Comparison of a novel GtfB-type 4,6-α-glucanotransferase from Fructilactobacillus sanfranciscensis Gs2 for converting starch in the gelatinized and granular systems

The starch-converting GH70 α-glucanotransferases, as the influential enzymatic tools for the synthesis of structurally diverse α-glucans, are of great value in the food industry. In this work, a new GtfB-type α-glucanotransferase from Fructilactobacillus sanfranciscensis Gs2 (FsGs2 GtfB) was mined from the NCBI database, which is highly conserved in four conserved regions I-IV for GtfB enzymes but possessed distinct characteristics in some residues at acceptor subsites +1/+2. Biochemical characterizations revealed that the FsGs2 GtfB enzyme demonstrated a novel 4,6-α-glucanotransferase activity, which was able to cleave the (α1 → 4) linkages in maltooligosaccharide/starch substrates and mainly synthesize branching (α1 → 4,6) linkages in the gelatinized system, resulting in the generation of low-molecular-mass short-clustered (α1 → 4,6)-glucans. Nevertheless, FsGs2 GtfB acted as a quasi-hydrolase instead of transglucosylase in the granular system, causing a notable decrease in the molecular mass of granular wheat starch and a tendency towards shortened chain lengths. Hence, the FsGs2 GtfB enzyme represents an additional potent enzymatic tool of reassembling the (α1 → 4) and (α1 → 6) linkages in starch molecules and enriches the diversity of α-glucans that can be synthesized.