The binding of cytochrome b 5 to liver microsomes.

The 40-44 hydrophobic peptide segment, present in cytochrome bS obtained by nonhydrolytic means, but absent from lipase or trypsin-extracted cytochrome bg, has been found to be required for the binding of a loto 20-fold molar excess of cytochrome bs to liver microsomes. At saturation, the bound cytochrome bs constitutes nearly 20% of the total protein of the vesicle preparations. This extra bound hemoprotein is indistinguishable from endogenous cytochrome bs as an electron acceptor from microsomal NADH cytochrome b5 reductase and as an electron donor for external cytochrome c or the rat liver microsomal stearyl coenzyme A desaturase. On the basis of earlier work of others and present data, it appears that these enzymes are localized on the outside of microsomal vesicles. A model, consistent with these data, proposes that the extra, as well as endogenous cytochrome b6, is bound to microsomes by interactions between the nonpolar peptide segment and the phospholipid of the membrane. This permits interactions of the polar catalytic heme binding segment at the surface of the outer membrane with either reductase, cytochrome c, or the desaturase system. The latter catalytic events require only the translational movement of the nonpolar portion of the protein in the lipid membrane of several molecular diameters.