Radiation damage of lysozyme in a biomimetic model: some insights by Raman spectroscopy

A biomimetic model for protein/lipid radical damage is described. The model shows that the protein degradation is accompanied by trans lipid isomerization due to thiyl radicals, derived from sulfur-containing residues and diffused through the lipid bilayer. γ-Irradiation was used to obtain free radical generation and lysozyme was chosen as enzyme example. A detailed study of the Raman spectra of lysozyme irradiated at different doses coupled to vesicle experiments, as well as enzymatic assays, is reported to gather a comprehensive view of the irradiation effect. The lysozyme resistance to degradation by γ-irradiation is remarkable and the protein structure play a significant role in blocking the ready access of free radicals both to the sulfur-containing residues and the active site. The degradation of sulfur moieties does not occur immediately, but as irradiation progressed, the involvement of these residues takes place. In fact, structural changes induced on the protein render these residues more exposed and susceptible of radical attack. Tyr are confirmed to be among the most sensitive residues towards oxidation. These results can be relevant for a better understanding of mechanisms of tandem radical damages occurring in a biological environment.