Structural and functional effects of cytochrome b5 interactions with human cytochrome P450 enzymes

The small heme-containing protein cytochrome b(5) can facilitate, inhibit, or have no effect on cytochrome P450 catalysis, often in a P450-dependent and substrate-dependent manner that is not well understood. Herein, solution NMR was used to identify b(5) residues interacting with different human drug-metabolizing P450 enzymes. NMR results revealed that P450 enzymes bound to either b(5) alpha 4-5 (CYP2A6 and CYP2E1) or this region and alpha 2-3 (CYP2D6 and CYP3A4) and suggested variation in the affinity for b(5). Mutations of key b(5) residues suggest not only that different b(5) surfaces are responsible for binding different P450 enzymes, but that these different complexes are relevant to the observed effects on P450 catalysis.