GTP cyclohydrolase I (gtpch I, EC 3.5.4.16) catalyzes the conversion of GTP to dihydroneopterin triphosphate and formic acid through a complex series of reactions. This reaction is the first committed step in the biosynthesis of FH4 (tetrahydrofolate) in plants and certain microorganisms, and BH4 (tetrahydrobiopterin) in mammals. Folates are comprised of a pABA ...