Hydroxylated derivatives of steroids play essential roles in medicine. Cytochrome P450 enzymes selectively hydroxylate methyne groups on steroids, which can dramatically change their polarity, biological activity and toxicity.

Cytochrome P450 enzymes (CYPs or P450s) are ubiquitous heme‐dependent enzymes that catalyze the monooxygenation of non‐activated C−H bonds to modify the structure of the substrate. In this study, we heterologously expressed CYP107X1 from Streptomyces avermitilis and conducted in vitro substrate screening using the alternative redox partners putidaredoxin and putidaredoxin reductase. CYP107X1 catalyzed the 16α‐hydroxylation of progesterone with regio‐ and stereoselectivity. The spectroscopic analyses showed that CYP107X1 bound progesterone with a relatively high Kd value of 65.3±38.9 μM. The Km and kcat values for progesterone were estimated to be 47.7±12.0 μM and 0.30 min−1, respectively. Furthermore, a crystal structure was obtained of CYP107X1 bound with glycerol from the buffer solution. Interestingly, a conserved threonine was replaced with asparagine in CYP107X1, indicating that it may adopt an unnatural proton transfer process and play a crucial role in its catalytic activity.