Versatile high resolution oligosaccharide microarrays for plant glycobiology and cell wall research.
Pedersen, H. L., Fangel, J. U., McCleary, B., Ruzanski, C., Rydahl, M. G., Ralet, M. C., Farkas, V., Von Schantz, L., Marcus, S. E., Andersen, M.C. F., Field, R., Ohlin, M., Knox, J. P., Clausen, M. H. & Willats, W. G. T. (2012). Journal of Biological Chemistry, 287(47), 39429-39438.
Microarrays are powerful tools for high throughput analysis, and hundreds or thousands of molecular interactions can be assessed simultaneously using very small amounts of analytes. Nucleotide microarrays are well established in plant research, but carbohydrate microarrays are much less established, and one reason for this is a lack of suitable glycans with which to populate arrays. Polysaccharide microarrays are relatively easy to produce because of the ease of immobilizing large polymers noncovalently onto a variety of microarray surfaces, but they lack analytical resolution because polysaccharides often contain multiple distinct carbohydrate substructures. Microarrays of defined oligosaccharides potentially overcome this problem but are harder to produce because oligosaccharides usually require coupling prior to immobilization. We have assembled a library of well characterized plant oligosaccharides produced either by partial hydrolysis from polysaccharides or by de novo chemical synthesis. Once coupled to protein, these neoglycoconjugates are versatile reagents that can be printed as microarrays onto a variety of slide types and membranes. We show that these microarrays are suitable for the high throughput characterization of the recognition capabilities of monoclonal antibodies, carbohydrate-binding modules, and other oligosaccharide-binding proteins of biological significance and also that they have potential for the characterization of carbohydrate-active enzymes.
Structural basis for entropy-driven cellulose binding by a type-A cellulose-binding module (CBM) and bacterial expansin.
Georgelis, N., Yennawar, N. H. & Cosgrove, D. J. (2012). Proceedings of the National Academy of Sciences, 109(37), 14830-14835.
Components of modular cellulases, type-A cellulose-binding modules (CBMs) bind to crystalline cellulose and enhance enzyme effectiveness, but structural details of the interaction are uncertain. We analyzed cellulose binding by EXLX1, a bacterial expansin with ability to loosen plant cell walls and whose domain D2 has type-A CBM characteristics. EXLX1 strongly binds to crystalline cellulose via D2, whereas its affinity for soluble cellooligosaccharides is weak. Calorimetry indicated cellulose binding was largely entropically driven. We solved the crystal structures of EXLX1 complexed with cellulose-like oligosaccharides to find that EXLX1 binds the ligands through hydrophobic interactions of three linearly arranged aromatic residues in D2. The crystal structures revealed a unique form of ligand-mediated dimerization, with the oligosaccharide sandwiched between two D2 domains in opposite polarity. This report clarifies the molecular target of expansin and the specific molecular interactions of a type-A CBM with cellulose.
Structural and enzymatic characterization of a glycoside hydrolase family 31 α-xylosidase from Cellvibrio japonicus involved in xyloglucan saccharification.
Johan, L., Atsushi, I., Farid, M. I., Azadeh, N., Harry, J. G., Gideon, J. D. & Harry, B. (2011). Biochemical Journal, 436(3), 567-580.
The desire for improved methods of biomass conversion into fuels and feedstocks has re-awakened interest in the enzymology of plant cell wall degradation. The complex polysaccharide xyloglucan is abundant in plant matter, where it may account for up to 20% of the total primary cell wall carbohydrates. Despite this, few studies have focused on xyloglucan saccharification, which requires a consortium of enzymes including endo-xyloglucanases, α-xylosidases, β-galactosidases and α-L-fucosidases, among others. In the present paper, we show the characterization of Xyl31A, a key α-xylosidase in xyloglucan utilization by the model Gram-negative soil saprophyte Cellvibrio japonicus. CjXyl31A exhibits high regiospecificity for the hydrolysis of XGOs (xylogluco-oligosaccharides), with a particular preference for longer substrates. Crystallographic structures of both the apo enzyme and the trapped covalent 5-fluoro-β-xylosyl-enzyme intermediate, together with docking studies with the XXXG heptasaccharide, revealed, for the first time in GH31 (glycoside hydrolase family 31), the importance of a PA14 domain insert in the recognition of longer oligosaccharides by extension of the active-site pocket. The observation that CjXyl31A was localized to the outer membrane provided support for a biological model of xyloglucan utilization by C. japonicus, in which XGOs generated by the action of a secreted endo-xyloglucanase are ultimately degraded in close proximity to the cell surface. Moreover, the present study diversifies the toolbox of glycosidases for the specific modification and saccharification of cell wall polymers for biotechnological applications.
An investigation of the substrate specificity of the xyloglucanase Cel74A from Hypocrea jecorina.
Desmet, T., Cantaert, T., Gualfetti, P., Nerinckx, W., Gross, L., Mitchinson, C. & Piens, K. (2007). FEBS Journal, 274(2), 356-363.
The substrate specificity of the xyloglucanase Cel74A from Hypocrea jecorina
(Trichoderma reesei) was examined using several polysaccharides and oligosaccharides. Our results revealed that xyloglucan chains are hydrolyzed at substituted Glc residues, in contrast to the action of all known xyloglucan endoglucanases (EC 18.104.22.168). The building block of xyloglucan, XXXG (where X is a substituted Glc residue, and G is an unsubstituted Glc residue), was rapidly degraded to XX and XG (Kcat = 7.2 s-1 and Km = 120 µm at 37°C and pH 5), which has only been observed before with the oligoxyloglucan-reducing-end-specific cellobiohydrolase from Geotrichum (EC 22.214.171.124). However, the cellobiohydrolase can only release XG from XXXGXXXG, whereas Cel74A hydrolyzed this substrate at both chain ends, resulting in XGXX. Differences in the length of a specific loop at subsite + 2 are discussed as being the basis for the divergent specificity of these xyloglucanases.