Characterization of the Paenibacillus beijingensis DSM 24997 GtfD and its glucan polymer products representing a new glycoside hydrolase 70 subfamily of 4,6-α-glucanotransferase enzymes

Previously we have reported that the Gram-negative bacterium Azotobacter chroococcum NCIMB 8003 uses the 4,6-alpha-glucanotransferase GtfD to convert maltodextrins and starch into a reuteran-like polymer consisting of (alpha 1 -> 4) glucan chains connected by alternating (alpha 1 -> 4)/(alpha 1 -> 6) linkages and (alpha 1 -> 4,6) branching points. This enzyme constituted the single evidence for this reaction and product specificity in the GH70 family, mostly containing glucansucrases encoded by lactic acid bacteria (http://www.CAZy.org). In this work, 4 additional GtfD-like proteins were identified in taxonomically diverse plant-associated bacteria forming a new GH70 subfamily with intermediate characteristics between the evolutionary related GH13 and GH70 families. The GtfD enzyme encoded by Paenibacillus beijingensis DSM 24997 was characterized providing the first example of a reuteran-like polymer synthesizing 4,6-alpha-glucanotransferase in a Gram-positive bacterium. Whereas the A. chroococcum GtfD activity on amylose resulted in the synthesis of a high molecular polymer, in addition to maltose and other small oligosaccharides, two reuteran-like polymer distributions are produced by P. beijingensis GtfD: a high-molecular mass polymer and a low-molecular mass polymer with an average M-w of 27 MDa and 19 kDa, respectively. Compared to the A. chroooccum GtfD product, both P. beijingensis GtfD polymers contain longer linear (alpha 1 -> 4) sequences in their structure reflecting a preference for transfer of even longer glucan chains by this enzyme. Overall, this study provides new insights into the evolutionary history of GH70 enzymes, and enlarges the diversity of natural enzymes that can be applied for modification of the starch present in food into less and/or more slowly digestible carbohydrate structures.