Structural modifications in metal complexes of a plant metallothionein caused by reductive radical stress: a Raman study.

The effect of radical stress in the biological environment is a very active field of research connecting various disciplines of life science. Thus, a comprehensive vision of all possible reactive species is necessary for contributing to the solution of puzzling questions on free radicals. In this contest, damages to Zn(II) and Cd(II) complexes of a plant metallothionein (Quercus suber - QsMT), because of reductive radical stress, were investigated by Raman spectroscopy. QsMT is a low-molecular-weight cysteine-rich protein obtained by in vivo synthesis to have a physiologically representative model. Gamma-irradiation was used to simulate the endogenous formation of reductive species. By changing the appropriate conditions of irradiation, a selection of the reacting radical species (hydrogen atom and hydrated electron) was carried out. Specific damages occur at sensitive amino acid sites, selectively, rather than indiscriminately. In particular, Cys resulted to be among the most sensitive residues toward radical attack, suggesting that the thiolate clusters of both metal and QsMTs act as efficient interceptors of reducing species. Under reductive stress, Zn-QsMT undergoes a significant thiolate group oxidation and the participation of ligands other than the cysteine-derived thiolate bonds (i.e. His) in zinc coordination becomes necessary for the protein stabilization. Regarding Met residues, they resulted to be more sensitive to the reductive radical attack when the protein binds Cd(II) ions, indicating that the protein structure can play a significant role in blocking the ready access of free radicals to the sulfur-containing residues, so strongly affecting the radiosensitivity of the protein. In conclusion, the results obtained from -irradiation experiments indicate that reductive stress causes changes in the primary QsMT structure and in the secondary and tertiary structures, and the radical-induced effects are dependent on the metal bound. Copyright © 2009 John Wiley & Sons, Ltd.