The mechanism of the hydrogen peroxide reduction by two molecules of glutathione catalyzed by the selenoprotein glutatione peroxidase (GPx) has been computationally studied. It has been shown that the first elementary reaction of this process, (E-SeH) + H2O2 → (E-SeOH) + H 2O (1), proceeds via a stepwise pathway with the overall barrier of 17.1 kcal/mol, which is in good agreement with the experimental barrier of 14.9 kcal/mol. During reaction 1, the Gln83 residue has been found to play a key role as a proton acceptor, which is consistent with experiments. The second elementary reaction, (E-SeOH) + GSH → (E-Se-SG) + HOH (2), proceeds with the barrier of 17.9 kcal/mol. The last elementary reaction, (E-Se-SG) + GSH → (E-SeH) + GS-SG (3), is initiated with the coordination of the second glutathione molecule. The calculations clearly suggest that the amide backbone of the Gly50 residue directly participates in this reaction and the presence of two water molecules is absolutely vital for the reaction to occur. This reaction proceeds with the barrier of 21.5 kcal/mol and is suggested to be a rate-determining step of the entire GPx-catalyzed reaction H2O 2 + 2GSH → GS-SG + 2H2O. The results discussed in the present study provide intricate details of every step of the catalytic mechanism of the GPx enzyme and are in good general agreement with experimental findings and suggestions.
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