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Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis

[Image: see text] 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 o...

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Autores principales: Richard, John P., Amyes, Tina L., Reyes, Archie C.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: American Chemical Society 2018
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6016548/
https://www.ncbi.nlm.nih.gov/pubmed/29595949
http://dx.doi.org/10.1021/acs.accounts.8b00059
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author Richard, John P.
Amyes, Tina L.
Reyes, Archie C.
author_facet Richard, John P.
Amyes, Tina L.
Reyes, Archie C.
author_sort Richard, John P.
collection PubMed
description [Image: see text] 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 10(23)-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 pK(a) that is associated with ligand binding. The binding of UMP to OMPDC results in an unusually large >12 unit decrease in the pK(a) = 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 CO(2) 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 CO(2) product may function to stabilize the decarboxylation transition state by providing hydrophobic solvation of this product.
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spelling pubmed-60165482019-03-29 Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis Richard, John P. Amyes, Tina L. Reyes, Archie C. Acc Chem Res [Image: see text] 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 10(23)-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 pK(a) that is associated with ligand binding. The binding of UMP to OMPDC results in an unusually large >12 unit decrease in the pK(a) = 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 CO(2) 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 CO(2) product may function to stabilize the decarboxylation transition state by providing hydrophobic solvation of this product. American Chemical Society 2018-03-29 2018-04-17 /pmc/articles/PMC6016548/ /pubmed/29595949 http://dx.doi.org/10.1021/acs.accounts.8b00059 Text en Copyright © 2018 American Chemical Society This is an open access article published under an ACS AuthorChoice License (http://pubs.acs.org/page/policy/authorchoice_termsofuse.html) , which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
spellingShingle Richard, John P.
Amyes, Tina L.
Reyes, Archie C.
Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis
title Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis
title_full Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis
title_fullStr Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis
title_full_unstemmed Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis
title_short Orotidine 5′-Monophosphate Decarboxylase: Probing the Limits of the Possible for Enzyme Catalysis
title_sort orotidine 5′-monophosphate decarboxylase: probing the limits of the possible for enzyme catalysis
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6016548/
https://www.ncbi.nlm.nih.gov/pubmed/29595949
http://dx.doi.org/10.1021/acs.accounts.8b00059
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