Binding and Dissociation of Cytochrome c to and from Membranes Containing Acidic Phospholipids

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Abstract
Membrane association and detachment of cytochrome c (cyt c) in millisecond to second time domain were investigated by stopped-flow fluorescence spectroscopy monitoring the efficiency of energy transfer from a pyrene-fatty acid containing phospholipid derivative, 1-palmitoyl-2-[10-(pyren-1-yl)decanoyl]-sn-glycero-3-phosphoglycerol (PPDPG, mole fraction X = 0.01) to the heme of the cyt c. Large unilamellar liposomes composed of egg phosphatidylcholine (eggPC) with varying content of the acidic phospholipid phosphatidylglycerol (eggPG) were employed. Unexpectedly, the rate of binding of cyt c to membranes was attenuated upon increasing the mole fraction of the acidic phospholipid (XPG). For example, at 50 μM phospholipid and 5 μM cyt c, when XPG was increased from 0.20 to 0.40 the half-time for the single-exponential decay in fluorescence increased from 4.7 to 8.6 ms. A similar observation was made for the membrane binding of another cationic protein, histone H1. We suggest that the formation of cooperative hydrogen-bonded networks by deprotonated and protonated PG in the vesicle surface retards the binding of cyt c to the liposome surface. However, once formed, the complex of cyt c with acidic phospholipids is stabilized by increasing XPG. Accordingly, significantly prolonged half-times of dissociation of cyt c from liposomes by NaCl, ATP, and different cationic proteins are measured upon increasing XPG. Differences between the latter cationic membrane binding ligands most likely reflect the varying relative contributions of hydrophobicity and Coulombic forces to their attachment to liposomes. Our data on the release and binding of cyt c to liposomes as a function of XPG and in the presence of ATP also provide the first direct experimental evidence for multiple lipid binding sites in cyt c.