Probably involved in acetate metabolism and not in the reduction of Fe(3+) chelates. May serve as a major route for NADP regeneration. Binds 1 [4Fe-4S] cluster. Heterotetramer with 2 alpha subunits. SfrAB-null strain is unable to grow in the presence of acetate and fumarate unless an additional electron donor is present. Strongly reduced NADPH-dependent benzyl viologen reductase activity.