Structural basis for the catalysis and inhibition of human steroid 5α‐reductase 2

Abstract

Human steroid 5α-reductases (SRD5As) are membrane-embedded NADPH-dependent steroid and lipid reductases with diverse physiological and pathological roles. Among them, SRD5A2 is a critical enzyme in steroid metabolism that catalyzes testosterone to dihydrotestosterone. Mutations on its gene have been linked to 5α-reductase deficiency and prostate cancer. Finasteride and dutasteride as SRD5A2 inhibitors are widely used anti-androgen drugs for benign prostate hyperplasia and androgenic alopecia, which have recently been indicated in the treatment of COVID-19. The molecular mechanisms underlying enzyme catalysis and inhibition had remained elusive for SRD5A2 and other eukaryotic integral membrane steroid reductases due to a lack of structural information. We recently solved a crystal structure of human SRD5A2 at 2.8 Å with finasteride using the lipid mesophase (or LCP) crystallization method. The structure reveals a unique structural topology of 7-transmembrane helices and an intermediate adduct of finasteride and NADPH as NADP-dihydrofinasteride, which binds in a largely enclosed binding cavity inside the membrane. To our knowledge, similar NADPH adducts have not been reported previously. Structural analysis together with computational and mutagenesis studies reveals molecular mechanisms for the 5α-reduction of testosterone and the semi-irreversible finasteride inhibition involving residues E57 and Y91 of SRD5A2. Molecular dynamics simulation results indicate high conformational dynamics of the cytosolic region of SRD5A2 including three cytosolic loops regulating the NADPH/NADP+ exchange and their entry into the transmembrane region of SRD5A2. Mapping disease-causing mutations of SRD5A2 to our structure suggests molecular mechanisms for their pathological effects. In summary, our results offer critical and unprecedented structural insights into the function of integral membrane steroid reductases and will facilitate drug development.