Antioxidant Activity of a Selenopeptide Modelling the Thioredoxin Reductase Active Site is Enhanced by NH···Se Hydrogen Bond in the Mixed Selenosulfide Intermediate

Abstract

Thioredoxin reductase (TrxR), one of the representative selenoenzymes, is an important antioxidant enzyme suppressing oxidative stress in living organisms. At the active site of human TrxR, the presence of a Sec•••His•••Glu catalytic triad was previously suggested. Method. In this study, a short selenopeptide mimicking this plausible triad, i.e., H-CUGHGE-OH (1), was designed, synthesized, and evaluated for the TrxR-like catalytic activity. In this study, a short selenopeptide mimicking this plausible triad, i.e., H-CUGHGE-OH (1), was designed, synthesized, and evaluated for the TrxR-like catalytic activity The molecular simulation in advance by REMC/SAAP3D predicted the preferential formation of Sec•••His•••Glu hydrogen bonding networks in the aqueous solution. Indeed, a significant antioxidant activity was observed for 1 in the activity assay using NADPH as a reductant and H2O2 as a substrate. Tracking the reaction between 1 and GSH by 77Se NMR revealed a reductive cleavage of the selenosulfide (Se-S) bond to generate the diselenide species. The observation suggested that in the transiently formed mixed Se-S intermediate, the NH•••Se hydrogen bond between the Sec and His residues leads a nucleophilic attack of the second thiol molecule not to the intrinsically more electrophilic Se atom but to the less electrophilic S atom of the Se-S bond. Ab initio calculations for the complex between MeSeSMe and an imidazolium ion at the MP2/6-31++G(d,p) level demonstrated that NH•••Se and NH•••S hydrogen bonds are equally favorable as the interaction modes. Thus, importance of the relative spatial arrangement of the Se-S bond with respect to the imidazole ring was suggested for the exertion of the TrxR-like catalytic activity. The proposed umpolung effect of NH•••Se hydrogen bond on the reactivity of a Se-S bond will be a useful tool for developing efficient TrxR models with high redox catalytic activity.