Unfolding and refolding of cytochrome c driven by the interaction with lipid micelles

Abstract

Binding of native cyt c to L-PG micelles leads to a partially unfolded conformation of cyt c. This micelle-bound state has no stable tertiary structure, but remains as α-helical as native cyt c in solution. In contrast, binding of the acid-unfolded cyt c to L-PG micelles induces folding of the polypeptide, resulting in a similar helical state to that originated from the binding of native cyt c to L-PG micelles. Far-ultraviolet (UV) circular dichroism (CD) spectra showed that this common micelle-associated helical state (H_L) has a native-like α-helix content, but is highly expanded without a tightly packed hydrophobic core, as revealed by tryptophan fluorescence, near-UV, and Soret CD spectroscopy. The kinetics of the interaction of native and acid-unfolded cyt c was investigated by stopped-flow tryptophan fluorescence. Formation of H_L from the native state requires the disruption of the tightly packed hydrophobic core in the native protein. This micelle-induced unfolding of cyt c occurs at a rate ∼0.1 s⁻¹, which is remarkably faster in the lipid environment compared with the expected rate of unfolding in solution. Refolding of acid-unfolded cyt c with L-PG micelles involves an early highly helical collapsed state formed during the burst phase (<3 ms), and the observed main kinetic event reports on the opening of this early compact intermediate prior to insertion into the lipid micelle.