In enzymology, a GTP cyclohydrolase II () is an enzyme that catalyzes the chemical reaction GTP + 3 H2O formate + 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)pyrimidine + diphosphate Thus, the two substrates of this enzyme are GTP and H2O, whereas its 3 products are formate, 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)pyrimidine, and diphosphate. This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amidines. The systematic name of this enzyme class is GTP 7,8-8,9-dihydrolase (diphosphate-forming). Other names in common use include guanosine triphosphate cyclohydrolase II, and GTP-8-formylhydrolase. This enzyme participates in riboflavin metabolism. Structural studies As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and . References EC 3.5.4 Enzymes of known structure