Fig. 1From: Bacterial cytochrome P450-catalyzed regio- and stereoselective steroid hydroxylation enabled by directed evolution and rational designDifferent redox systems for electron transfer in CYPs. a Three-protein systems: P450 and reductase can either be soluble or membrane-bound. b Two-protein systems: cytochrome P450 reductase (CPR) and P450 are membrane-bound. c One-protein system: soluble or membrane-bound systems formed by fusions of a CPR-like reductase and P450Back to article page