Electrostatic modulation of the kinetics of electron transfer from cytochrome c to cobalt phenanthroline by binding to lipid bilayers: Effects of ionic strength and extent of incorporation of various negatively charged lipids

Abstract

The effect of electrostatically binding ferrous cytochrome c to anionic liposomes, composed of dimyristoyl phosphatidylglycerol (DMPG −), dioleoyl phosphatidylglycerol (DOPG −), or cardiolipin (CL 2−) mixed with varying amounts of egg phosphatidylcholine (PC), on the kinetics of cytochrome oxidation by the positively charged cobalt phenanthroline ion has been measured using stopped-flow spectrophotometry. The rate of electron transfer is enhanced as much as 3000-fold by increasing the number of negatively charged binding sites on the liposome surface, and by as much as 1000-fold by decreasing the ionic strength of the buffer. The sigmoidal shape of the curve of rate constant vs mole percent anionic lipid is consistent with a positively cooperative effect of the negative surface charge. The rate stimulation is greater for DOPG −- and CL 2−-containing liposomes than for DMPG − vesicles; this is most likely due to structural differences in the respective liposomes. The results do not provide any support for a role of structural changes in the bound cytochrome in influencing oxidation kinetics, a possibility suggested by recent spectroscopic measurements, although relatively small conformational effects cannot be completely ruled out.