much smaller decreases in the kinetic parameters for ScOMPDC-catalyzed decarboxylation of FOMP, which are limited by the rate of enzyme conformational changes. By contrast, the Y217A mutation results in large decreases in kcat/Km for ScOMPDC-catalyzed decarboxylation of both OMP and FOMP, because of the comparable effects of this mutation on rate-determining decarboxylation of enzyme-bound OMP and on the rate-determining enzyme conformational change for decarboxylation of FOMP. We propose that kcat = 8.2 s(-1) for decarboxylation of FOMP by the Y217A mutant is equal to the rate constant for cage formation from the complex between FOMP and the open enzyme, that the tyrosyl phenol group stabilizes the closed form of ScOMPDC by hydrogen bonding to the substrate phosphodianion, and that the phenyl group of Y217 and F217 facilitates formation of the transition state for the rate-limiting conformational change. An analysis of kinetic data for mutant enzyme-catalyzed decarboxylation of OMP and FOMP provides estimates for the rate and equilibrium constants for the conformational change that traps FOMP at the enzyme active site.

Dataset Information

Anatomy of a proficient enzyme: the structure of orotidine 5'-monophosphate decarboxylase in the presence and absence of a potential transition state analog.

Publications

Anatomy of a proficient enzyme: the structure of orotidine 5'-monophosphate decarboxylase in the presence and absence of a potential transition state analog.

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