Abstract Steroid hormones are essential in stress response, immune system regulation, and reproduction in mammals. Steroids with 3-oxo-Δ 4 structure, such as testosterone, androstenedione and progesterone, could be catalyzed by steroid 5α-reductases (SRD5As) to generate their corresponding 3-oxo-5α steroids, which are essential for multiple physiological and pathological processes. Abnormal activities of SRD5As will lead to benign prostatic hyperplasia, alopecia, prostatic cancer or infertility due to the poor quality of sperms. However, the detailed reduction mechanisms of SRD5As remain elusive. Here we report the crystal structure of PbSRD5A, which shares 60.6% and 51.5% sequence similarities with human SRD5A1 and −2 respectively, from Proteobacteria bacterium in complex with the cofactor NADPH at 2.0 Å resolution. PbSRD5A exists as a monomer comprised of seven transmembrane segments (TMs). The TM1-4 enclose a hydrophobic cavity for steroids substrates binding, whereas TM5-7 coordinate with cofactor NADPH through extensive hydrogen bonds network. Homology-based structural models of HsSRD5A1 and −2, together with extensive biochemical characterizations, for the first time unveiled the substrate recognition of SRD5As and provide an important framework for further understanding of the mechanism of NADPH mediated steroids 3-oxo-Δ 4 reduction. Based on these analyses, the design of therapeutic molecules targeting SRD5As with improved specificity and therapeutic efficacy would be possible. One Sentence Summary Structural and biochemical characterizations decipher the evolutionarily conserved mechanism in steroid 5α-reductases catalyzing NADPH mediated steroids reduction.

Crystal structure of steroid reductase SRD5A reveals conserved steroid reduction mechanism