Accepted name: GTP cyclohydrolase IIa
Reaction: GTP + 3 H2O = 2-amino-5-formylamino-6-(5-phospho-D-ribosylamino)pyrimidin-4(3H)-one + 2 phosphate
Systematic name: GTP 8,9-hydrolase (phosphate-forming)
Comments: Requires Mg2+. This enzyme catalyses the hydrolysis of the imidazole ring of guanosine 5′-triphosphate, N7-methylguanosine 5′-triphosphate or inosine 5′-triphosphate. Xanthosine 5′-triphosphate and ATP are not substrates. It also catalyses the hydrolysis of diphosphate to form two equivalents of phosphate. Unlike GTP cyclohydrolase II (EC, this enzyme does not release formate, but does hydrolyse the diphosphate from GTP to phosphate.
1.  Graham, D.E., Xu, H. and White, R.H. A member of a new class of GTP cyclohydrolases produces formylaminopyrimidine nucleotide monophosphates. Biochemistry 41 (2002) 15074–15084. [PMID: 12475257]
[EC created 2003, modified 2011]
Accepted name: inosine/xanthosine triphosphatase
Reaction: (1) inosine 5′-triphosphate + H2O = inosine 5′-diphosphate + phosphate
(2) xanthosine 5′-triphosphate + H2O = xanthosine 5′-diphosphate + phosphate
Glossary: inosine 5′-triphosphate = ITP
xanthosine 5′-triphosphate = XTP
Other name(s): yjjX (gene name)
Systematic name: inosine/xanthosine 5′-triphosphate phosphohydrolase
Comments: The enzyme, characterized from the bacterium Escherichia coli, preferentially hydrolyses inosine triphosphate and xanthosine triphosphate, which are formed by oxidative deamination damage. By hydrolysing these damaged nucleotides, the enzyme prevents their incorporation into RNA.
1.  Zheng, J., Singh, V.K. and Jia, Z. Identification of an ITPase/XTPase in Escherichia coli by structural and biochemical analysis. Structure 13 (2005) 1511–1520. [PMID: 16216582]
[EC created 2020]