. "Reviewed: D'Eustachio, P, 2011-08-23"^^ . "Edited: Jassal, B, 2011-08-17"^^ . "Authored: Jassal, B, 2011-08-17"^^ . "The first and rate-limiting enzyme in tetrahydrobiopterin de novo biosynthesis is GTP cyclohydrolase I (GCH1, GTPCHI). Three different isoforms are produced but only isoform 1 is functionally active (G\u00FCtlich et al. 1994). GCH1 is functional as a homodecamer. First, a monomer of GCH1 forms a dimer. Then five dimers arrange into a ring-like structure to form the homodecamer (Nar et al. 1995)."^^ . . . . . . . "GCH1 reduces GTP to dihydroneopterin triphosphate"^^ . "3.5.4.16"^^ . . . . .