Synthesis of L-Rhamnose andN-Acetyl-D-Glucosamine Derivatives Entering in the Composition of Bacterial Polysaccharides by Use of Glucansucrases

Computer-aided engineering of a branching sucrase for the glucodiversification of a tetrasaccharide precursor of S. flexneri antigenic oligosaccharides

Enzyme engineering approaches have allowed to extend the collection of enzymatic tools available for synthetic purposes. However, controlling the regioselectivity of the reaction remains challenging, in particular when dealing with carbohydrates bearing numerous reactive hydroxyl groups as substrates. Here, we used a computer-aided design framework to engineer the active site of a sucrose-active [Formula: see text]-transglucosylase for the 1,2-cis-glucosylation of a lightly protected chemically synthesized tetrasaccharide, a common precursor for the synthesis of serotype-specific S. flexneri O-antigen fragments. By targeting 27 amino acid positions of the acceptor binding subsites of a GH70 branching sucrase, we used a RosettaDesign-based approach to propose 49 mutants containing up to 15 mutations scattered over the active site. Upon experimental evaluation, these mutants were found to produce up to six distinct pentasaccharides, whereas only two were synthesized by the parental enzyme. Interestingly, we showed that by introducing specific mutations in the active site of a same enzyme scaffold, it is possible to control the regiospecificity of the 1,2-cis glucosylation of the tetrasaccharide acceptor and produce a unique diversity of pentasaccharide bricks. This work offers novel opportunities for the development of highly convergent chemo-enzymatic routes toward S. flexneri haptens.

Amylosucrase as a transglucosylation tool: From molecular features to bioengineering applications

Amylosucrase (EC 2.4.1.4, ASase), an outstanding sucrose-utilizing transglucosylase in the glycoside hydrolase family 13, can produce glucans with only α-1,4 linkages. Generally, on account of a double-displacement mechanism, ASase can catalyze polymerization, isomerization, and hydrolysis reactions with sucrose as the sole substrate, and has transglycosylation capacity to attach glucose molecules from sucrose to extra glycosyl acceptors. Based on extensive enzymology research, this review presents the characteristics of various ASases, including their microbial metabolism, preparation, and enzymatic properties, and exhibits structure-based strategies in the improvement of activity, specificity, and thermostability. As a vital transglucosylation tool of producing sugars, carbohydrate-based bioactive compounds, and materials, the bioengineering applications of ASases are also systematically summarized.

Structural and functional insights into RHA-P, a bacterial GH106 α-L-rhamnosidase from Novosphingobium sp. PP1Y

α-L-Rhamnosidases (α-RHAs, EC 3.2.1.40) are glycosyl hydrolases (GHs) hydrolyzing terminal α-l-rhamnose residues from different substrates such as heteropolysaccharides, glycosylated proteins and natural flavonoids. Although the possibility to hydrolyze rhamnose from natural flavonoids has boosted the use of these enzymes in several biotechnological applications over the past decades, to date only few bacterial rhamnosidases have been fully characterized and only one crystal structure of a rhamnosidase of the GH106 family has been described. In our previous work, an α-l-rhamnosidase belonging to this family, named RHA-P, was isolated from the marine microorganism Novosphingobium sp. PP1Y. The initial biochemical characterization highlighted the biotechnological potential of RHA-P for bioconversion applications. In this work, further functional and structural characterization of the enzyme is provided. The recombinant protein was obtained fused to a C-terminal His-tag and, starting from the periplasmic fractions of induced recombinant cells of E. coli strain BL21(DE3), was purified through a single step purification protocol. Homology modeling of RHA-P in combination with a site directed mutagenesis analysis confirmed the function of residues D503, E506, E644, likely located at the catalytic site of RHA-P. In addition, a kinetic characterization of the enzyme on natural flavonoids such as naringin, rutin, hesperidin and quercitrin was performed. RHA-P showed activity on all flavonoids tested, with a catalytic efficiency comparable or even higher than other bacterial α-RHAs described in literature. The results confirm that RHA-P is able to hydrolyze both α-1,2 and α-1,6 glycosidic linkages, and suggest that the enzyme may locate different polyphenolic aromatic moities in the active site.

Glucansucrases: three-dimensional structures, reactions, mechanism, α-glucan analysis and their implications in biotechnology and food applications

Glucansucrases are extracellular enzymes that synthesize a wide variety of α-glucan polymers and oligosaccharides, such as dextran. These carbohydrates have found numerous applications in food and health industries, and can be used as pure compounds or even be produced in situ by generally regarded as safe (GRAS) lactic acid bacteria in food applications. Research in the recent years has resulted in big steps forward in the understanding and exploitation of the biocatalytic potential of glucansucrases. This paper provides an overview of glucansucrase enzymes, their recently elucidated crystal structures, their reaction and product specificity, and the structural analysis and applications of α-glucan polymers. Furthermore, we discuss key developments in the understanding of α-glucan polymer formation based on the recently elucidated three-dimensional structures of glucansucrase proteins. Finally we discuss the (potential) applications of α-glucans produced by lactic acid bacteria in food and health related industries.