The conformational changes of apocytochrome c upon binding to phospholipid vesicles and micelles of phospholipid based detergents: a circular dichroism study.

The influence of lipid aggregates on the secondary structure of the mitochondrial precursor protein apocytochrome c was investigated by circular dichroism techniques. A conformational change of the protein from a random coil to partially alpha-helical structures was observed upon binding to negatively charged DOPS SUVs. Also DOPC SUVs showed to induce such a conformational change, but to a lesser extent. The detergents decyl-, lauryl and myristoyl-phosphoglycol or -phosphocholine, were synthesized as micel forming phospholipid analogs and are shown to mimic the phospholipids well in their ability to induce alpha-helices in the protein. A full assignment of the regions where the possible alpha-helices are formed is proposed by making use of derived fragments of apocytochrome c, prediction methods and the known X-ray structure of cytochrome c. Besides a helix at the N-terminus (residues 1-22) and at the C-terminal part (residues 80-101), two regions in the middle section (residues 49-54 and 59-70) are suggested to be helical. It is inferred that the two cysteines in the positions 14 an 17 at the N-terminal part are facing in the same direction, which could facilitate the covalent attachment of the heme group to the precursor in the translocation process.