Ryosuke Masuda

Abstract Selenocysteine (Sec)‐derived cyclic selenenyl amides, formed by the intramolecular cyclization of Sec selenenic acids (Sec–SeOHs), have been postulated to function as protective forms in the bypass mechanism of glutathione peroxidase (GPx). However, their chemical properties have not been experimentally elucidated in proteins or small‐molecule systems. Recently, we reported the first nuclear magnetic resonance observation of Sec–SeOHs and their cyclization to the corresponding cyclic selenenyl amides by using selenopeptide model systems incorporated in a molecular cradle. Herein, we elucidate the structures and reactivities of Sec‐derived cyclic selenenyl amides. The crystal structures and reactions toward a cysteine thiol or a 1,3‐diketone‐type chemical probe indicated the highly electrophilic character of cyclic selenenyl amides. This suggests that they can serve not only as protective forms to suppress the inactivation of Sec–SeOHs in GPx but also as highly electrophilic intermediates in the reactions of selenoproteins.

Although much attention has been paid to chemical elucidation of the catalytic cycle of glutathione peroxidase (GPx), it has been hampered by instability of selenocysteine selenenic acid (Sec–SeOH) intermediates. In this study, not only chemical processes of the canonical catalytic cycle but also those involved in the bypass mechanism, including the intramolecular cyclization of a Sec–SeOH to the corresponding five-membered ring selenenyl amide were demonstrated experimentally by utilizing selenopeptide model systems in which reactive intermediates can be stabilized by a nano-sized molecular cradle. The resulting cyclic selenenyl amide exhibited higher durability under oxidative conditions than in the state of a Sec–SeOH, corroborating its role as the protective form of GPx. The cyclization of Sec–SeOHs of the Sec-Gly-Thr and Sec-Gly-Lys models, which mimic the catalytic site of isozymes GPx1 and GPx4, respectively, was found to proceed at lower temperature than in the Sec-Gly-Gly model, which corresponds to the generalized form of the tripeptides in the catalytic site of GPx. The role of the hydrogen-bond accepting moieties in the cyclization process was elucidated by DFT calculation. It was indicated that, if the selenocysteine centers are incorporated in appropriate microenvironments, the bypass mechanism can function efficiently.