Interactions of cytochrome c (cyt c) with cardiolipin (CL) partially unfold the protein activating its peroxidase function, a critical event in the execution of apoptosis. However, structural features of the altered protein species in the heterogeneous ensemble are difficult to probe with ensemble averaging. Analyses of the dye-to-heme distance distributions P(r) from time-resolved FRET (TR-FRET) have uncovered two distinct types of CL-bound cyt c conformations, extended and compact. We have combined TR-FRET, fluorescence correlation spectroscopy (FCS), and bio-layer interferometry to develop a systematic understanding of the functional partioning between the two conformations. The two subpopulations are in equilibrium with each other, with a sub-millisecond rate of conformational exchange reflecting the protein folding into a compact non-native state, as well as protein interactions with the lipid surface. Electrostatic interactions with the negatively-charged lipid surface that correlate with physiologically-relevant changes in CL concentrations strongly affect the kinetics of cyt c binding and conformational exchange. A predominantly peripheral binding mechanism, rather than deep protein insertion into the membrane, provides a rationale for the general denaturing effect of the CL surface and the large-scale protein unfolding. These findings closely relate to cyt c folding dynamics and suggest a general strategy for extending the time window in monitoring the kinetics of folding.
展开▼
