PMID:15511226
| Citation |
Lunn, FA and Bearne, SL (2004) Alternative substrates for wild-type and L109A E. coli CTP synthases: kinetic evidence for a constricted ammonia tunnel. Eur. J. Biochem. 271:4204-12 |
|---|---|
| Abstract |
Cytidine 5'-triphosphate (CTP) synthase catalyses the ATP-dependent formation of CTP from uridine 5'-triphosphate using either NH(3) or l-glutamine as the nitrogen source. The hydrolysis of glutamine is catalysed in the C-terminal glutamine amide transfer domain and the nascent NH(3) that is generated is transferred via an NH(3) tunnel [Endrizzi, J.A., Kim, H., Anderson, P.M. & Baldwin, E.P. (2004) Biochemistry43, 6447-6463] to the active site of the N-terminal synthase domain where the amination reaction occurs. Replacement of Leu109 by alanine in Escherichia coli CTP synthase causes an uncoupling of glutamine hydrolysis and glutamine-dependent CTP formation [Iyengar, A. & Bearne, S.L. (2003) Biochem. J.369, 497-507]. To test our hypothesis that L109A CTP synthase has a constricted or a leaky NH(3) tunnel, we examined the ability of wild-type and L109A CTP synthases to utilize NH(3), NH(2)OH, and NH(2)NH(2) as exogenous substrates, and as nascent substrates generated via the hydrolysis of glutamine, gamma-glutamyl hydroxamate, and gamma-glutamyl hydrazide, respectively. We show that the uncoupling of the hydrolysis of gamma-glutamyl hydroxamate and nascent NH(2)OH production from N(4)-hydroxy-CTP formation is more pronounced with the L109A enzyme, relative to the wild-type CTP synthase. These results suggest that the NH(3) tunnel of L109A, in the presence of bound allosteric effector guanosine 5'-triphosphate, is not leaky but contains a constriction that discriminates between NH(3) and NH(2)OH on the basis of size. |
| Links |
PubMed Online version:10.1111/j.1432-1033.2004.04360.x |
| Keywords |
Ammonia/chemistry; Binding Sites; Carbon-Nitrogen Ligases/chemistry; Catalysis; Cytidine Triphosphate/chemistry; Escherichia coli/enzymology; Glutamates/chemistry; Glutaminase/chemistry; Guanosine Triphosphate/chemistry; Hydroxamic Acids/chemistry; Kinetics; Leucine/chemistry; Models, Chemical; Models, Molecular; Mutation; Protein Conformation; Protein Structure, Tertiary; Substrate Specificity; Temperature |
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