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cytochrome P450 2R1 CYP2R1, also called vitamin D 25-hydroxylase (EC 1.14.14.24), is a microsomal enzyme that is required for the activation of vitamin D; it catalyzes the initial step converting vitamin D into 25-hydroxyvitamin D (25(OH)D), the major circulating metabolite of vitamin D. The 1alpha-hydroxylation of 25(OH)D by CYP27B1 generates the fully active vitamin D metabolite, 1,25-dihydroxyvitamin D (1,25(OH)2D). Mutations in the CYP2R1 gene are associated with an atypical form of vitamin D-deficiency rickets, which has been classified as vitamin D dependent rickets type 1B. CYP2R1 belongs to family 2 of the large cytochrome P450 (P450, CYP) superfamily of heme-containing proteins that catalyze a variety of oxidative reactions of a large number of structurally different endogenous and exogenous compounds in organisms from all major domains of life. CYPs bind their diverse ligands in a buried, hydrophobic active site, which is accessed through a substrate access channel formed by two flexible helices and their connecting loop.
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