Bacteria on steroids: the enzymatic mechanism of an NADH-dependent dehydrogenase that regulates the conversion of cortisol to androgen in the gut microbiome

The ability to metabolize both endogenous and exogenous compounds to a variety of metabolic products is not exclusive to our human cells. In fact, the bacterial communities that inhabit our digestive system are responsible for a network of steroid transformations that can produce hormones in the gut, which are then absorbed to act in the host. These communities have been shown to impact our health in numerous ways, affecting disease predisposition, pathogenesis, physical fitness, and dietary responsiveness. Steroid biotransformations by gut bacteria are predicted to impact the host endocrine system. A particular set of transformations facilitated by microbial enzymes has been shown to result in the formation of 11-oxy-androgens from host-derived cortisol. Since androgens have been implicated in disease and immune modulations, understanding the structure and catalytic mechanism of enzymes involved in cortisol metabolism is a key step to hasten the development of strategies that reduce the formation of disease-promoting bioactive steroids in certain individuals. Here, we combine experimental and computational techniques to describe DesC, an enzyme capable of creating 20-dihydrocortisol and siphoning cortisol away from pathways that produce androgens. DesC diverges significantly from previously described bacterial and eukaryotic counterparts, catalyzing an NADH-dependent 20-hydroxysteroid dehydrogenase reaction but presenting little sequence and structure similarity to them. The structural information obtained by X-ray crystallography and hybrid QM/MM simulations, validated through mutagenesis studies, show the reaction occurs through a multi-step proton relay mechanism. Free energy calculations were then used to describe the kinetics of the reaction mechanism. The mechanistic information presented here can be employed in the development of therapeutics to divert microbial pathways away from disease-promoting steroids.