Xylanases are hydrolytic enzymes which randomly cleave the β 1,4 backbone of the complex plant cell wall polysaccharide xylan. Diverse forms of these enzymes exist, displaying varying folds, mechanisms of action, substrate specificities, hydrolytic activities (yields, rates and products) and physicochemical characteristics. Research has mainly focused on only two of the xylanase containing glycoside hydrolase families, namely families 10 and 11, yet enzymes with xylanase activity belonging to families 5, 7, 8 and 43 have also been identified and studied, albeit to a lesser extent. Driven by industrial demands for enzymes that can operate under process conditions, a number of extremophilic xylanases have been isolated, in particular those from thermophiles, alkaliphiles and acidiphiles, while little attention has been paid to cold-adapted xylanases. Here, the diverse physicochemical and functional characteristics, as well as the folds and mechanisms of action of all six xylanase containing families will be discussed. The adaptation strategies of the extremophilic xylanases isolated to date and the potential industrial applications of these enzymes will also be presented.

Xylanases, xylanase families and extremophilic xylanases

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of ‘glycomedicine’. Glycosylation refers to the addition of carbohydrate chains to proteins and lipids. In this Review, the authors discuss the broad role of glycans in immunity, cancer, xenotransplantation and glomerular filtration and the potential of ‘glycomedicine’.

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Glycosylation in health and disease.

Recent Developments of Transition-State Analogue Glycosidase Inhibitors of Non-Natural Product Origin

α- and β-Glucosidase inhibitors: chemical structure and biological activity

Mechanistic insights into glycosidase chemistry.

Not "from above", but "from the side": Configuration-retaining β-glycosidases protonate their substrate either anti or syn to the endocyclic C1-O bond as the first step in the enzymic cleavage of the glycosidic bond (see schematic drawing). Insights into the mechanism of action of glycosidases have been gained by a combination of the synthesis of inhibitors, the study of the kinetics of their inhibition, and the analysis of the crystal structures of glycosidases and glycosidase-ligand complexes.

Recent Insights into Inhibition, Structure, and Mechanism of Configuration-Retaining Glycosidases.

The structures of a series of complexes designed to mimic intermediates along the reaction coordinate for β-galactosidase are presented. These complexes clarify and enhance previous proposals regarding the catalytic mechanism. The nucleophile, Glu537, is seen to covalently bind to the galactosyl moiety. Of the two potential acids, Mg2+ and Glu461, the latter is in better position to directly assist in leaving group departure, suggesting that the metal ion acts in a secondary role. A sodium ion plays a part in substrate binding by directly ligating the galactosyl 6-hydroxyl. The proposed reaction coordinate involves the movement of the galactosyl moiety deep into the active site pocket. For those ligands that do bind deeply there is an associated conformational change in which residues within loop 794−804 move up to 10 A closer to the site of binding. In some cases this can be inhibited by the binding of additional ligands. The resulting restricted access to the intermediate helps to explain why allolactos...

A Structural View of the Action of Escherichia Coli (Lacz) Beta-Galactosidase

Neue Erkenntnisse über Hemmung, Struktur und Mechanismus konfigurationserhaltender Glycosidasen

The structure of the β-glycosidase inhibitors 1–7 and 10–13 suggests that protonation of O–C(1) (the glycosidic O-center) of the substrate by a carboxy group of the retaining β-glycosidases does not occur in the plane perpendicular to the ring of the glycon (β-side; ‘from the top’), but in the plane of the ring (‘from the side’). The triazoles 17 and 18 have been prepared in six steps from the L-xylofuranose 21. They possess a CH group instead of the N-center of the related tetrazoles 4 and 5, corresponding to the glycosidic O-atom, and a very similar structure, both in solution and in the solid state. Unlike the tetrazoles, however, which are good-to-medium inhibitors of retaining β-glycosidases, the triazoles do not inhibit the β-glycosidases from sweet almonds, snail, and bovine liver, and only slightly inhibit the β-glucosidase from Caldocellum saccharolyticum. This is in keeping with the proposed direction of protonation in the plane of the saccharide ring and with modelling studies, docking 4 into the active site of the white clover cyanogenic β-glucosidase and 6 into the E. coli β-galactosidase and the Lactococcus lactis 6-phospho-β-galactosidase.

Synthesis of Fused Triazoles as Probes for the Active Site of Retaining β‐Glycosidases: From Which Direction Is the Glycoside Protonated?

The difference between the strong inhibition of retaining β-glucosidases by the tetrazole 1 and the weak inhibition by the triazole 3 has been explained by the protonation by the enzymic catalytic acid of N(3) of 1, replaced by CH in 3. One also expects a contribution to the inhibition from the charge-dipole interaction between the enzymic catalytic nucleophile and the azole ring. The extent of this contribution was estimated from the calculated, distance-dependent heats of formation of the acetate-azole complexes. The calculations were validated by comparison of the charge-dipole interaction between phosphate and the inhibitors 1 and 3 in the glycogen phosphorylase b (GPb)-azole-phosphate complexes, as derived from differences in the Ki values for 1 and 3, while the structural invariance of the complexes was demonstrated by X-ray analysis. The difference between the charge-dipole interactions of (dihydrogen) phosphate and 1 or 3 as derived from Δ Ki is 1.1 kcal mol−1, while the calculated difference is 1.3 kcal mol−1. The calculated difference for the interaction of 1 or 3 with acetate, representing the catalytic nucleophile in β-glycosidases, is 2.0 kcal mol−1, while the differences of the binding energies as derived from the Ki values for the inhibition by 1 or 3 of different β-glycosidases range from 2.4 to 5.3 5 kcal mol−1. The calculated difference for 1 and the imidazole 6 is 2.5 kcal mol−1 in favour of 1, whereas the Ki-derived difference is 3.7 kcal mol−1 in favour of 6, equal to the calculated difference between 1 and the protonated imidazole 6. Thus, protonation by the catalytic acid and the charge-dipole interaction with the catalytic nucleophile contribute cooperatively to the binding of inhibitors possessing a trigonal anomeric centre bonded to a heteroatom.

Cooperative Interactions of the Catalytic Nucleophile and the Catalytic Acid in the Inhibition of β-Glycosidases. Calculations and their validation by comparative kinetic and structural studies of the inhibition of glycogen phosphorylase b

Tom D. Heightman

Papers

Recent Insights into Inhibition, Structure, and Mechanism of Configuration-Retaining Glycosidases.

A Structural View of the Action of Escherichia Coli (Lacz) Beta-Galactosidase

Neue Erkenntnisse über Hemmung, Struktur und Mechanismus konfigurationserhaltender Glycosidasen

Synthesis of Fused Triazoles as Probes for the Active Site of Retaining β‐Glycosidases: From Which Direction Is the Glycoside Protonated?

Cooperative Interactions of the Catalytic Nucleophile and the Catalytic Acid in the Inhibition of β-Glycosidases. Calculations and their validation by comparative kinetic and structural studies of the inhibition of glycogen phosphorylase b