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

Sophie Bernad ^*, Silke Oellerich , Tewfik Soulimane , Sylvie Noinville ^*, Marie-Hélène Baron ^*, Maité Paternostre  and Sophie Lecomte ^*

^* Laboratoire de Dynamique, Interactions et Réactivité, CNRS-Université Paris VI, Thais, France; Max-Planck Institut für Strahlenchemie, Mülheim, Germany; Paul Scherrer Institut, Life Sciences, Structural Biology, OSRA/008, Villigen, Switzerland; and URA 2096, CEA-CNRS, DBJC/SBFM CEA-Saclay, Gif/Yvette, France

Correspondence: Address reprint requests to Dr. Sophie Lecomte, Laboratoire de Dynamique, Interactions et Réactivité, UMR-7075, CNRS-Université Paris VI, 2 Rue Henri Dunant, F-94320 Thais, France. Tel.: 33-1-4978-1113; Fax: 33-1-4978-1118; E-mail: sophie.lecomte{at}glvt-cnrs.fr.

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 ß-structure. This hydrophobic effect may be the key to the difference between the behaviors of the two cytochromes when bound to DOPG vesicles.