Cytidine 5¢-triphosphate (CTP) synthase catalyses the ATP-dependent formation of CTP from uridine 5¢-triphosphate using either NH3orL-glutamine as the nitrogen source. The hydrolysis of glutamine is catalysed in the C-terminal glu-tamine amide transfer domain and the nascent NH3that is generated is transferred via an NH3 tunnel [Endrizzi, ., Kim, H., Anderson, . & Baldwin, . (2004) Biochemistry43, 6447–6463] to the active site of the N-ter-minal synthasedomainwhere theaminationreactionoccurs
