Component of the arsenate respiratory reductase (Arr) complex, which catalyzes the reduction of arsenate (As(V)) to arsenite (As(III)) (PubMed:9738904). ArrB is probably the electron transfer subunit (By similarity). Binds 4 [4Fe-4S] cluster. Heterodimer composed of one large subunit (ArrA) and one small subunit (ArrB). Fully induced in the presence of arsenate. May also use zinc as a cofactor.