GCHFR binds to GCH1 and negatively regulates its activity Authored: Jassal, B, 2011-08-17 Edited: Jassal, B, 2011-08-17 High levels of the end product, BH4, negatively regulates GCH1. It does this via GTP cyclohydrolase 1 feedback regulatory protein (GCHFR). BH4-dependant GCHFR in the form of a homopentamer complexes with the decameric GCH1 enzyme in the ratio 2:1 to inactivate it. L-phenylalanine reverses this inhibition. These regulatory steps control the biosynthesis of BH4. (Swick & Kapatos 2006, Chavan et al. 2006, Harada et al. 1993). Pubmed16696853 Pubmed16778797 Pubmed8502995 Reactome Database ID Release 431474158 Reactome, http://www.reactome.org ReactomeREACT_111115 Reviewed: D'Eustachio, P, 2011-08-23 has a Stoichiometric coefficient of 2 GCH1 decamer 2GCHFR:GCH1 GCHFR pentamer