A study of the influence of the of yeast iso-2-cytochrome c on hydrophobiccore residues phosphate binding: A probe of the hydrophobic core-surface charge interactions

To gain insight into the role of hydrophobic core-surface charge interactio ns in stabilizing cytochrome c, we investigated the influence of hydrophobi c core residues on phosphate binding by mutating residues in yeast iso-2-cy tochrome c to those corresponding to iso-l-cytochrome c in various combinat ions. Heat transition of ultraviolet CD was followed as a function of pH in the presence and absence of phosphate. Thermodynamic parameters were deduc ed. It was found that the 120V/V43A/M98L mutation in the hydrophobic core, whose locations are remote from the putative phosphate sites, modulates pho sphate interactions, The modulation is pH dependent. The 120V/M98L and V43A mutation effects are nonadditive. The results lead to a model analogous to that of Tsao, Evans, and Wennerstrom, where a domain associated with the o rdered hydrophobic core is sensitive to the fields generated by the surface charges. Such an explanation would be in accord with the observed differen ce in thermal stability between iso-2 and horse cytochromes c.