Peripheral and integral binding of cytochrome c to phospholipids vesicles

The interactions of ferric cytochrome c (Cyt-c) with dioleoyl-phosphatidylglycerol (DOPG) at low ionic strength have been studied by viscosity and turbidity measurements as well as by resonance Raman, circular dichroism, and UV-vis-absorption spectroscopy to monitor the structural changes of the liposomes and the protein upon complex formation. The observed mutual structural changes in the liposomes and the protein are associated with three different modes of protein binding. At high lipid/protein (L/P) ratios, Cyt-c binds electrostatically to the anionic headgroups of the phospholipids which induces structural changes of the protein. Decreasing the L/P-ratio weakens the electrostatic interactions such that membrane anchoring of Cyt-c can effectively compete with peripheral binding. This mode of binding is accompanied by an increase of long-range liposome-liposome interactions. Upon lowering the L/P-ratio below the ratio for full protein coverage of the vesicles, further Cyt-c binding is achieved via interactions with the protein monolayer. This mode of binding initiates phase separation of the liposome aggregates from the aqueous buffer. Our results indicate that the crucial parameter controlling the interplay between the binding modes appears to be the membrane surface potential which in turn sensitively depends on the protein coverage.