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 (HL)
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 HL 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.