GTP cyclohydrolase I (GCH1) is a member of the GTP cyclohydrolase family. GCH1 is the first and rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis, catalyzing the conversion of GTP into 7,8-dihydroneopterin triphosphate. BH4 is an essential cofactor required by aromatic amino acid hydroxylases as well as nitric oxide synthases [taken from NCBI Entrez Gene (Gene ID: 2643)].
