Xylan

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Structure of xylan in hardwood.[1]
Plant cell wall is composed of cellulose, hemicellulose, pectin and glycoproteins [2]. Hemicelluloses (a heterogeneous group of polysaccharides) cross-link glycans interlocking the cellulose fibers and form a mesh like structure to deposit other polysaccharides.

Xylan (CAS number: 9014-63-5) is a group of hemicelluloses that represents third most abundant biopolymer on Earth and found in secondary cell walls of dicots and all plant cell walls of grasses [3].

Composition[edit]

Xylans are polysaccharides made up of β-1,4-linked xylose (a pentose sugar) residues with side branches of α-arabinofuranose and α-glucuronic acids and contribute to cross-linking of cellulose microfibrils and lignin through ferulic acid residues[4]. On the basis of substituted groups xylan can be categorized into three classes i) glucuronoxylan (GX) ii) neutral arabinoxylan (AX) and iii) glucuronoarabinoxylan (GAX) [5].

Xylans play an important role in the integrity of the plant cell wall and increase cell wall recalcitrance to enzyme digestion [6] thus, help plants to defend against herbivores and pathogens (biotic stress). Xylan also plays a significant role in plant growth and development. Typically, xylans content in hardwoods is 10 - 35 % while they are 10 - 15 % in softwoods of the hemicelluloses. The main xylan component in hardwoods is O-acetyl-4-O-methylglucuronoxylan while arabino-4-O-methylglucuronoxylans are a major component in softwoods. In general, softwood xylans differ from hardwood xylans by the lack of acetyl groups and the presence of arabinose units linked by α-(1,3)-glycosidic bonds to the xylan backbone [7].

Biosynthesis[edit]

Studies on Arabidopsis mutants revealed that several Glycosyltransferase are involved in the biosynthesis of xylans[8][9][10]. Glycosyltransferases (GTs) catalyze the formation of glycosidic bonds between sugar molecules using nucleotide sugar as donor molecule [9]. In eukaryotes, GTs represent about 1% to 2% of gene products [11]. GTs are assembled into complexes existing in the Golgi apparatus. However, no xylan synthase complexes have been isolated from Arabidopsis tissues (dicot). The first gene involved in the biosynthesis of xylan was revealed on xylem mutants (irx) in Arabidopsis thaliana because of some mutation affecting xylan biosynthesis genes. As a result, abnormal plant growth due to thinning and weakening of secondary xylem cell walls was seen [10]. Arabidopsis mutant irx9 (At2g37090), irx14 (At4g36890), irx10/gut2 (At1g27440), irx10-L/gut1 (At5g61840) showed defect in xylan backbone biosynthesis [12]. Arabidopsis mutants irx7, irx8, and parvus are thought to be related to the reducing end oligosaccharide biosynthesis [13]. Thus, many genes have been associated with xylan biosynthesis but their biochemical mechanism is still unknown. Zeng et al. (2010) immuno-purified Xylan synthase activity from etiolated wheat (Triticum aestivum) microsomes [14]. Jiang et al. (2016) reported a xylan synthase complex (XSC) from wheat that has a central core formed of two members of the GT43 and GT47 families (CAZy database). They purified Xylan synthase activity from wheat seedlings through proteomics analysis and showed that two members of TaGT43 and TaGT47 are sufficient for the synthesis of a xylan-like polymer in vitro [15].

Applications[edit]

Xylan is used in different ways as part of our daily lives. For example, the quality of cereal flours and the hardness of dough are largely affected by the amount of xylan [5] thus, playing a significant role in bread industry. The main constituent of xylan can be converted into xylitol (a xylose derivative) which is used as a natural food sweetener, which helps to reduce dental cavities and acts as a sugar substitute for diabetic patients. It has many more applications in the livestock industry, because poultry feed has a high percentage of xylan [5]. Some macrophytic green algae contain xylan (specifically homoxylan[16]) especially those within the Codium and Bryopsis genera[17] where it replaces cellulose in the cell wall matrix. Similarly, it replaces the inner fibrillar cell-wall layer of cellulose in some red algae.

Xylan is one of the foremost anti-nutritional factors in common use feedstuff raw materials. Xylooligosaccharides produced from xylan are considered as "functional food" or dietary fibers [18] due their potential prebiotic properties [19]. Xylan can be converted in xylooligosaccharides by chemical hydrolysis using acids [20] or by enzymatic hydrolysis using endo-xylanases [21]. Some enzymes from yeast can exclusively converts xylan into only xylooligosaccharides-DP-3 to 7 [22].

Xylan is a major components of plant secondary cell walls which is a major source of renewable energy especially for second generation biofuels[23]. However, xylose (backbone of xylan) is a pentose sugar that is hard to ferment during biofuel conversion because microorganisms like yeast cannot ferment pentose naturally[24].

References[edit]

  1. ^ Horst H. Nimz, Uwe Schmitt, Eckart Schwab, Otto Wittmann, Franz Wolf "Wood" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a28_305
  2. ^ Carpita, Nicholas C. (2011-01-01). "Update on Mechanisms of Plant Cell Wall Biosynthesis: How Plants Make Cellulose and Other (1→4)-β-d-Glycans". Plant Physiology. 155 (1): 171–184. doi:10.1104/pp.110.163360. ISSN 0032-0889. PMID 21051553.
  3. ^ Mellerowicz, E. J.; Gorshkova, T. A. (2011-11-16). "Tensional stress generation in gelatinous fibres: a review and possible mechanism based on cell-wall structure and composition". Journal of Experimental Botany. 63 (2): 551–565. doi:10.1093/jxb/err339. ISSN 0022-0957.
  4. ^ Balakshin, Mikhail; Capanema, Ewellyn; Gracz, Hanna; Chang, Hou-min; Jameel, Hasan (2011-02-05). "Quantification of lignin–carbohydrate linkages with high-resolution NMR spectroscopy". Planta. 233 (6): 1097–1110. doi:10.1007/s00425-011-1359-2. ISSN 0032-0935.
  5. ^ a b c Faik, Ahmed (2010-06-01). "Xylan Biosynthesis: News from the Grass". Plant Physiology. 153 (2): 396–402. doi:10.1104/pp.110.154237. ISSN 0032-0889. PMID 20375115.
  6. ^ Faik, Ahmed (2013), ""Plant Cell Wall Structure-Pretreatment" the Critical Relationship in Biomass Conversion to Fermentable Sugars", SpringerBriefs in Molecular Science, Springer Netherlands, pp. 1–30, doi:10.1007/978-94-007-6052-3_1, ISBN 9789400760516, retrieved 2018-11-11
  7. ^ Sixta, Herbert, ed. (2006). Handbook of pulp. 1. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA. pp. 28–30. ISBN 978-3-527-30999-3.
  8. ^ Brown, David M.; Zhang, Zhinong; Stephens, Elaine; Dupree, Paul; Turner, Simon R. (2009-01-29). "Characterization of IRX10 and IRX10-like reveals an essential role in glucuronoxylan biosynthesis in Arabidopsis". The Plant Journal. 57 (4): 732–746. doi:10.1111/j.1365-313x.2008.03729.x. ISSN 0960-7412.
  9. ^ a b "Plant glycosyltransferases". Current Opinion in Plant Biology. 4 (3): 219–224. 2001-06-01. doi:10.1016/S1369-5266(00)00164-3. ISSN 1369-5266.
  10. ^ a b Wu, Ai-Min; Hörnblad, Emma; Voxeur, Aline; Gerber, Lorenz; Rihouey, Christophe; Lerouge, Patrice; Marchant, Alan (2010-06-01). "Analysis of the Arabidopsis IRX9/IRX9-L and IRX14/IRX14-L Pairs of Glycosyltransferase Genes Reveals Critical Contributions to Biosynthesis of the Hemicellulose Glucuronoxylan". Plant Physiology. 153 (2): 542–554. doi:10.1104/pp.110.154971. ISSN 0032-0889. PMID 20424005.
  11. ^ Lairson, L.L.; Henrissat, B.; Davies, G.J.; Withers, S.G. (2008-06-02). "Glycosyltransferases: Structures, Functions, and Mechanisms". Annual Review of Biochemistry. 77 (1): 521–555. doi:10.1146/annurev.biochem.76.061005.092322. ISSN 0066-4154.
  12. ^ Wu, Ai-Min; Hörnblad, Emma; Voxeur, Aline; Gerber, Lorenz; Rihouey, Christophe; Lerouge, Patrice; Marchant, Alan (2010-06-01). "Analysis of the Arabidopsis IRX9/IRX9-L and IRX14/IRX14-L Pairs of Glycosyltransferase Genes Reveals Critical Contributions to Biosynthesis of the Hemicellulose Glucuronoxylan". Plant Physiology. 153 (2): 542–554. doi:10.1104/pp.110.154971. ISSN 0032-0889. PMID 20424005.
  13. ^ Peña, Maria J.; Zhong, Ruiqin; Zhou, Gong-Ke; Richardson, Elizabeth A.; O'Neill, Malcolm A.; Darvill, Alan G.; York, William S.; Ye, Zheng-Hua (2007-02-01). "Arabidopsis irregular xylem8 and irregular xylem9: Implications for the Complexity of Glucuronoxylan Biosynthesis". The Plant Cell. 19 (2): 549–563. doi:10.1105/tpc.106.049320. ISSN 1040-4651. PMID 17322407.
  14. ^ Zeng, Wei; Chatterjee, Mohor; Faik, Ahmed (2008-05-01). "UDP-Xylose-Stimulated Glucuronyltransferase Activity in Wheat Microsomal Membranes: Characterization and Role in Glucurono(arabino)xylan Biosynthesis". Plant Physiology. 147 (1): 78–91. doi:10.1104/pp.107.115576. ISSN 0032-0889. PMID 18359844.
  15. ^ Jiang, Nan; Wiemels, Richard E.; Soya, Aaron; Whitley, Rebekah; Held, Michael; Faik, Ahmed (2016-04-01). "Composition, Assembly, and Trafficking of a Wheat Xylan Synthase Complex". Plant Physiology. 170 (4): 1999–2023. doi:10.1104/pp.15.01777. ISSN 0032-0889. PMID 26917684.
  16. ^ Ebringerová, Anna; Hromádková, Zdenka; Heinze, Thomas (2005-01-01). Heinze, Thomas, ed. Hemicellulose. Advances in Polymer Science. Springer Berlin Heidelberg. pp. 1–67. ISBN 9783540261124.
  17. ^ "Xylan Glycoproducts for life sciences - Engineering and production". www.elicityl-oligotech.com. Retrieved 2016-04-20.
  18. ^ Alonso JL, Dominguez H, Garrote G, Parajo JC, Vazques MJ (2003). "Xylooligosaccharides: properties and production technologies". Electron. J. Environ. Agric. Food Chem. 2 (1): 230–232.
  19. ^ Broekaert, W.F.; Courtin, C.M.; Verbeke, C.; Van de Wiele, T.; Verstraete, W.; Delcour, J.A (2011). "Prebiotic and Other Health-Related Effects of Cereal-Derived Arabinoxylans, Arabinoxylan-Oligosaccharides, and Xylooligosaccharides". Critical Reviews in Food Science and Nutrition. 51: 178–194. doi:10.1080/10408390903044768.
  20. ^ Akpinar, O; Erdogan, K; Bostanci, S. "Production of xylooligosaccharides by controlled acid hydrolysis of lignocellulosic materials". Carbohydrate Research. 344 (5): 660–666. doi:10.1016/j.carres.2009.01.015.
  21. ^ Linares-Pastén, J.A.; Aronsson, A.; Nordberg Karlsson, E. (2017). "Structural Considerations on the Use of Endo-Xylanases for the Production of prebiotic Xylooligosaccharides from Biomass". Current Protein & Peptide Science. 18: 1–20. doi:10.2174/1389203717666160923155209. ISSN 1875-5550.
  22. ^ Adsul, MG; Bastawde, KG; Gokhale, GV (2009). "Biochemical characterization of two xylanases from yeast Pseudozyma hubeiensis producing only xylooligosaccharides". Bioresource Technology. 100 (24): 6488–6495. doi:10.1016/j.biortech.2009.07.064.
  23. ^ "Cell wall biomechanics: a tractable challenge in manipulating plant cell walls 'fit for purpose'!". Current Opinion in Biotechnology. 49: 163–171. 2018-02-01. doi:10.1016/j.copbio.2017.08.013. ISSN 0958-1669.
  24. ^ "Xylan biosynthesis". Current Opinion in Biotechnology. 26: 100–107. 2014-04-01. doi:10.1016/j.copbio.2013.11.013. ISSN 0958-1669.