page_title: Orotate phosphoribosyltransferase
section_title: Enzyme Mechanism
hierarchical_section_title: Orotate phosphoribosyltransferase / Enzyme Mechanism
caption_attribution_description: English: Orotate Phosphoribosyltransferase overall structure with orotic acid, magnesium and PRPP in active site pocket.
context_page_description: Orotate phosphoribosyltransferase or orotic acid phosphoribosyltransferase is an enzyme involved in pyrimidine biosynthesis. It catalyzes the formation of orotidine 5'-monophosphate from orotate and phosphoribosyl pyrophosphate. In yeast and bacteria, orotate phosphoribosyltransferase is an independent enzyme with a unique gene coding for the protein, whereas in mammals and other multicellular organisms, the catalytic function is carried out by a domain of the bifunctional enzyme UMP synthase.
context_section_description: The reaction of orotic acid (orotate) to orotidylate is catalyzed by orotate phophoribosyltransferase with the cofactor PRPP, which is a cofactor commonly used for nucleotide synthesis. It transfers pyrophosphoryl groups very favorably with a ΔG of -8.3 + 0.5 kcal/mol. Two main interactions attract PRPP to assist orotate phophoribosyltransferase in this reaction. First, orotate phophoribosyltransferase has Aspartic acid- Aspartic acid residues next to its PRPP-binding motif which interact with the ribosyl 2-/3- hydroxyl groups that stabilize the movement of Carbon-1 of the bound ribosyl group. The stabilization occurs through a hydrogen bonding network of these hydroxyl groups with pyrophosphate, water and magnesium. Second, the side-chains of the C-terminal end of the PRPP-binding motif interact favorably with PRPP’s 5-phosphate.
  In B. subtilis, PRPP is bound to these two sites with a Kd of 33mM. When orotate is present, pyrophosphate binding affinity is increased fourfold and the reaction undergoes burst kinetics, with rapid phosphoribosyl transfer and then slow release of products. This slow release is thought to be due to the solvent-exposed loop of orotate phosphoribosyltransferase that protects the active site during the first step. The loop opening happens in two-steps with the PRPP dissociation unfavorable and slow since the loop closes 85% of the time.
  Three key pyrimidine nucleosides include uridine, cytidine and thymidine, and they play major roles in nucleic acid biosynthesis as well as carbohydrate and lipid metabolism. Pyrimidine phosphoribosyltransferases such as orotate phosphoribosyltransferase activate their substrates by forming SN1-like transition states, facilitating migration of the ribosyl anomeric carbon region to MgPPᵢ. Like other pyrimidine phosphoribosyltransferases, orotate phosphoribosyltransferase has a flexible loop that moves to position groups in the ideal positions for catalysis. They also use many water molecules to hold everything in place during the reaction.
caption: Orotate phosphoribosyltransferase overall structure with orotic acid, magnesium and PRPP in the active site pocket.
exif: {}