GTP cyclohydrolase I (GTP-CH-I) catalyzes the conversion of GTP into dihydroneopterin triphosphate. The enzyme product is the precursor of tetrahydrofolate in eubacteria, fungi, and plants and of the folate analogs in methanogenic bacteria. In vertebrates and insects it is the biosynthtic precursor of tetrahydrobiopterin (BH4) which is involved in the formation of catacholamines, nitric oxide, and the stimulation of T lymphocytes. The biosynthetic reaction of BH4 is controlled by a regulatory protein GFRP which mediates feedback inhibition of GTP-CH-I by BH4. This inhibition is reversed by phenylalanine. The decameric GTP-CH-I forms a complex with two pentameric GFRP in the presence of phenylalanine or a combination of GTP and BH4, respectively.
Comment:a complete active site of the GTP-CH-I decamer is formed by two monomers from one pentamer and one monomer from the other pentamer contribute to the GTP and Zn2+ binding site
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