GTP + H2O = 7,8-dihydroneopterin 3'-triphosphate + formate + H(+) Cofactor biosynthesis; 7,8-dihydroneopterin triphosphate biosynthesis; 7,8-dihydroneopterin triphosphate from GTP: step 1/1. Toroid-shaped homodecamer, composed of two pentamers of five dimers. Belongs to the GTP cyclohydrolase I family.