title: Inhibition of E. coli CTP synthase by the "positive" allosteric effector GTP.
abstract: Cytidine 5'-triphosphate (CTP) synthase catalyzes the ATP-dependent formation of CTP from UTP using either ammonia or l-glutamine as the source of nitrogen. When glutamine is the substrate, GTP is required as a positive allosteric effector to promote catalysis of glutamine hydrolysis. We show that at concentrations exceeding approximately 0.15 mM, GTP actually behaves as a negative allosteric effector of E. coli CTP synthase, inhibiting glutamine-dependent CTP formation. In addition, GTP inhibits NH(3)-dependent CTP formation in a concentration-dependent manner. However, GTP does not inhibit the enzyme's intrinsic glutaminase activity. Although the activation of CTP synthase by GTP does not display cooperative behavior, inhibition of both CTP synthase-catalyzed ammonia- and glutamine-dependent CTP synthesis by GTP do exhibit positive cooperativity. These results suggest that GTP binding affects CTP synthase catalysis in two ways: it activates enzyme-catalyzed glutamine hydrolysis and it inhibits the utilization of NH(3) as a substrate by the synthase domain.
authors.affiliation: Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 1X5.
authors.forename: Jennifer E
authors.initials: JE
authors.lastname: MacDonnell
authors.forename: Faylene A
authors.initials: FA
authors.lastname: Lunn
authors.forename: Stephen L
authors.initials: SL
authors.lastname: Bearne
mesh_terms: D000494:Allosteric Regulation; D000641:Ammonia / Q000378:metabolism; D019731:Carbon-Nitrogen Ligases / Q000037:antagonists & inhibitors*; D003570:Cytidine Triphosphate / Q000096:biosynthesis*; D004926:Escherichia coli / Q000201:enzymology*; D005972:Glutaminase / Q000037:antagonists & inhibitors; D005973:Glutamine / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism* / Q000494:pharmacology*; D006868:Hydrolysis; D007700:Kinetics; D014544:Uridine Triphosphate / Q000378:metabolism / Q000494:pharmacology
chemical_list: D005973:Glutamine; D003570:Cytidine Triphosphate; D000641:Ammonia; D006160:Guanosine Triphosphate; D005972:Glutaminase; D019731:Carbon-Nitrogen Ligases; C019951:CTP synthetase; D014544:Uridine Triphosphate