Abnormal Enhancement of Protein Disulfide Isomerase-like Activity of a Cyclic Diselenide Conjugated with a Basic Amino Acid by Inserting a Glycine Spacer

SIMPLE SUMMARY: A polypeptide chain, which is a biological polymer composed of amino acids, can have a physiological function as a “protein” by forming a unique three-dimensional structure through protein folding. Structural failure of proteins in cells frequently induces various diseases, such as neurodegenerative diseases and diabetes. Therefore, cells are equipped with various enzymes, such as protein disulfide isomerase (PDI), to promote correct folding and avoid misfolding. In both cells and test tubes, protein folding is a key process in determining the overall efficiency of protein synthesis. Therefore, it is crucial to develop an artificial folding catalyst, which behaves like PDI, for both drug discovery and protein engineering. Herein, we report the structural optimization of a cyclic diselenide molecule to improve its intrinsic PDI-like catalytic activity. ABSTRACT: In a previous study, we reported that (S)-1,2-diselenane-4-amine (1) catalyzes oxidative protein folding through protein disulfide isomerase (PDI)-like catalytic mechanisms and that the direct conjugation of a basic amino acid (Xaa: His, Lys, or Arg) via an amide bond improves the catalytic activity of 1 by increasing its diselenide (Se–Se) reduction potential (E′°). In this study, to modulate the Se–Se redox properties and the association of the compounds with a protein substrate, new catalysts, in which a Gly spacer was inserted between 1 and Xaa, were synthesized. Exhaustive comparison of the PDI-like catalytic activities and E′° values among 1, 1-Xaa, and 1-Gly-Xaa showed that the insertion of a Gly spacer into 1-Xaa either did not change or slightly reduced the PDI-like activity and the E′° values. Importantly, however, only 1-Gly-Arg deviated from this generality and showed obviously increased E°′ value and PDI-like activity compared to the corresponding compound with no Gly spacer (1-Arg); on the contrary, its catalytic activity was the highest among the diselenide compounds employed in this study, while this abnormal enhancement of the catalytic activity of 1-Gly-Arg could not be fully explained by the thermodynamics of the Se–Se bond and its association ability with protein substrates.