Cold setting of gelatin–antioxidant peptides composite hydrogels using a new psychrophilic recombinant transglutaminase (rTGase)

This study used a new psychrophilic transglutaminase (TGase) to catalyze the formation of protein and peptide hydrogels by forming isopeptide bonds. The psychrophilic recombinant TGase (rTGase) was overexpressed in E. coli and purified using a Ni-Sepharose column followed by stepwise dialysis to recover the cold-active enzyme (specific activity of 1.63 U/mg). The rTGase catalyzed the formation of cold-set hydrogels at 4 °C, which were comprised of high molecular weight gelatin and antioxidant peptides. The antioxidant bioactivity of the cold-set gelatin–peptides composite hydrogels was significantly improved with rTGase at 4 °C for 10 days. To understand the mechanism of the psychrophilic rTGase on the enhanced bioactivity of cross-linked hydrogels, in silico molecular docking studies were performed to assess the binding of antioxidant peptides into the rTGase active pocket, as well as to analyze the interactions of Gln and Lys residues in the antioxidant peptides with amino acid residues in the rTGase catalytic site. The formation of covalent cross-linkages in the gelatin–gelatin, gelatin–peptide and peptide–peptide matrices, and possible structures of composite hydrogels formed by rTGase catalysis in cold conditions, were deduced. The new psychrophilic rTGase enabled stabilization of the antioxidant bioactivity of protein- and peptide-containing cold-set hydrogels formed at low temperatures.