Oxidation reactivity of zinc-cysteine clusters in metallothionein.

Evaluating the reactivity of the metal-thiolate clusters in metallothionein (MT) is a key step in understanding the biological functions of this protein. The effects of the metal clustering and protein environment on the thiolate reactivity with hydrogen peroxide (H(2)O(2)) were investigated by performing quantum theory calculations with chemical accuracy at two levels of complexity. At the first level, the reactivity with H(2)O(2) of a model system ([(Zn)(3)(MeS)(9)](3-), MeS is methanethiolate) of the β domain cluster of MT was evaluated using density functional theory (DFT) with the mPW1PW91 functional. At the second level of complexity, the protein environment was included in the reactant system and the calculations were performed with the hybrid ONIOM method combining the DFT-mPW1PW91 and the semiempirical PM6 levels of theory. In these conditions, the energy barrier for the oxidation of the most reactive terminal thiolate was 21.5 kcal mol(-1). This is 3 kcal mol(-1) higher than that calculated for the terminal thiolate in the model system [(Zn)(3)(MeS)(9)](3-) and about 7 kcal mol(-1) higher than that obtained for the free thiolate. In spite of this rise of the energy barrier induced by the protein environment, the thiolate oxidation by H(2)O(2) is confirmed as a possible way for metal release from MT. On the other hand, the results suggest that the antioxidant role of MT in the living cell cannot be as important as that of glutathione (which bears a free thiol).