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 3.5.4.25), this enzyme does not release formate, but does hydrolyse the diphosphate from GTP to phosphate.
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The expected taxonomic range for this enzyme is: Archaea, Bacteria
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SYSTEMATIC NAME
IUBMB Comments
GTP 8,9-hydrolase (phosphate-forming)
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 3.5.4.25), this enzyme does not release formate, but does hydrolyse the diphosphate from GTP to phosphate.
enzyme has diphosphate phosphohydrolase and GTP cyclohydrolase activities, no activity with 8-bromo-GTP, 8-azido-GTP, alpha,beta-methylene-GTP, ATP, GDP, GMP and XTP
calcium is detected up to 10fold in molar excess of the protein. Treatment of the protein with Chelex (Na+ form) reduces the Ca2+ to 4fold excess but does not eliminate it
required for cyclohydrolase activity, maximal activity at 4 mM, no other metal supports full activity, maximal diphosphate phosphohydrolase activity in the presence of 5 mM MgCl2
three metal ions are located in the active site, two of which occupy positions that are analogous to those occupied by divalent metal ions in the structures of a number of palm domain containing proteins, such as DNA polymerase I. Two conserved Asp residues that coordinate the metal ions, which are also found in palm domain containing proteins, are observed in GCH III. Site-directed variants (Asp->Asn) of these residues in GCH III are less active than wild-type. The third metal ion, most likely a potassium ion, is involved in substrate recognition through coordination of O6 of GTP. The arrangement of the metal ions in the active site suggests that GCH III utilizes two metal ion catalysis
three metal ions are located in the active site, two of which occupy positions that are analogous to those occupied by divalent metal ions in the structures of a number of palm domain containing proteins, such as DNA polymerase I. Two conserved Asp residues that coordinate the metal ions, which are also found in palm domain containing proteins, are observed in GCH III. Site-directed variants (Asp->Asn) of these residues in GCH III are less active than wild-type. The third metal ion, most likely a potassium ion, is involved in substrate recognition through coordination of O6 of GTP. The arrangement of the metal ions in the active site suggests that GCH III utilizes two metal ion catalysis
each monomer is composed of an N- and a C-terminal domain that adopt nearly superimposible structures, suggesting that the protein has arisen by gene duplication
each monomer is composed of an N- and a C-terminal domain that adopt nearly superimposible structures, suggesting that the protein has arisen by gene duplication
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CRYSTALLIZATION (Commentary)
ORGANISM
UNIPROT
LITERATURE
large square orthorhombic crystals containing the enzyme/GTP complex are obtained when both GTP and K+ is present in the crystallization drop and when Mg2+ is absent
Morrison, S.D.; Roberts, S.A.; Zegeer, A.M.; Montfort, W.R., Bandarian, V.
A new use for a familiar fold: the X-ray crystal structure of GTP-bound GTP cyclohydrolase III from Methanocaldococcus jannaschii reveals a two metal ion catalytic mechanism
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History
3.5.4.29 GTP cyclohydrolase IIa
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