NADPH-dependent Fe(3+) reductase. Probably involved in acetate metabolism and not in the reduction of Fe(3+) chelates. Unable to utilize NADH or formate as electron donors. Unlike formate dehydrogenases, contains no W, Mo or Se. Substrates are Fe(3+)-nitrilotriacetate >> Fe(3+)-citrate or Fe(3+)-EDTA > Fe(3+)-oxyhydroxide. Catalyzes also the reduction of NADP(+) with reduced methyl viologen as electron donor. No activity with menadione as electron acceptor. May be alternatively involved in transferring electrons from reduced ferredoxin to NADP(+) and may serve as a major route for NADP regeneration. Not regulated by FAD or FMN. Optimum pH is 5.5. Optimum temperature is 45 degrees Celsius. Heterotetramer with 2 beta subunits. Expressed constitutively. SfrA lacks the residues that are catalytically important in formate dehydrogenases, including a Cys residue which is a ligand to the heavy-metal atom (Mo or W) of the molybdopterin cofactor. PubMed:17906154 suggests that SfrAB is involved in acetate metabolism and does not participate directly in the reduction of Fe(3+) chelates, as was initially proposed in PubMed:11443080.