Requires Fe2+. A zinc protein. The enzyme is involved in methanopterin biosynthesis in methanogenic archaea. cf. GTP cyclohydrolase I (EC 220.127.116.11), GTP cyclohydrolase II (EC 18.104.22.168) and GTP cyclohydrolase IIa (EC 22.214.171.124).
0.5:1 molar ratio of zinc to enzyme monomer. The zinc content of the mutants H200N and H293N is similar to that of the wild type. Mutant H101N shows an increased amount of zinc (1:1 molar ratio of zinc to monomer)
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the enzyme readily loses activity upon exposure to air. MptA loses all activity within 4 days at -20°C. This activity can be restored by adding specific divalent metal ions to the oxygen-inactivated enzyme under anaerobic conditions in the presence of dithiothreitol. Only Fe2+ is capable of restoring activity at the physiologically relevant concentrations of 0.020 mM. Higher concentrations of Mn2+ (0.2 mM) are able to restore activity to 24% of that seen with the Fe2+. At very high concentrations (2 mM) Mn2+ is able to restore MptA activity to a higher level than Fe2+. The Mn2+ enzyme is not inactivated by exposure to oxygen. The addition of reducing agents to the assay mixture is able to partially restore activity in oxygen inactivated MptA