GTP cyclohydrolase 1 is the first enzyme in the biosynthetic pathway leading to folic acid. GTP + H2O = 7,8-dihydroneopterin 3'-triphosphate + formate + H(+) GTP shows a positive allosteric effect, and tetrahydrobiopterin inhibits the enzyme activity. 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.