Component of the arsenate respiratory reductase (Arr) complex, which catalyzes the reduction of arsenate (As(V)) to arsenite (As(III)) (PubMed:12939408, PubMed:17951391, PubMed:30104376). ArrA is the arsenate-binding subunit (PubMed:30104376). The periplasmic localization of this complex may allow the cell to couple arsenate reduction to energy production before arsenate can be transported to the cell cytoplasm and enter the ars detoxification pathway, an energy-requiring process (PubMed:17951391). A + arsenite + H2O = AH2 + arsenate + H(+) Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Phosphate is a competitive inhibitor. kcat is 9810 sec(-1). Optimum pH is 7.5. Heterodimer composed of one large subunit (ArrA) and one small subunit (ArrB). The arrA-arrB operon is induced under anaerobic conditions in the presence of nanomolar concentrations of arsenite or low micromolar concentrations of arsenate (PubMed:16237022). Expression is repressed under aerobic conditions and in the presence of nitrate (PubMed:16237022). The peak of expression occurs during the exponential phase of growth (PubMed:16237022). Predicted to be exported by the Tat system. The position of the signal peptide cleavage has not been experimentally proven. Deletion mutants are incapable of growing on arsenate, but are still able to grow on a wide variety of other electron acceptors as efficiently as the wild-type. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.