Hydrolysis of wheat flour arabinoxylan, acid-debranched wheat flour arabinoxylan and arabino-xylo-oligosaccharides by β-xylanase, α-L-arabinofuranosidase and β-xylosidase.
McCleary, B. V., McKie, V. A., Draga, A., Rooney, E., Mangan, D. & Larkin, J. (2015). Carbohydrate Research, 407, 79-96.
A range of α-L-arabinofuranosyl-(1-4)-β-D-xylo-oligosaccharides (AXOS) were produced by hydrolysis of wheat flour arabinoxylan (WAX) and acid debranched arabinoxylan (ADWAX), in the presence and absence of an AXH-d3 α-L-arabinofuranosidase, by several GH10 and GH11 β-xylanases. The structures of the oligosaccharides were characterised by GC-MS and NMR and by hydrolysis by a range of α-L-arabinofuranosidases and β-xylosidase. The AXOS were purified and used to characterise the action patterns of the specific α-L-arabinofuranosidases. These enzymes, in combination with either Cellvibrio mixtus or Neocallimastix patriciarum β -xylanase, were used to produce elevated levels of specific AXOS on hydrolysis of WAX, such as 32-α-L-Araf-(1-4)-β-D-xylobiose (A3X), 23-α-L-Araf-(1-4)-β-D-xylotriose (A2XX), 33-α-L-Araf-(1-4)-β-D-xylotriose (A3XX), 22-α-L-Araf-(1-4)-β-D-xylotriose (XA2X), 32-α-L-Araf (1-4)-β-D-xylotriose (XA3X), 23-α-L-Araf-(1-4)-β-D-xylotetraose (XA2XX), 33-α-L-Araf-(1-4)-β-D-xylotetraose (XA3XX), 23 ,33-di-α-L-Araf-(1-4)-β-D-xylotriose (A2+3XX), 23,33-di-α-L-Araf-(1-4)-β-D-xylotetraose (XA2+3XX), 24,34-di-α-L-Araf-(1-4)-β-D-xylopentaose (XA2+3XXX) and 33,34-di-α-L-Araf-(1-4)-β-D-xylopentaose (XA3A3XX), many of which have not previously been produced in sufficient quantities to allow their use as substrates in further enzymic studies. For A2,3XX, yields of approximately 16% of the starting material (wheat arabinoxylan) have been achieved. Mixtures of the α-L-arabinofuranosidases, with specific action on AXOS, have been combined with β-xylosidase and β-xylanase to obtain an optimal mixture for hydrolysis of arabinoxylan to L-arabinose and D-xylose.
A Comparison of Polysaccharide Substrates and Reducing Sugar Methods for the Measurement of endo-1,4-β-Xylanase
McCleary, B. V. & McGeough, P. (2015). Appl. Biochem. Biotechnol., 177(5), 1152-1163.
The most commonly used method for the measurement of the level of endo-xylanase in commercial enzyme preparations is the 3,5-dinitrosalicylic acid (DNS) reducing sugar method with birchwood xylan as substrate. It is well known that with the DNS method, much higher enzyme activity values are obtained than with the Nelson-Somogyi (NS) reducing sugar method. In this paper, we have compared the DNS and NS reducing sugar assays using a range of xylan-type substrates and accurately compared the molar response factors for xylose and a range of xylo-oligosaccharides. Purified beechwood xylan or wheat arabinoxylan is shown to be a suitable replacement for birchwood xylan which is no longer commercially available, and it is clearly demonstrated that the DNS method grossly overestimates endo-xylanase activity. Unlike the DNS assay, the NS assay gave the equivalent colour response with equimolar amounts of xylose, xylobiose, xylotriose and xylotetraose demonstrating that it accurately measures the quantity of glycosidic bonds cleaved by the endo-xylanase. The authors strongly recommend cessation of the use of the DNS assay for measurement of endo-xylanase due to the fact that the values obtained are grossly overestimated due to secondary reactions in colour development.
Evaluation of the xylan breakdown potential of eight mesophilic endoxylanases.
Cuyvers, S., Dornez, E., Moers, K., Pollet, A., Delcour, J. A. & Courtin, C. M. (2011). Enzyme and Microbial Technology, 49(3), 305-311.
In biomass degradation using simultaneous saccharification and fermentation (SSF), there is a need for efficient biomass degrading enzymes that can work at lower temperatures suitable for yeast fermentation. As xylan is an important lignocellulosic biomass constituent, this study aimed at investigating the possible differences in xylan breakdown potential of endoxylanases using eight different endoxylanases at conditions relevant for SSF. Both solubilising and degrading capacities of the endoxylanases were investigated using water-insoluble and water-soluble oat spelt xylan as model substrates for biomass xylan. Results showed that selecting for combinations of endoxylanases that are efficient at solubilising xylan on the one hand and degrading it to large extent on the other hand, coupled to high specific activities, seems the best option for complete xylan breakdown in lignocellulosic biomass conversion using SSF.
The use of Xylanases from different microbial origin in bread baking and their effects on bread qualities.
Al-Widyan, O., Khataibeh, M. H. & Abu-Alruz, K. (2008). Journal of Applied Sciences, 8(4), 672-676.
Effects of xylanases on bread quality were examined. Enzymes used were endo-xylanase (EC 126.96.36.199) from different sources of microorganisms. Baked loaves were assessed for Loaves volume, colour and staling rate. Xylanases produced from rumen microorganisms M6 had clearly positive effects on loaf volume of bread as well as anti-firming potential. M3 (produced from Trichoderma longibrachiatum) improved crumb softness. The use of xylanase for breadmaking lowered firmness of bread crumb effectively compared with control loaf. It can be summarized that xylanases had significant positive effects on bread characteristics. In particular, they had advantage in retarding the staling rate of bread. It is recommended that the optimum dosage of enzymes, method of application in industrial scale especially with xylanase should be studied further in order to gain the great advantages of enzyme addition in breadmaking.
His374 of wheat endoxylanase inhibitor TAXI‐I stabilizes complex formation with glycoside hydrolase family 11 endoxylanases.
Fierens, K., Gils, A., Sansen, S., Brijs, K., Courtin, C. M., Declerck, P. J., De Ranter, C. J., Gebruers, K., Rabijns, A., Robben, J., Van Campenhout, S., Volckaert, G. & Delcour, J. A. (2005). FEBS Journal, 272(22), 5872-5882.
Wheat endoxylanase inhibitor TAXI-I inhibits microbial glycoside hydrolase family 11 endoxylanases. Crystallographic data of an Aspergillus niger endoxylanase-TAXI-I complex showed His374 of TAXI-I to be a key residue in endoxylanase inhibition [Sansen S, De Ranter CJ, Gebruers K, Brijs K, Courtin CM, Delcour JA & Rabijns A (2004) J Biol Chem 279, 36022–36028]. Its role in enzyme–inhibitor interaction was further investigated by site-directed mutagenesis of His374 into alanine, glutamine or lysine. Binding kinetics and affinities of the molecular interactions between A. niger, Bacillus subtilis, Trichoderma longibrachiatum endoxylanases and wild-type TAXI-I and TAXI-I His374 mutants were determined by surface plasmon resonance analysis. Enzyme–inhibitor binding was in accordance with a simple 1 : 1 binding model. Association and dissociation rate constants of wild-type TAXI-I towards the endoxylanases were in the range between 1.96 and 36.1 × 104m-1·s-1 and 0.72–3.60 × 10-4·s-1, respectively, resulting in equilibrium dissociation constants in the low nanomolar range. Mutation of TAXI-I His374 to a variable degree reduced the inhibition capacity of the inhibitor mainly due to higher complex dissociation rate constants (three- to 80-fold increase). The association rate constants were affected to a smaller extent (up to eightfold decrease). Substitution of TAXI-I His374 therefore strongly affects the affinity of the inhibitor for the enzymes. In addition, the results show that His374 plays a critical role in the stabilization of the endoxylanase–TAXI-I complex rather than in the docking of inhibitor onto enzyme.
Matrix solubilization and cell wall weakening by β-expansin (group‐1 allergen) from maize pollen.
Tabuchi, A., Li, L. C. & Cosgrove, D. J. (2011). The Plant Journal, 68(3), 546-559.
Beta-expansins accumulate to high levels in grass pollen, a feature apparently unique to grasses. These proteins, which are major human allergens, facilitate pollen tube penetration of the maize stigma and style (the silk). Here we report that treatment of maize silk cell walls with purified β-expansin from maize pollen led to solubilization of wall matrix polysaccharides, dominated by feruloyated highly substituted glucuronoarabinoxylan (60%) and homogalacturonan (35%). Such action was selective for cell walls of grasses, and indicated a target preferentially found in grass cell walls, probably the highly substituted glucuronoarabinoxylan. Several tests for lytic activities by β-expansin were negative and polysaccharide solubilization had weak temperature dependence, which indicated a non-enzymatic process. Concomitant with matrix solubilization, β-expansin treatment induced creep, reduced the breaking force and increased the plastic compliance of wall specimens. From comparisons of the pH dependencies of these processes, we conclude that matrix solubilization was linked closely to changes in wall plasticity and breaking force, but not so closely coupled to cell wall creep. Because matrix solubilization and increased wall plasticity have not been found with other expansins, we infer that these novel activities are linked to the specialized role of grass pollen β-expansins in promotion of penetration of the pollen tube through the stigma and style, most likely by weakening the middle lamella.
Changes in cell wall biomechanical properties in the xyloglucan-deficient xxt1/xxt2 mutant of Arabidopsis.
Park, Y. B. & Cosgrove, D. J. (2012). Plant Physiology, 158(1), 465-475.
The main load-bearing network in the primary cell wall of most land plants is commonly depicted as a scaffold of cellulose microfibrils tethered by xyloglucans. However, a xyloglucan-deficient mutant (xylosyltransferase1/xylosyltransferase2 [xxt1/xxt2]) was recently developed that was smaller than the wild type but otherwise nearly normal in its development, casting doubt on xyloglucan’s role in wall structure. To assess xyloglucan function in the Arabidopsis (Arabidopsis thaliana) wall, we compared the behavior of petiole cell walls from xxt1/xxt2 and wild-type plants using creep, stress relaxation, and stress/strain assays, in combination with reagents that cut or solubilize specific components of the wall matrix. Stress/strain assays showed xxt1/xxt2 walls to be more extensible than wild-type walls (supporting a reinforcing role for xyloglucan) but less extensible in creep and stress relaxation processes mediated by α-expansin. Fusicoccin-induced “acid growth” was likewise reduced in xxt1/xxt2 petioles. The results show that xyloglucan is important for wall loosening by α-expansin, and the smaller size of the xxt1/xxt2 mutant may stem from the reduced effectiveness of α-expansins in the absence of xyloglucan. Loosening agents that act on xylans and pectins elicited greater extension in creep assays of xxt1/xxt2 cell walls compared with wild-type walls, consistent with a larger mechanical role for these matrix polymers in the absence of xyloglucan. Our results illustrate the need for multiple biomechanical assays to evaluate wall properties and indicate that the common depiction of a cellulose-xyloglucan network as the major load-bearing structure is in need of revision.
New glycosidase substrates for droplet-based microfluidic screening.
Najah, M., Mayot, E., Mahendra-Wijaya, I. P., Griffiths, A. D., Ladame, S. & Drevelle, A. (2013). Analytical Chemistry, 85(20), 9807-9814.
Droplet-based microfluidics is a powerful technique allowing ultra-high-throughput screening of large libraries of enzymes or microorganisms for the selection of the most efficient variants. Most applications in droplet microfluidic screening systems use fluorogenic substrates to measure enzymatic activities with fluorescence readout. It is important, however, that there is little or no fluorophore exchange between droplets, a condition not met with most commonly employed substrates. Here we report the synthesis of fluorogenic substrates for glycosidases based on a sulfonated 7-hydroxycoumarin scaffold. We found that the presence of the sulfonate group effectively prevents leakage of the coumarin from droplets, no exchange of the sulfonated coumarins being detected over 24 h at 30°C. The fluorescence properties of these substrates were characterized over a wide pH range, and their specificity was studied on a panel of relevant glycosidases (cellulases and xylanases) in microtiter plates. Finally, the β-D-cellobioside-6,8-difluoro-7-hydroxycoumarin-4-methanesulfonate substrate was used to assay cellobiohydrolase activity on model bacterial strains (Escherichia coli and Bacillus subtilis) in a droplet-based microfluidic format. These new substrates can be used to assay glycosidase activities in a wide pH range (4–11) and with incubation times of up to 24 h in droplet-based microfluidic systems.