Unfolding pathway of cytochromec oxidase induced by ionic surfactants: Circular dichroism and picosecond time-resolved fluorescence studies

The unfolding of the membrane protein, cytochromec oxidase (CcO) induced by ionic surfactants have been studied by using circular dichroism, optical absorbance and time resolved tryptophan fluorescence spectroscopic methods. Ionic surfactant cetyltrimethyl ammonium bromide (CTAB) was found to cause denaturation of this membrane protein leading to release of both, the hemea residues from CcO indicated by both CD and optical titration. Upon dissociation of the hemes from the protein matrix; the tryptophan fluorescence intensity of CcO increased drastically and the fluorescence lifetimes became much longer compared to the short lifetimes observed in the native protein. The shortest lifetime of 70 ps observed in the native protein due to strong quenching (energy transfer) of the heme groups, increased ∼10-fold in the CTAB-unfolded protein indicating complete removal of the heme groups from the protein matrix. Remarkable differences were observed between the mode of actions of ionic surfactants and the commonly used denaturant guanidine hydrochloride. Improved data analysis of maximum entropy method showed that the lifetime distribution pattern in the two cases of unfolding were very different. The lifetimes in guanidine hydrochloride unfolded CcO were much shorter and more widely distributed indicating that the hemes are probably not separated away from the protein matrix and that the unfolded state is highly heterogeneous. Our results further showed that the lauryl maltoside inhibits denaturation of CcO by the ionic surfactant and the initial step of the denaturation possibly involves quantitative replacement of the lauryl maltoside by the ionic surfactant at the surface of the enzyme.