GTP cyclohydrolase 1 is the first enzyme in the biosynthetic pathway leading to folic acid. 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. Homodimer. Expressed in leaves and unripe fruits. Belongs to the GTP cyclohydrolase I family.