Background: Cholesterol degradation is challenging due to its complex structure and low water solubility. Results: C25 dehydrogenase is a novel molybdenum/iron-sulfur/heme-containing enzyme that hydroxylates the tertiary C25 of the steroid side chain. Conclusion: C25 dehydrogenase and related enzymes identified in the genome of Sterolibacterium denitrificans replace oxygenases in anaerobic and even aerobic steroid metabolism. Significance: O 2 -independent hydroxylations by molybdoenzymes probably represent a general strategy to activate steroid substrates anaerobically.

The hydroxylation of vitamin D3 (VD3, cholecalciferol) side chains to give 25-hydroxyvitamin D3 (25OHVD3) is a crucial reaction in the formation of the circulating and biologically active forms of VD3 . It is usually catalyzed by cytochrome P450 monooxygenases that depend on complex electron donor systems. Cell-free extracts and a purified Mo enzyme from a bacterium anaerobically grown with cholesterol were employed for the regioselective, ferricyanide-dependent hydroxylation of VD3 and proVD3 (7-dehydrocholesterol) into the corresponding tertiary alcohols with greater than 99 % yield. Hydroxylation of VD3 strictly depends on a cyclodextrin-assisted isomerization of VD3 into preVD3 , the actual enzymatic substrate. This facile and robust method developed for 25OHVD3 synthesis is a novel example for the concept of substrate-engineered catalysis and offers an attractive alternative to chemical or O2  /electron-donor-dependent enzymatic procedures.

The hydroxylation of vitamin D3 (VD3, cholecalciferol) side chains to give 25‐hydroxyvitamin D3 (25OHVD3) is a crucial reaction in the formation of the circulating and biologically active forms of VD3. It is usually catalyzed by cytochrome P450 monooxygenases that depend on complex electron donor systems. Cell‐free extracts and a purified Mo enzyme from a bacterium anaerobically grown with cholesterol were employed for the regioselective, ferricyanide‐dependent hydroxylation of VD3 and proVD3 (7‐dehydrocholesterol) into the corresponding tertiary alcohols with greater than 99 % yield. Hydroxylation of VD3 strictly depends on a cyclodextrin‐assisted isomerization of VD3 into preVD3, the actual enzymatic substrate. This facile and robust method developed for 25OHVD3 synthesis is a novel example for the concept of substrate‐engineered catalysis and offers an attractive alternative to chemical or O2 /electron‐donor‐dependent enzymatic procedures.