Characterization of a new 4,6-α-glucanotransferase from Limosilactobacillus fermentum NCC 3057 with ability of synthesizing low molecular mass isomalto-/maltopolysaccharide

4,6-α-Glucanotransferase (4,6-α-GT) converts starch into product with increased α(1–6) glycosidic bonds ratio, and this product is a new type of soluble dietary fiber with property of escaping small intestine digestion. 4,6-α-GT gained interest recently because of their potential use in enzymatic synthesis of soluble dietary fiber. In this study, a putative GtfB sequence from Limosilactobacillus fermentum NCC 3057 was identified. This sequence was truncated and expressed in Escherichia coli to obtain the protein L. fermentum NCC 3057 GtfBΔN. GtfBΔN showed optimal activity at 35 °C and pH 6.0, and it converted amylose V to isomalto-/maltopolysaccharide (IMMP) with low molecular mass (3.1 kDa). This IMMP product contains 72% α(1–6) glycosidic bonds, and it showed 64% indigestible content in vitro digestion experiment. These results indicate that the product of L. fermentum NCC 3057 GtfBΔN is a soluble dietary fiber. Finally the X-ray crystal structure of GtfBΔN (2.4 Å) was resolved. Based on the GtfBΔN structure, we offer an insight about that three loops of domain C may be related to the molecular mass of IMMP product. •A new 4,6-α-glucanotransferase (GtfB) was identified and characterized.•GtfB produces IMMP with 72% α(1–6) bonds and 64% indigestible component.•IMMP is a soluble dietary fiber with low molecular mass.•The crystal structure of GtfB was solved.