The SfrAB enzymatic complex is probably involved in acetate metabolism and does not participate directly in the reduction of Fe(3+) chelates. May serve as a major route for NADP regeneration. Not regulated by FAD or FMN. Heterotetramer with 2 beta 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.