Thermodynamics of the binding of the C-terminal repeat domain of Streptococcus sobrinus glucosyltransferase-I to dextran

Bacterial α-Glucan and Branching Sucrases from GH70 Family: Discovery, Structure-Function Relationship Studies and Engineering

Glucansucrases and branching sucrases are classified in the family 70 of glycoside hydrolases. They are produced by lactic acid bacteria occupying very diverse ecological niches (soil, buccal cavity, sourdough, intestine, dairy products, etc.). Usually secreted by their producer organisms, they are involved in the synthesis of α-glucans from sucrose substrate. They contribute to cell protection while promoting adhesion and colonization of different biotopes. Dextran, an α-1,6 linked linear α-glucan, was the first microbial polysaccharide commercialized for medical applications. Advances in the discovery and characterization of these enzymes have remarkably enriched the available diversity with new catalysts. Research into their molecular mechanisms has highlighted important features governing their peculiarities thus opening up many opportunities for engineering these catalysts to provide new routes for the transformation of sucrose into value-added molecules. This article reviews these different aspects with the ambition to show how they constitute the basis for promising future developments.

GH13 amylosucrases and GH70 branching sucrases, atypical enzymes in their respective families

Amylosucrases and branching sucrases are α-retaining transglucosylases found in the glycoside-hydrolase families 13 and 70, respectively, of the clan GH-H. These enzymes display unique activities in their respective families. Using sucrose as substrate and without mediation of nucleotide-activated sugars, amylosucrase catalyzes the formation of an α-(1 → 4) linked glucan that resembles amylose. In contrast, the recently discovered branching sucrases are unable to catalyze polymerization of glucosyl units as they are rather specific for dextran branching through α-(1 → 2) or α-(1 → 3) branching linkages depending on the enzyme regiospecificity. In addition, GH13 amylosucrases and GH70 branching sucrases are naturally promiscuous and can glucosylate different types of acceptor molecules including sugars, polyols, or flavonoids. Amylosucrases have been the most investigated glucansucrases, in particular to control product profiles or to successfully develop tailored α-transglucosylases able to glucosylate various molecules of interest, for example, chemically protected carbohydrates that are planned to enter in chemoenzymatic pathways. The structural traits of these atypical enzymes will be described and compared, and an overview of the potential of natural or engineered enzymes for glycodiversification and chemoenzymatic synthesis will be highlighted.

Structural Insights into the Carbohydrate Binding Ability of an α-(1→2) Branching Sucrase from Glycoside Hydrolase Family 70

The α-(1→2) branching sucrase ΔN123-GBD-CD2 is a transglucosylase belonging to glycoside hydrolase family 70 (GH70) that catalyzes the transfer ofd-glucosyl units from sucroseto dextrans or gluco-oligosaccharides via the formation of α-(1→2) glucosidic linkages. The first structures of ΔN123-GBD-CD2 in complex withd-glucose, isomaltosyl, or isomaltotriosyl residues were solved. The glucose complex revealed three glucose-binding sites in the catalytic gorge and six additional binding sites at the surface of domains B, IV, and V. Soaking with isomaltotriose or gluco-oligosaccharides led to structures in which isomaltosyl or isomaltotriosyl residues were found in glucan binding pockets located in domain V. One aromatic residue is systematically identified at the bottom of these pockets in stacking interaction with one glucosyl moiety. The carbohydrate is also maintained by a network of hydrogen bonds and van der Waals interactions. The sequence of these binding pockets is conserved and repeatedly present in domain V of several GH70 glucansucrases known to bind α-glucans. These findings provide the first structural evidence of the molecular interaction occurring between isomalto-oligosaccharides and domain V of the GH70 enzymes.

A survey of the year 2007 literature on applications of isothermal titration calorimetry

Elucidation of the energetic principles of binding affinity and specificity is a central task in many branches of current sciences: biology, medicine, pharmacology, chemistry, material sciences, etc. In biomedical research, integral approaches combining structural information with in-solution biophysical data have proved to be a powerful way toward understanding the physical basis of vital cellular phenomena. Isothermal titration calorimetry (ITC) is a valuable experimental tool facilitating quantification of the thermodynamic parameters that characterize recognition processes involving biomacromolecules. The method provides access to all relevant thermodynamic information by performing a few experiments. In particular, ITC experiments allow to by-pass tedious and (rarely precise) procedures aimed at determining the changes in enthalpy and entropy upon binding by van't Hoff analysis. Notwithstanding limitations, ITC has now the reputation of being the "gold standard" and ITC data are widely used to validate theoretical predictions of thermodynamic parameters, as well as to benchmark the results of novel binding assays. In this paper, we discuss several publications from 2007 reporting ITC results. The focus is on applications in biologically oriented fields. We do not intend a comprehensive coverage of all newly accumulated information. Rather, we emphasize work which has captured our attention with originality and far-reaching analysis, or else has provided ideas for expanding the potential of the method.