Preparation and crystallization of the stimulatory and inhibitory complexes of GTP cyclohydrolase I and its feedback regulatory protein GFRP

Mammalian GTP cyclohydrolase I is a decameric enzyme in the first and rate- limiting step in the biosynthesis of tetrahydrobiopterin, which is an essen tial cofactor for enzymes producing neurotransmitters such as catecholamine s and for nitric oxide synthases. The enzyme is dually regulated by its fee dback regulatory protein GFRP in the presence of its stimulatory effector p henylalanine and its inhibitory effector biopterin. Here, both the stimulat ory and inhibitory complexes of rat GTP cyclohydrolase I bound to GFRP were crystallized by vapour diffusion. Diffraction data sets at resolutions of 3.0 and 2.64 Angstrom were collected for the stimulatory and inhibitory com plexes, respectively. Each complex consists of two GTPCHI pentamer rings an d two GFRP pentamer rings, with pseudo-52 point-group symmetry.