CYP109E1 from Bacillus megaterium has a 25- and 24-hydroxylation activity towards vitamin D3. A CYP109E1 based whole-cell system for the conversion of vitamin D3 was established in Bacillus megaterium MS941 and the products of the reaction were identified by NMR analysis. Docking of vitamin D3 was performed to identify important amino acid residues in the active site responsible for selective product formation. Improvement of 25-hydroxyvitamin D3 production by the I85A mutant.