Tetrathionate hydrolase (4THase) plays an important role in dissimilatory sulfur oxidation in the acidophilic iron‐ and sulfur‐oxidizing bacterium Acidithiobacillus ferrooxidans. The structure of recombinant 4THase from A. ferrooxidans (Af‐Tth) was determined by X‐ray crystallography to a resolution of 1.95 Å. Af‐Tth is a homodimer, and its monomer structure exhibits an eight‐bladed β‐propeller motif. Two insertion loops participate in dimerization, and one loop forms a cavity with the β‐propeller region. We observed unexplained electron densities in this cavity of the substrate‐soaked structure. The anomalous difference map generated using diffraction data collected at a wavelength of 1.9 Å indicated the presence of polymerized sulfur atoms. Asp325, a highly conserved residue among 4THases, was located near the polymerized sulfur atoms. 4THase activity was completely abolished in the site‐specific Af‐Tth D325N variant, suggesting that Asp325 plays a crucial role in the first step of tetrathionate hydrolysis. Considering that the Af‐Tth reaction occurs only under acidic pH, Asp325 acts as an acid for the tetrathionate hydrolysis reaction. The polymerized sulfur atoms in the active site cavity may represent the intermediate product in the subsequent step.

中文翻译：

---

基于氧化亚铁硫杆菌四硫酸盐水解酶晶体结构的四硫酸盐水解反应机理

四硫酸盐水解酶 (4THase) 在嗜酸性铁和硫氧化细菌 Acidithiobacillus ferrooxidans 的异化硫氧化中起重要作用。来自A. ferrooxidans ( Af -Tth) 的重组 4THase 的结构通过 X 射线晶体学测定，分辨率为 1.95 Å。AF-Tth 是同型二聚体，其单体结构呈现出八叶β-螺旋桨基序。两个插入环参与二聚化，一个环与β-螺旋桨区域形成空腔。我们在基板浸泡结构的这个空腔中观察到了无法解释的电子密度。使用在 1.9 Å 波长处收集的衍射数据生成的异常差异图表明存在聚合硫原子。Asp325 是 4THase 中高度保守的残基，位于聚合硫原子附近。位点特异性Af -Tth D325N 变体中的 4THase 活性被完全消除，这表明 Asp325 在连四硫酸盐水解的第一步中起着至关重要的作用。考虑到Af-Tth 反应仅在酸性 pH 下发生，Asp325 充当连四硫酸盐水解反应的酸。活性位点腔中的聚合硫原子可以代表后续步骤中的中间产物。