硫氧化酶系统（Sulfur oxidizing，Sox）在脱氮硫杆菌(Thiobacillus denitrificans，Td)的硫代谢过程中起着至关重要的作用，其核心为SoxAXYZB，硫化物需先经SoxAX蛋白催化并转移到SoxYZ上才能进行Sox通路后续的硫氧化反应，而目前尚无脱氮硫杆菌SoxAX蛋白结构解析的报道。本文采用反向折叠法构建了SoxAX蛋白的二聚体结构并验证其合理性，并通过分子对接实验探讨SoxAX蛋白模型与硫代硫酸盐、硫化氢和亚硫酸盐等不同底物结合在构型与能量上的差异。结果表明，构建的SoxAX二聚体蛋白结构相对合理，氢键是维持SoxAX二聚体结合的主要作用力，10个短强氢键和1个π键作用力参与维持二聚体结构的稳定，SoxA的Arg160等6个残基及SoxX的Asn15等8个残基对维持二聚体结构有重要作用。分子对接试验显示氢键是维持底物与SoxAX结合的主要作用力，与底物结合的关键氨基酸为Arg210、Cys214和Gln217。在三种底物与酶结合的亲和力中以硫化氢最高、硫代硫酸盐次之、亚硫酸盐最低。

The sulfur oxidizing (Sox) enzyme complex plays a vital role in the sulfur metabolism of Thiobacillus denitrificans (Td), with the SoxAXYZB complex enzyme at its core. Inorganic sulfur compound substrates need to be successfully combined on the SoxAX complex and transferred to the carrier SoxYZ for further subsequent sulfur oxidation. However, the three-dimensional (3D) structure of Td SoxAX has not yet been elucidated. The protein threading method was adopted in this study in order to construct the Td SoxAX dimer, and the result was verified. Differences in the configuration and energy of binding between the SoxAX protein model and the substrates thiosulfate, hydrogen sulfide, and sulfite were then explored using molecular docking experiments. The result shows that the 3D structure of the constructed SoxAX dimer was relatively reliable. Hydrogen bonds were the major forces that maintained the interaction between SoxA and SoxX. Ten short and strong hydrogen bonds and one pi-pi interaction were involved in stabilizing the dimer structure, and six residues including Arg160 in SoxA and eight residues including Asn15 in SoxX played an important role in the stability of the dimer. Molecular docking studies showed that hydrogen bonds were the major force maintaining the binding between SoxAX and the three different substrates. The key residues for substrate binding were Arg210, Cys214, and Gln217. Among the three substrates, sulfide showed the highest binding energy, followed by thiosulfate and sulfite.

广东省自然科学基金项目（2016A030313457）