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. Leads to folinic acid auxotrophy, and lacks any detectable specific enzymatic activity. Present with 2840 molecules/cell in log phase SD medium. Belongs to the GTP cyclohydrolase I family.