GTP cyclohydrolase II () is an enzyme characterised from Escherichia coli that catalyzes the hydrolysis of guanosine triphosphate to give 2,5-diamino-6-hydroxy-4-(5-phosphoribosylamino)pyrimidine (1), formic acid, and orthophosphoric acid (Pi):
This enzyme is a hydrolase, one 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 use include guanosine triphosphate cyclohydrolase II, and GTP-8-formylhydrolase. The reaction is part of riboflavin biosynthesis.
Structural studies As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and .
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