GTP cyclohydrolase I catalyses the biosynthesis of formic acid and dihydroneopterin triphosphate from GTP. This reaction is the first step in the biosynthesis of tetrahydrofolate in prokaryotes, of tetrahydrobiopterin in vertebrates, and of pteridine-containing pigments in insects. The enzyme is a homodecameric complex (a dimer of pentamers) containing one zinc ion per subunit. The pentamerisation leads to the formation of a 20-stranded antiparallel beta-barrel.

This superfamily entry consists of the N-terminal domain found in GTP cyclohydrolases I, which has been well conserved throughout evolution across bacterial and eukaryotic species. The GTP-CH-I monomer folds into an alpha+beta structure with a predominantly helical N-terminal part.

PFAM:PF01227, INTERPRO:IPR020602,PMID:12559918,PMID:7663943