Orotidine 5'-phosphate decarboxylase

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Orotidine-5'-phosphate decarboxylase
OMP decarboxylase.png
E. coli OMP decarboxylase.[1]
Identifiers
EC number 4.1.1.23
CAS number 9024-62-8
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum
Gene Ontology AmiGO / EGO

Orotidine 5’-phosphate decarboxylase (OMP decarboxylase) or orotidylate decarboxylase is an enzyme involved in pyrimidine biosynthesis. It catalyzes the decarboxylation of orotidine monophosphate (OMP) to form uridine monophosphate (UMP). The function of this enzyme is essential to the de novo biosynthesis of the pyrimidine nucleotides uridine triphosphate, cytidine triphosphate, and thymidine triphosphate. OMP decarboxylase has been a frequent target for scientific investigation because of its demonstrated extreme catalytic efficiency and its usefulness as a selection marker for yeast strain engineering.

Schematic of reaction catalyzed by OMP decarboxylase

Catalysis[edit]

OMP decarboxylase is known for being an extraordinarily efficient catalyst capable of accelerating the uncatalyzed reaction rate by a factor of 1017. To put it in perspective, the enzyme can catalyze the substrate in 18 milliseconds, in a reaction that would take 78 million years to complete without the enzyme.[2] This extreme enzymatic efficiency is especially interesting because OMP decarboxylases uses no cofactor and contains no metal sites[3] or prosthetic groups.[4] The catalysis relies on a handful of charged amino acid residues positioned within the active site of the enzyme.

Image representing the structure of the active site of OMP decarboxylase when bound to the inhibitor BMP. Note the Lys and Asp residues surrounding the 6-hydroxyl of the substrate. (Image captured from pymol viewer snapshot of 1LOR crystal structure)[5]

The exact mechanism by which OMP decarboxylase catalyzes its reaction has been a subject of rigorous scientific investigation. The driving force for the loss of the carboxyl linked to the C6 of the pyrimidine ring comes from the close proximity of an aspartate residue carboxyl group in the enzyme's active site, which destabilizes the ground state relative to the transition state of the uncatalyzed reaction. There have been multiple hypotheses about what form the transition state takes before protonation of the C6 carbon occurs to yield the final product. Many studies investigated the binding of a potent inhibitor of OMP decarboxylase, 6-hydroxy uridine monophosphate (BMP, a barbituric acid derivative), within the active site, to identify which essential amino acid residues are directly involved with stabilization of the transition state. (See figure of enzyme bound to BMP) Several mechanisms for enzymatic decarboxylation of OMP have been proposed, including protonation at O2 to form a zwitterionic species as an intermediate,[6] anion stabilization of O4,[7] or nucleophilic attack at C5.[8] Current consensus suggests that the mechanism proceeds through a stabilized carbanion at the C6 after loss of carbon dioxide. This mechanism was suggested from studies investigating kinetic isotope effects in conjunction with competitive inhibition and active site mutagenesis.[9][10][11][12] In this mechanism the short-lived carbanion species is stabilized by a nearby lysine residue, before it is quenched by a proton. (See schematic of catalytic mechanism)

Reaction schematic showing the mechanism of OMP decarboxylase catalysis through a putative vinyl carbanion at the C6 position. This carbanion is likely stabilized by the nearby protonated lysine residue. The Lys93 and Asp91 residue numbering corresponds to the sequence for OMP decarboxylase from S cerevisiae.[13]

OMP decarboxylase vs UMP synthase[edit]

In yeast and bacteria, OMP decarboxylase is a single-function enzyme. However, in mammals, OMP decarboxylase is part of a single protein with two catalytic activities. This bifunctional enzyme is named UMP synthase and it also catalyzes the preceding reaction in pyrimidine nucleotide biosynthesis, the transfer of ribose 5-phosphate from 5-phosphoribosyl-1-pyrophosphate to orotate to form OMP. In organisms utilizing OMP decarboxylase, this reaction is catalyzed by orotate phosphoribosyltransferase.[14]

Importance in Yeast Genetics[edit]

Mutations in the gene encoding OMP decarboxylase in yeast (URA3) leads to auxotrophy in uracil. in addition, a function OMP decarboxylase renders yeast strains sensitive to the molecule 5-fluoro orotic acid (5-FOA).[15] The establishment of the URA3 gene as a selection marker with both positive and negative selection strategies has made the controlled expression of OMP decarboxylase a significant laboratory tool for the investigation of yeast genetics.

References[edit]

  1. ^ PDB 1EIX; Harris P, Navarro Poulsen JC, Jensen KF, Larsen S (April 2000). "Structural basis for the catalytic mechanism of a proficient enzyme: orotidine 5'-monophosphate decarboxylase". Biochemistry 39 (15): 4217–24. doi:10.1021/bi992952r. PMID 10757968. 
  2. ^ Radzicka A, Wolfenden R (January 1995). "A proficient enzyme". Science 267 (5194): 90–3. doi:10.1126/science.7809611. PMID 7809611. 
  3. ^ Miller BG, Smiley JA, Short SA, Wolfenden R (August 1999). "Activity of yeast orotidine-5'-phosphate decarboxylase in the absence of metals". J. Biol. Chem. 274 (34): 23841–3. doi:10.1074/jbc.274.34.23841. PMID 10446147. 
  4. ^ Miller BG, Wolfenden R (2002). "Catalytic proficiency: the unusual case of OMP decarboxylase". Annu. Rev. Biochem. 71: 847–85. doi:10.1146/annurev.biochem.71.110601.135446. PMID 12045113. 
  5. ^ Wu N, Pai EF (August 2002). "Crystal structures of inhibitor complexes reveal an alternate binding mode in orotidine-5'-monophosphate decarboxylase". J. Biol. Chem. 277 (31): 28080–7. doi:10.1074/jbc.M202362200. PMID 12011084. 
  6. ^ Beak P, Siegel B (1976). "Mechanism of decarboxylation of 1,3-dimethylorotic acid. A model for orotidine 5'-phosphate decarboxylase.". J Am Chem Soc. 98 (12): 3601–6. doi:10.1021/ja00428a035. PMID 1270703. 
  7. ^ Lee JK, Houk KN (May 1997). "A proficient enzyme revisited: the predicted mechanism for orotidine monophosphate decarboxylase". Science 276 (5314): 942–5. doi:10.1126/science.276.5314.942. PMID 9139656. 
  8. ^ Silverman, R.B. Groziak, M.P. (1982). "Model Chemistry for a Covalent Mechanism of Action of Orotidine 5'-Phosphate Decarboxylase". J. Am. Chem. Soc. 104 (23): 6434–6439. doi:10.1021/ja00387a047. 
  9. ^ Lee JK, Tantillo DJ (2004). "Topics in Current Chemistry 238: Orotidine Monophosphate Decarboxylase: A Mechanistic Dialogue". Eds. (New York: Springer-Verlag). 
  10. ^ Richavy MA Cleland WW (2000). "Determination of the Mechanism of Orotidine 5'-Monophosphate Decarboxylase by Isotope Effects". Biochemistry 39 (16): 4569–4574. doi:10.1021/bi000376p. PMID 10769111. 
  11. ^ Toth K, Amyes TL, Wood BM, Chan K, Gerlt JA, Richard JP (October 2007). "Product Deuterium Isotope Effect for Orotidine 5'-Monophosphate Decarboxylase: Evidence for the Existence of a Short-Lived Carbanion Intermediate". J. Am. Chem. Soc. 129 (43): 12946–7. doi:10.1021/ja076222f. PMC 2483675. PMID 17918849. 
  12. ^ Amyes TL, Wood BM, Chan K, Gerlt JA, Richard JP (February 2008). "Formation and Stability of a Vinyl Carbanion at the Active Site of Orotidine 5′-Monophosphate Decarboxylase: pKa of the C-6 Proton of Enzyme-Bound UMP". J. Am. Chem. Soc. 130 (5): 1574–5. doi:10.1021/ja710384t. PMC 2652670. PMID 18186641. 
  13. ^ Van Vleet JL, Reinhardt LA, Miller BG, Sievers A, Cleland WW (January 2008). "Carbon isotope effect study on orotidine 5'-monophosphate decarboxylase: support for an anionic intermediate". Biochemistry 47 (2): 798–803. doi:10.1021/bi701664n. PMID 18081312. 
  14. ^ Yablonski MJ, Pasek DA, Han BD, Jones ME, Traut TW. (1996). "Intrinsic activity and stability of bifunctional human UMP synthase and its two separate catalytic domains, orotate phosphoribosyltransferase and orotidine-5'-phosphate decarboxylase". J Biol Chem. 271 (18): 10704–10708. doi:10.1074/jbc.271.18.10704. PMID 8631878. 
  15. ^ Boeke JD, LaCroute F, Fink GR (1984). "A positive selection for mutants lacking orotidine-5'-phosphate decarboxylase activity in yeast: 5-fluoro-orotic acid resistance". Mol Gen Genet 197 (2): 345–346. doi:10.1007/BF00330984. PMID 6394957.