Arsenic contamination of drinking water affects more than 140 million people worldwide. While toxic to humans, inorganic forms of arsenic (arsenite and arsenate), can be used as energy sources for microbial respiration. AioX and its orthologues (ArxX and ArrX) represent the first members of a new sub-family of periplasmic-binding proteins that serve as the first component of a signal transduction system, that’s role is to positively regulate expression of arsenic metabolism enzymes. As determined by X-ray crystallography for AioX, arsenite binding only requires subtle conformational changes in protein structure, providing insights into protein-ligand interactions. The binding pocket of all orthologues is conserved but this alone is not sufficient for oxyanion selectivity, with proteins selectively binding either arsenite or arsenate. Phylogenetic evidence, clearly demonstrates that the regulatory proteins evolved together early in prokaryotic evolution and had a separate origin from the metabolic enzymes whose expression they regulate.

The anaerobic bacterium Chrysiogenes arsenatis respires using the oxyanion arsenate (AsO4 3-) as the terminal electron acceptor, where it is reduced to arsenite (AsO3 3-) while concomitantly oxidizing various organic (e.g. acetate and formate) or inorganic (e.g. hydrogen) 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 show 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.

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