4,6-alpha-Glucanotransferase activity occurs more widespread in Lactobacillus strains and constitutes a separate GH70 subfamily

Family 70 glycoside hydrolase glucansucrase enzymes exclusively occur in lactic acid bacteria and synthesize a wide range of alpha-d-glucan (abbreviated as alpha-glucan) oligo- and polysaccharides. Of the 47 characterized GH70 enzymes, 46 use sucrose as glucose donor. A single GH70 enzyme was recently found to be inactive with sucrose and to utilize maltooligosaccharides [(1 -> 4)-alpha-d-glucooligosaccharides] as glucose donor substrates for alpha-glucan synthesis, acting as a 4,6-alpha-glucanotransferase (4,6-alpha GT) enzyme. Here, we report the characterization of two further GH70 4,6-alpha GT enzymes, i.e., from Lactobacillus reuteri strains DSM 20016 and ML1, which use maltooligosaccharides as glucose donor. Both enzymes cleave alpha 1 -> 4 glycosidic linkages and add the released glucose moieties one by one to the non-reducing end of growing linear alpha-glucan chains via alpha 1 -> 6 glycosidic linkages (alpha 1 -> 4 to alpha 1 -> 6 transfer activity). In this way, they convert pure maltooligosaccharide substrates into linear alpha-glucan product mixtures with about 50% alpha 1 -> 6 glycosidic bonds (isomalto/maltooligosaccharides). These new alpha-glucan products may provide an exciting type of carbohydrate for the food industry. The results show that 4,6-alpha GTs occur more widespread in family GH70 and can be considered as a GH70 subfamily. Sequence analysis allowed identification of amino acid residues in acceptor substrate binding subsites +1 and +2, differing between GH70 GTF and 4,6-alpha GT enzymes.