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  • Title: Inhibition of E. coli CTP synthase by the "positive" allosteric effector GTP.
    Author: MacDonnell JE, Lunn FA, Bearne SL.
    Journal: Biochim Biophys Acta; 2004 Jun 01; 1699(1-2):213-20. PubMed ID: 15158730.
    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.
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