Involved in the detoxification of arsenic. Oxidizes As(III)O3(3-) (arsenite) to the somewhat less toxic As(V)O4(3-) (arsenate). arsenite + H2O + 2 oxidized [azurin] = arsenate + 3 H(+) + 2 reduced [azurin] Binds 1 [3Fe-4S] cluster per subunit. Binds 1 molybdenum-bis(molybdopterin guanine dinucleotide) (Mo-bis-MGD) cofactor per subunit. Heterodimer consisting of a large and a small subunit. Belongs to the prokaryotic molybdopterin-containing oxidoreductase family.