Interaction of Horse Heart and Thermus thermophilus Type c Cytochromes with Phospholipid Vesicles and Hydrophobic Surfaces

The binding of horse heart cytochrome c (cyt-c) and Thermus thermophilus cytochrome c552 (cyt-c552) to dioleoyl phosphatidylglycerol (DOPG) vesicles was investigated using Fourier transform infrared (FTIR) spectroscopy and turbidity measurements. FTIR spectra revealed that the tertiary structures of both cytochromes became more open when bound to DOPG vesicles, but this was more pronounced for cyt-c. Their secondary structures were unchanged. Turbidity measurements showed important differences in their behavior bound to the negatively charged DOPG vesicles. Both cytochromes caused the liposomes to aggregate and flocculate, but the ways they did so differed. For cyt-c, more than a monolayer was adsorbed onto the liposome surface prior to aggregation due to charge neutralization, whereas cyt c552 caused aggregation at a protein/lipid ratio well below that required for charge neutralization. Therefore, although cyt-c may cause liposomes to aggregate by electrostatic interaction, cyt-c552 does not act in this way. FTIR-attenuated total reflection spectroscopy (FTIR-ATR) revealed that cyt-c lost much of its secondary structure when bound to the hydrophobic surface of octadecyltrichlorosilane, whereas cyt-c552 folds its domains into a β-structure. This hydrophobic effect may be the key to the difference between the behaviors of the two cytochromes when bound to DOPG vesicles.