Trienzymatic Complex System for Isomerization of Agar-Derived D-Galactose into D-Tagatose as a Low-Calorie Sweetener

D-Tagatose is a rare monosaccharide that is used in products in the food industry as a low-calorie sweetener. To facilitate biological conversion of D-tagatose, the agarolytic enzyme complexes based on the principle of the cellulosome structure were constructed through dockerin-cohesin interaction with the scaffoldin. The construction of agarolytic complexes composed of L-arabinose isomerase caused efficient isomerization activity on the agar-derived sugars. In a trienzymatic complex, the chimeric beta-agarase (cAgaB) and anhydro-galactosidase (cAhgA) from Zobellia galactanivorans could synergistically hydrolyze natural agar substrates and L-arabinose isomerase (LsAraA Doc) from Lactobacillus sakei 23K could convert D-galactose into D-tagatose. The trienzymatic complex increased the concentration of D-tagatose from the agar substrate to 4.2 g/L. Compared with the monomeric enzyme, the multimeric enzyme showed a 1.4-fold increase in tagatose production, good thermostability, and reusability. A residual activity of 75% remained, and 52% of conversion was noted after five recycles. These results indicated that the dockerin-fused chimeric enzymes on the scaffoldin successfully isomerized D-galactose into D-tagatose with synergistic activity. Thus, the results demonstrated the possibility of advancing efficient strategies for utilizing red algae as a biomass source to produce D-tagatose in the industrial food field that uses marine biomass as the feedstock.