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
(IC). 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 IC
through a more extended state associated with the membrane
surface (IS). While IC
appears to be as compact as salt-induced compact states
in solution with substantial α-helix content, the final
lipid-inserted state (Hmic) 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.