Certain enzymes of the GH70 family dextransucrases synthesize very high molar mass dextran polymers, whereas others produce a mixed population of very high and low molar mass products directly from sucrose substrate. Identifying the determinants dictating polymer elongation would allow the tight control of dextran size. To explore this central question, we focus on the recently discovered DSR-M enzyme from Leuconostoc citreum NRRL B-1299, which is the sole enzyme that naturally, exclusively, and very efficiently produces only low molar mass dextrans from sucrose. Extensive biochemical and structural characterization of a truncated form of DSR-M (DSR-MΔ2, displaying the same biochemical behavior as the parental enzyme) and X-ray structural analysis of complexes with sucrose and isomaltotetraose molecules together with accurate monitoring of the resulting polymer formation reveal that DSR-MΔ2 adopts a nonprocessive mechanism attributed to (i) a high propensity to recognize sucrose as a preferred acceptor at the initial stage of catalysis, (ii) an ability to elongate oligodextrans irrespective of their size, and (iii) the presence of a domain V showing a weak ability to bind to the growing dextran chains. In this study, we present the 3D structure with the largest defined domain V reported to date in the GH70 family and map sugar binding pockets on the basis of the structure of the complex obtained with isomaltotetraose. Altogether, these findings give insights into the interplay between the domain V and the catalytic site during polymerization. They open promising strategies for GH70 enzyme engineering aiming at modulating glucan size.

Leuconostoc citreum NRRL B-742 has been known for years to produce a highly ␣-(133)-branched dextran for which the synthesis had never been elucidated. In this work a gene coding for a putative ␣-transglucosylase of the GH70 family was identified in the reported genome of this bacteria and functionally characterized. From sucrose alone, the corresponding recombinant protein, named BRS-B, mainly catalyzed sucrose hydrolysis and leucrose synthesis. However, in the presence of sucrose and a dextran acceptor, the enzyme efficiently transferred the glucosyl residue from sucrose to linear ␣-(136) dextrans through the specific formation of ␣-(133) linkages. To date, BRS-B is the first reported ␣-(133) branching sucrase. Using a suitable sucrose/dextran ratio, a comb-like dextran with 50% of ␣-(133) branching was synthesized, suggesting that BRS-B is likely involved in the comb-like dextran produced by L. citreum NRRL B-742. In addition, data mining based on the search for specific sequence motifs allowed the identification of two genes putatively coding for branching sucrases in the genome of Leuconostoc fallax KCTC3537 and Lactobacillus kunkeei EFB6. Biochemical characterization of the corresponding recombinant enzymes confirmed their branching specificity, revealing that branching sucrases are not only found in L. citreum species. According to phylogenetic analyses, these enzymes are proposed to constitute a new subgroup of the GH70 family.Numerous lactic acid bacteria from the Leuconostoc genus isolated from different habitats, such as sugar juice, fermenting vegetables, or dairy products, have long been known to produce slimes in sucrose solution (1, 2). These slimy compounds were rapidly characterized as dextrans, homopolymers of ␣-D-glucopyranosyl units mainly linked by ␣-(136) linkages. Dextrans with a high content of ␣-(136) linkages and a low degree of branching found their first industrial applications in the 1940s as a source of synthetic blood volume expanders (3). These findings motivated Jeanes et al. (22) to investigate the diversity of polymers produced during sucrose fermentation by different strains of lactic acid bacteria. A total of 96 strains were screened, and their dextrans were structurally characterized. In this study one strain, Leuconostoc mesenteroides NRRL B-742, also known as L. mesenteroides ATCC 13146 and renamed Leuconostoc citreum NRRL B-742 (4), received particular attention. Indeed, this strain, first isolated from a can of spoiled tomatoes in 1927 (1), was shown to produce two types of dextrans that differed by their structures and degree of solubility from sucrose. The less soluble polymer contained 75% of ␣-(136) linkages and 15% of ␣-(134) linkages, whereas the more soluble polymer displayed an uncommon comb-like structure consisting of a linear backbone of ␣-(136)-D-glucopyranosyl residues grafted with one ␣-(133)-linked glucosyl unit on every glucosyl moiety (5-9). Concomitantly with those findings, dextrans were shown to be synthesized by ␣-transglucosylases, commonly named g...