Interaction of cytochrome c with phospholipid monolayers. Orientation and penetration of protein as functions of the packing density of film, nature of the phospholipids, and ionic content of the aqueous phase.

Energy-transfer fluorescence quenching has been used to observe the binding of cytochrome c to a lipid assembly. The probe (donor), dansylphosphatidylethanolamine, was dispersed either in dipalmitoylphosphatidylcholine, in phosphatidic acid, or in a mixture of the two lipids. The heme of the protein was the acceptor. When the phospholipids were spread in monolayer at the air-water interface, orientation and penetration parameters of the protein relative to the membrane were obtained. The cytochrome is bound with an orientation such that its heme crevice is fully accessible to the aqueous space. It penetration in the lipid layer is dependent on the ionic content of the subphase and the initial packing of the film. The perturbation induced in the lipid matrix by the binding appear very localized. The same results were obtained with lipid microvesicles. The type of binding of cytochrome c to phospholipids observed here implies that there are specific areas on the protein which appear to be different from those involved in its interaction with cytochrome oxidase and other cytochromes. These conclusions are relevant to the existence of different classes of binding sites for cytochrome c in the mitochondrial membrane.