Folding of apocytochrome c induced by the interaction with negatively charged lipid micelles proceeds via a collapsed intermediate state

Abstract

Unfolded apocytochrome c acquires an α‐helical conformation upon interaction with lipid. Folding kinetic results below and above the lipid's CMC, together with energy transfer measurements of lipid bound states, and salt‐induced compact states in solution, show that the folding transition of apocytochrome c from the unfolded state in solution to a lipid‐inserted helical conformation proceeds via a collapsed intermediate state ( I C ). This initial compact state is driven by a hydrophobic collapse of the polypeptide chain in the absence of the heme group and may represent a heme‐free analogue of an early compact intermediate detected on the folding pathway of cytochrome c in solution. Insertion into the lipid phase occurs via an unfolding step of I C through a more extended state associated with the membrane surface ( I S ). While I C appears to be as compact as salt‐induced compact states in solution with substantial α‐helix content, the final lipid‐inserted state ( H mic ) is as compact as the unfolded state in solution at pH 5 and has an α‐helix content which resembles that of native cytochrome c .