GTP cyclohydrolase I

GTP cy·clo·hy·dro·lase I (siґklo-hiґdro-lās) [EC 3.5.4.16] an enzyme of the hydrolase class that catalyzes the first step in the biosynthesis of tetrahydrobiopterin from guanosine triphosphate. Deficiency of the enzyme, an autosomal recessive trait caused by mutation of the GCH1 gene encoding the enzyme, is a form of tetrahydrobiopterin deficiency (q.v.) with hyperphenylalaninemia; an autosomal dominant disorder with sex-influenced reduced penetrance caused by mutation of same gene manifests as a dopa-responsive dystonia without hyperphenylalaninemia, with reduced rate of tetrahydrobiopterin synthesis.