GTP cyclohydrolase IIa () is an enzyme that catalyzes the hydrolysis of guanosine triphosphate to 2-amino-5-formylamino-6-(5-phospho-D-ribosylamino)pyrimidin-4(3H)-one (1) and orthophosphate Pi:
The reaction is part of the biosynthetic pathway to riboflavin in bacteria.
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 8,9-hydrolase (phosphate-forming).
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