Steroid hormones are essential in stress response, immune system regulation, and reproduction in mammals. Steroids with 3-oxo-Δ⁴ structure, such as testosterone or progesterone, are catalyzed by steroid 5α-reductases (SRD5As) to generate their corresponding 3-oxo-5α steroids, which are essential for multiple physiological and pathological processes. SRD5A2 is already a target of clinically relevant drugs. However, the detailed mechanism of SRD5A-mediated reduction remains elusive. Here we report the crystal structure of PbSRD5A from Proteobacteria bacterium, a homolog of both SRD5A1 and SRD5A2, 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 substrate binding cavity, whereas TM5-7 coordinate cofactor NADPH through extensive hydrogen bonds network. Homology-based structural models of HsSRD5A1 and -2, together with biochemical characterization, define the substrate binding pocket of SRD5As, explain the properties of disease-related mutants and provide an important framework for further understanding of the mechanism of NADPH mediated steroids 3-oxo-Δ⁴ reduction. Based on these analyses, the design of therapeutic molecules targeting SRD5As with improved specificity and therapeutic efficacy would be possible.

类固醇激素对哺乳动物的应激反应，免疫系统调节和生殖至关重要。类固醇与3-氧代- Δ ⁴的结构，例如睾酮或孕酮，由类固醇5α还原酶（SRD5As）催化生成其相应的3-氧代-甾族化合物5α，这对于多种生理和病理过程是必不可少的。SRD5A2已经成为临床相关药物的靶标。但是，SRD5A介导的还原的详细机制仍然难以捉摸。在这里我们报道了来自Proteobacteria细菌的PbSRD5A的晶体结构SRD5A1和SRD5A2的同系物，与辅因子NADPH复杂，分辨率为2.0Å。PbSRD5A作为包含七个跨膜链段（TM）的单体存在。TM1-4封闭疏水底物结合腔，而TM5-7通过广泛的氢键网络协调辅因子NADPH。HsSRD5A1和-2的基于同源性的结构模型，以及生化特征，定义了SRD5A的底物结合口袋，解释了疾病相关突变体的特性，并为进一步理解NADPH介导的类固醇3-氧代磷灰石的机理提供了重要的框架。 -Δ ⁴还原。基于这些分析，设计靶向SRD5A的具有改善的特异性和治疗功效的治疗分子将是可能的。