Enables several functions, including GTP binding activity; GTP cyclohydrolase I activity; and metal ion binding activity. Involved in several processes, including dihydrobiopterin metabolic process; response to pain; and vasodilation. Part of protein-containing complex. Human ortholog(s) of this gene implicated in BH4-deficient hyperphenylalaninemia B; bipolar disorder; dystonia; and dystonia 5. Orthologous to human GCH1 (GTP cyclohydrolase 1). [provided by Alliance of Genome Resources, Apr 2022]