Structure and dynamics of lipid-associated states of apocytochrome c

Apocytochrome c (apocyt c), which in aqueous solution is largely unstructur ed, acquires an alpha-helical conformation upon association with lipid memb ranes. The extent of alpha-helix induced in apocyt c is lipid-dependent and this folding process is driven by both electrostatic and hydrophobic lipid -protein interactions. The structural and dynamic properties of apocyt c in lipid membranes were investigated by attenuated total reflection Fourier t ransform infrared spectroscopy combined with amide H-D exchange kinetics. A pocyt c acquires a higher content of alpha-helical structure with negativel y charged membranes than with zwitterionic ones. For all membranes studied here, the helices of these partially folded states of apocyt c have a prefe rential orientation perpendicular to the plane of the lipid membrane. The H -D exchange revealed that a small fraction of amide protons of apocyt c, po ssibly associated with a stable folded domain protected by the lipid, remai ned protected from exchange over 20 min. However, a large fraction of amide protons exchanged in less than 20 min, indicating that the helical states of apocyt c in lipid membranes are very dynamic.