Effect of trehalose on alkaline transition of cytochrome-c

This work deals with a study of alkaline transition of ferricytochrome-e in water and in water-trehalose solutions using two different techniques: spe ctrophotometry in the region of the heme absorption spectrum and dielectric spectroscopy at radio frequencies. The first is a local probe and provides qualitative information on the identity of axial ligands to the heme iron. The second is able to reveal small variations in conformation and/or hydra tion state of the protein by two parameters: electric dipole moment and eff ective hydrodynamic radius. Optical absorption measurements of cytochrome-e in water as a function of pH show, in the basic range, a transition of the protein toward a partially denatured state, characterized by a displacemen t of methionine-S-Fe linkage in the heme. Dielectric measurements show that a simultaneous increase of the hydrodynamic radius and a decrease of the e lectric dipole moment of the protein accompany this alkaline transition. Th us, in water, conformational changes of cytochrome at basic pH result both in local structural changes in the heme pocket and in more global changes o f the protein structure. In the water-trehalose solutions, optical measurem ents reveal a transition similar to that in water. On the contrary, dielect ric parameters keep constant in the same pH range. Despite the displacement of the methionine-S-Fe linkage in the heme, the presence of trehalose stab ilizes the global conformation of protein. This is coherent with the role o f trehalose as bioprotector.