Abstract

The metabolism of steroids by the gut microbiome affects hormone homeostasis, impacting host development, mental health, and reproductive functions. In this study, we identify the {Delta}4-3-ketosteroid 5{beta}-reductase, 3{beta}-hydroxysteroid dehydrogenase/{Delta}5-4 isomerase, and {Delta}6-3-ketosteroid reductase enzyme families encoded by common human gut bacteria. Through phylogenetic reconstruction and mutagenesis, We show that 5{beta}-reductase and {Delta}6-3-ketosteroid reductase have evolved to specialize in converting diverse 3-keto steroid hormones into their 5{beta}- and {Delta}6-reduced derivatives. We also find that the novel 3{beta}-hydroxysteroid dehydrogenase/{Delta}5-4 isomerase is fused with 5{beta}-reductase in multiple species, streamlining the multi-step conversion of pregnenolone, a steroid hormone precursor, into epipregnanolone. Through metagenomic analysis, we reveal that these enzymes are prevalent in healthy populations, being enriched in females over males. These findings provide the molecular basis for studying microbial steroid metabolism in the gut, offering insights into its potential impact on hormonal health in hosts, especially in the context of womens health.