title: Uridine-5'-phosphate synthase: evidence for substrate cycling involving this bifunctional protein.
abstract: Uridine 5'-phosphate (UMP) synthase contains two sequential catalytic activities for the synthesis of orotidine 5'-phosphate (OMP) from orotate (EC 2.4.2.10, orotate phosphoribosyltransferase) and the decarboxylation of OMP to form UMP (EC 4.1.1.23, OMP decarboxylase). Previous kinetic studies had indicated that partial channeling of OMP might occur [T.W. Traut and M.E. Jones (1977) J. Biol. Chem. 252, 8374-8381]; in the presence of a nucleotidase, there was no measurable formation of orotidine from OMP under conditions where OMP was maintained at a steady-state concentration [T.W. Traut (1980) Arch. Biochem. Biophys. 200, 590-594]. Recently claims were made that (i) the steady-state activities of UMP synthase could be modeled by Michaelis-Menten kinetics, and (ii) the nucleotidase activity in Ehrlich ascites cells was insufficient to degrade any significant amount of OMP [R.W. McClard and K.M. Shokat (1987) Biochemistry 26, 3378-3384]. The present studies show that UMP synthase has cooperative kinetics toward OMP, and that a substrate cycle involving orotate phosphoribosyltransferase, cytoplasmic nucleotidase, and uridine phosphorylase maintains the cyclic interconversion: orotate----OMP----orotidine----orotate, etc. It is therefore the complex steady-state kinetics of UMP synthase in the presence of OMP, and the existence of a substrate cycle that account for the results which were interpreted as channeling in the earlier studies.
authors.affiliation: Department of Biochemistry, University of North Carolina School of Medicine, Chapel Hill 27599.
authors.forename: T W
authors.initials: TW
authors.lastname: Traut
mesh_terms: D000818:Animals; D002262:Carboxy-Lyases / Q000378:metabolism*; D002286:Carcinoma, Ehrlich Tumor / Q000201:enzymology; D007700:Kinetics; D051379:Mice; D009097:Multienzyme Complexes / Q000378:metabolism*; D009962:Orotate Phosphoribosyltransferase / Q000378:metabolism*; D009963:Orotic Acid / Q000378:metabolism; D009964:Orotidine-5'-Phosphate Decarboxylase / Q000378:metabolism*; D010430:Pentosyltransferases / Q000378:metabolism*; D014542:Uridine Monophosphate / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D009097:Multienzyme Complexes; C005417:orotidylic acid; D009963:Orotic Acid; C027157:uridine 5'-monophosphate synthase; D014542:Uridine Monophosphate; D010430:Pentosyltransferases; D009962:Orotate Phosphoribosyltransferase; D002262:Carboxy-Lyases; D009964:Orotidine-5'-Phosphate Decarboxylase