Comparison of Electron Transfer Kinetics between Redox Proteins Free in Solution and Electrostatically Complexed to a Lipid Bilayer Membrane

The second-order rate constants obtained in solution for the reduction of horse cytochrome c (cytc; net charge +7) by either Clostridium beijerinckii flavodoxin semiquinone (Fld; net charge −16) or reduced spinach ferredoxin (Fd; net charge −15) decrease monotonically with increasing ionic strength, as expected for reactions between oppositely charged species. Although the rate constant for the Fld reaction is almost two orders of magnitude larger at low ionic strength than that for Fd, the values extrapolated to infinite ionic strength are closely similar, indicating comparable reactivities when electrostatic effects are eliminated. Furthermore, Fld has a much larger value for the electrostatic interaction energy, and thus a larger apparent active site charge, than does Fd, accounting for the rate constant disparity at low ionic strength. Electrostatically binding cytc at low ionic strength to a negatively charged Lipid bilayer vesicle (membranes containing mixtures of egg phosphatidylcholine (PC) and cardiolipin (CL)) results in a marked decrease of the observed electron transfer rate constant (kobs) for reduction of the cytochrome by both Fld and Fd. The magnitude of this decrease is proportional to the mole percent of CL present in the membrane (10- to 20-fold change over 5-60 mol%). With Fld, kobs decreases monotonically with increasing ionic strength at a fixed CL concentration. With Fd an increase in kobs occurs as the ionic strength is increased, which maximizes at intermediate ionic strength at a value larger than that obtained in the absence of lipid vesicles. When Fld is electrostatically bound to a positively charged vesicle composed of 40 mol% dioctadecyldimethylammonium ion (DODAC) and 60 mol% PC, again kobs for electron transfer to cytc is decreased over that obtained in solution, and the magnitude is diminished monotonically by increasing ionic strength. In contrast, kobs for electron transfer from Fd to cytc is unaffected by the presence of the positively charged membrane. The implications of these results for the role of membrane surface charge in modulating protein-protein interactions is discussed.