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: spectrophotometry 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 hydration state of the protein by two parameters: electric dipole moment and effective 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 displacement of methionine-S-Fe linkage in the heme. Dielectric measurements show that a simultaneous increase of the hydrodynamic radius and a decrease of the electric dipole moment of the protein accompany this alkaline transition. Thus, in water, conformational changes of cytochrome at basic pH result both in local structural changes in the heme pocket and in more global changes of the protein structure. In the water-trehalose solutions, optical measurements reveal a transition similar to that in water. On the contrary, dielectric parameters keep constant in the same pH range. Despite the displacement of the methionine-S-Fe linkage in the heme, the presence of trehalose stabilizes the global conformation of protein. This is coherent with the role of trehalose as bioprotector.