Study on Characteristics of Catalytic Products of 4,6-α-Glucantransferase from Limosilactobacillus fermentum

Modifying the molecular structure of dextrin can yield reconstituted dextrin products with different digestive properties, including slow-digesting dextrin and resistant dextrin, which are important starch-based water-soluble dietary fibers (DF). 4,6-α-glucan transferase can use starch as a substrate and catalyze the synthesis of resistant dextrin through a transglycosylation reaction. However, the currently identified enzymes of this type generally had low conversion efficiency and unsatisfactory product yield during the catalytic process, which limited their industrial application. A 4,6-α-glucan transferase (GT7) from Limosilactobacillus fermentum was screened and successfully heterologously expressed in Escherichia coli. Subsequently, it was applied to the industrial preparation of resistant dextrin, and the properties of the reconstituted dextrin were identified and systematically analyzed. The results showed that the recombinant strain Escherichia coli BL21(DE3)/pET-24a(+)-GT7 produced an enzyme with activity of 79.3 U/mL. After optimizing the process conditions, with an enzyme dosage of 3000 U/g and a reaction time of 18 h, high-performance liquid chromatography detection showed that the relative peak area of DF in the reconstituted dextrin sample reached 65%. The average molecular weight of the reconstituted dextrin was 7.6 kDa, and the proportion of α-1, 6-glycosidic bonds was 92%. The reconstituted dextrin sample was a high-molecular-weight linear dextrin, with the glucan chains mainly connected by α-1,6-glycosidic bonds and a small amount of structures connected by α-1,4-glycosidic bonds at the chain ends. The solubility of the reconstituted dextrin sample was 70.5 g/100 mL, and the viscosity was 14.7 mPa·s. It had Maillard reaction ability and good freeze-thaw stability (freeze-thaw cycles ≤3), as well as high storage stability. In vitro anti-digestion experiments showed that the resistant starch content in the reconstituted dextrin sample was 73.01%. It was hoped that this research could provide technical support for the screening and efficient expression of 4,6-α-glucan transferase, and the industrial preparation and property evaluation of resistant dextrin.