Crystal structure of CYP105A1 (P450SU-1) in complex with 1α,25-dihydroxyvitamin D3

Vitamin D-3 (VD3) a prohormone in mammals, plays a crucial role in the maintenance of calcium and phosphorus concentrations in serum. Activation of VD3 requires 25-hydroxylation in the liver and 1 alpha-hydroxylation in the kidney by cytochrome P450 (CYP) enzymes. Bacterial CYP105A1 converts VD3 into 1 alpha,25-dihydroxyvitamin D-3 (1 alpha,25(OH)(2)D-3) in two independent reactions, despite its low sequence identity with mammalian enzymes (< 21 % identity). The present study determined the crystal structures of a highly active mutant (R84A) of CYP105A1 from Streptomyces griseolus in complex and not in complex with 1 alpha,25(OH)(2)D-3. The compound 1 alpha,25(OH)(2)D-3 is positioned 11 A from the iron atom along the I helix within the pocket. A similar binding mode is observed in the structure of the human CYP2R1-VD3 complex, indicating a common substrate-binding mechanism for 25-hydroxylation. A comparison with the structure of wild-type CYP105A1 suggests that the loss of two hydrogen bonds in the R84A mutant increases the adaptability of the B' and F helices, creating a transient binding site. Further mutational analysis of the active site reveals that 25- and 1 alpha-hydroxylations share residues that participate in these reactions. These results provide the structural basis for understanding the mechanism of the two-step hydroxylation that activates VD3.