Subtle Change in the Charge Distribution of Surface Residues May Affect the Secondary Functions of Cytochrome c [Molecular Biophysics]

April 14th, 2015 by Paul, S. S., Sil, P., Haldar, S., Mitra, S., Chattopadhyay, K.

Although the primary function of cytochrome c (cytc) is electron transfer, the protein carries out an additional secondary function, involving its interaction with membrane cardiolipin (cdl), its peroxidase activity and the initiation of apoptosis. While the primary function of cytc is essentially conserved, its secondary function varies depending on the source of the protein. We report here a detailed experimental and computational study, which aims to understand, at the molecular level, the difference in secondary functions of cytc obtained from horse heart (mammalian) and Saccharomyces cerevisiae (yeast). The conformational landscape of cytc has been found to be heterogeneous, consisting of an equilibrium between compact and extended conformers as well as oligomeric species. Since the determination of relative populations of these conformers is difficult to obtain by ensemble measurements, we have used fluorescence correlation spectroscopy (FCS), a method that offers single molecule resolution. The population of different species are found to depend on multiple factors, including the protein source, the presence of cdl and urea, and their concentrations. The complex interplay between the conformational distribution and oligomerization plays a crucial role in the variation of the pre-apoptotic regulations of cytc observed from different sources. Finally, computational studies reveal that the variation in the charge distribution at the surface and the charge reversal sites may be the key determinants of the conformational stability of cytc.