Tetrahydrobiopterin, the cofactor required for hydroxylation of aromatic amino acids regulates its own synthesis in via feedback inhibition of GTP cyclohydrolase I. This mechanism is mediated by the regulatory subunit called GTP cyclohydrolase I feedback regulatory protein (GFRP) [1].

1: Crystal structure of rat GTP cyclohydrolase I feedback regulatory protein, GFRP. Bader G, Schiffmann S, Herrmann A, Fischer M, Gutlich M, Auerbach G, Ploom T, Bacher A, Huber R, Lemm T; J Mol Biol 2001;312:1051-1057. PMID:11580249