Metal complexes of a plant metallothionein under radical stress: assessment of structural modifications and transfer of radical damage

Damages to Zn2+and Cd2+ complexes of a metallothionein (MT) from a plant (Quercus suber - Qs), due to radical stress exposure, were investigated. QsMT, obtained by in vivo synthesis, is a low-molecular weight cysteine-rich protein with high capacity for binding metal ions. Although MTs do not appear to be essential for life, there is mounting evidences for a survival advantage of MT in situations of stress, including exposure to radicals and toxic metals. Gamma-irradiation was used to simulate the conditions of an endogenous radical stress. The degradation of the metal complexes was followed by Raman spectroscopy and the occurrence of tandem protein/lipid damage was shown by using a biomimetic model based on unsaturated lipid vesicle suspensions [1,2]. The Ho and eaq- attacks on the metal-QsMT aggregates are able to induce significant structural changes such as partial deconstruction and/or rearrangement of the metal clusters, and breaking of the protein backbone. Sulfur-containing residues resulted to be selectively attacked; in particular, Cys resulted to be among the most sensitive residues towards radical attack, suggesting that the thiolate clusters of both metal-QsMTs act as efficient interceptors of reducing species (Fig. 1). Under reductive stress Zn-QsMT undergoes a significant thiolate group oxidation. The participation of His to metal coordination became necessary for protein stabilization after radical stress. The radical-induced effects were dependent on the divalent metal bound. The reactions of reductive reactive species with Met residues and/or sulfur-containing ligands afford diffusible sulfur-centered radicals, which migrate from the aqueous phase to the lipid bilayer and transform the cis double bond of the oleate moiety to the trans isomer (Fig. 1).