May positively regulate nitric oxide synthesis in endothelial cells. May be involved in dopamine synthesis (By similarity). May modify pain sensitivity and persistence. GTP + H2O = 7,8-dihydroneopterin 3'-triphosphate + formate + H(+) GTP shows a positive allosteric effect, and tetrahydrobiopterin inhibits the enzyme activity. Zinc is required for catalytic activity. Inhibited by Mg(2+). 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. Interacts with AHSA1 and GCHFR/GFRP. By cytokines such as insulin, interferon-gamma, and phytohemagglutinin in adrenal gland, macrophages, and T-cell respectively. Phosphorylated. Belongs to the GTP cyclohydrolase I family.