Component of the arsenate respiratory reductase (Arr) complex, which catalyzes the reduction of arsenate (As(V)) to arsenite (As(III)) (PubMed:9738904). Can use acetate as the electron donor (PubMed:9738904). ArrA is the arsenate-binding subunit (By similarity). A + arsenite + H2O = AH2 + arsenate + H(+) Binds 1 [4Fe-4S] cluster. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Heterodimer composed of one large subunit (ArrA) and one small subunit (ArrB). Fully induced in the presence of arsenate. Predicted to be exported by the Tat system. The position of the signal peptide cleavage has been experimentally proven. May also use zinc as a cofactor. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.