Hybrid P450 systems in which P450 monooxygenases are reconstituted with non-native or surrogate redox partners have become important for the engineering of this class of versatile enzymes. P450sca-2 from Streptomyces carbophilus stereoselectively hydroxylates mevastatin to yield pravastatin, a cholesterol-lowering drug. While S. carbophilus has been successfully applied in the industrial biotransformation process for pravastatin, the molecular study and engineering of P450sca-2 has been very limited. We have previously established a functional P450sca-2/Pdx/Pdr hybrid system. In this study, on the basis of a more active P450sca-2 mutant (R8-5C), five sites located in the substrate binding pocket, substrate access entrance, and presumed Pdx interaction interface were rationally chosen, and systematically subjected to site-directed saturation mutagenesis (SDSM), and three rounds of iterative saturation mutagenesis (ISM). A best mutant (Variant III) was obtained, which showed a whole cell biotransformation activity (377.5mg/L) and an overall apparent k(cat) (6.37min(-1)) that was 7.1- and 10.0-fold that of the starting template R8-5C, respectively. Kinetic characterization revealed that most of the improvements seen for the SDSM and ISM mutants came from enhanced overall electron transfer, with the two sites at the interface between P450sca-2 and Pdx (T119 and N363) being most critical. Our study underscores the important role of electron transfer in a hybrid P450 system, and also demonstrates the utility of ISM in optimizing the redox partner interface. This should facilitate engineering of this and other important hybrid P450 systems. Biotechnol. Bioeng. 2013;110: 2815-2825. (c) 2013 Wiley Periodicals, Inc.