Structural and Functional Investigation of the Periplasmic Arsenate-Binding Protein ArrX from Chrysiogenes arsenatis

The anaerobic bacterium Chrysiogenes arsenatis respires using the oxyanion arsenate (AsO43–) as the terminal electron acceptor, where it is reduced to arsenite (AsO33–) while concomitantly oxidizing various organic (e.g., acetate) electron donors. This respiratory activity is catalyzed in the periplasm of the bacterium by the enzyme arsenate reductase (Arr), with expression of the enzyme controlled by a sensor histidine kinase (ArrS) and a periplasmic-binding protein (PBP), ArrX. Here, we report for the first time, the molecular structure of ArrX in the absence and presence of bound ligand arsenate. Comparison of the ligand-bound structure of ArrX with other PBPs shows a high level of conservation of critical residues for ligand binding by these proteins; however, this suite of PBPs shows different structural alterations upon ligand binding. For ArrX and its homologue AioX (from Rhizobium sp. str. NT-26), which specifically binds arsenite, the structures of the substrate-binding sites in the vicinity of a conserved and critical cysteine residue contribute to the discrimination of binding for these chemically similar ligands.

厌氧细菌砷酸化产氢菌（Chrysiogenes arsenatis）以含氧阴离子砷酸盐（arsenate，AsO₄³⁻）作为末端电子受体进行呼吸代谢，将其还原为亚砷酸盐（arsenite，AsO₃³⁻），同时协同氧化多种有机电子供体（如乙酸盐）。该呼吸代谢活性由该菌周质空间中的砷酸还原酶（arsenate reductase，Arr）催化完成，而该酶的表达受组氨酸激酶传感器（sensor histidine kinase，ArrS）与周质结合蛋白（periplasmic-binding protein，PBP）ArrX共同调控。本研究首次报道了ArrX在未结合配体与结合配体砷酸盐状态下的分子结构。将ArrX的配体结合结构与其他周质结合蛋白（PBP）进行比对后发现，这类蛋白用于配体结合的关键残基具有高度保守性；但该类周质结合蛋白在配体结合后会呈现出不同的结构变化。对于ArrX及其特异性结合亚砷酸盐的同源蛋白AioX（来自根瘤菌属菌株NT-26，Rhizobium sp. str. NT-26），底物结合位点在保守关键半胱氨酸残基附近的结构特征，决定了二者对这类化学结构相似配体的结合特异性区分。