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 (/ˈzlən/[3]) (CAS number: 9014-63-5) is a group of hemicelluloses that represents the third most abundant biopolymer on Earth. It is found in plants, in the secondary cell walls of dicots and all cell walls of grasses.[4]

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.[5] On the basis of substituted groups xylan can be categorized into three classes i) glucuronoxylan (GX) ii) neutral arabinoxylan (AX) and iii) glucuronoarabinoxylan (GAX).[6]

Biosynthesis[edit]

Studies on Arabidopsis mutants revealed that several Glycosyltransferases are involved in the biosynthesis of xylans.[7][8][9] Glycosyltransferases (GTs) catalyze the formation of glycosidic bonds between sugar molecules using nucleotide sugar as donor molecule.[8] In eukaryotes, GTs represent about 1% to 2% of gene products.[10] 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.[9] Arabidopsis mutant irx9 (At2g37090), irx14 (At4g36890), irx10/gut2 (At1g27440), irx10-L/gut1 (At5g61840) showed defect in xylan backbone biosynthesis.[11] Arabidopsis mutants irx7, irx8, and parvus are thought to be related to the reducing end oligosaccharide biosynthesis.[12] 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.[13] 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.[14]

Catabolism[edit]

Xylanase catalyzes the catabolism of xylan into xylose. Given that plants contain a lot of xylan, xylanase is thus important to the nutrient cycle.

Role in plant cell structure[edit]

Xylans play an important role in the integrity of the plant cell wall and increase cell wall recalcitrance to enzymatic digestion;[15] thus, they 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%, whereas they are 10-15% in softwoods. The main xylan component in hardwoods is O-acetyl-4-O-methylglucuronoxylan, whereas 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.[16]

The microanatomy, molecular physiology, and physical chemistry of the interactions between the three main structural biopolymers xylan, cellulose, and lignin in providing the rigidity of plant cell walls are topics of current research,[17][18] that may provide solutions in bioengineering, for example in biofuels manufacturing from maize, rice, and switchgrass.[18]

Commercial 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[6] 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.[6] Some macrophytic green algae contain xylan (specifically homoxylan[19]) especially those within the Codium and Bryopsis genera[20] 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[21] due their potential prebiotic properties.[22] Xylan can be converted in xylooligosaccharides by chemical hydrolysis using acids[23] or by enzymatic hydrolysis using endo-xylanases.[24] Some enzymes from yeast can exclusively converts xylan into only xylooligosaccharides-DP-3 to 7.[25]

Xylan is a major components of plant secondary cell walls which is a major source of renewable energy especially for second generation biofuels.[26] 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.[27]

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. PMC 3075763. PMID 21051553.
  3. ^ Houghton Mifflin Harcourt, The American Heritage Dictionary of the English Language, Houghton Mifflin Harcourt.
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  5. ^ 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. PMID 21298285.
  6. ^ 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. PMC 2879768. PMID 20375115.
  7. ^ 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. PMID 18980662.
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  12. ^ 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. PMC 1867335. PMID 17322407.
  13. ^ 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. PMC 2330321. PMID 18359844.
  14. ^ 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. PMC 4825154. PMID 26917684.
  15. ^ 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
  16. ^ 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.
  17. ^ Simmons, TJ; Mortimer, JC; Bernardinelli, OD; Pöppler, AC; et al. (2016), "Folding of xylan onto cellulose fibrils in plant cell walls revealed by solid-state NMR", Nature Communications, 7: 13902, doi:10.1038/ncomms13902, PMC 5187587, PMID 28000667.
  18. ^ a b Kang, X; Kirui, A; Dickwella Widanage, MC; Mentink-Vigier, F; et al. (2019), "Lignin-polysaccharide interactions in plant secondary cell walls revealed by solid-state NMR", Nature Communications, 10 (1): 347, doi:10.1038/s41467-018-08252-0, PMC 6341099, PMID 30664653.
  19. ^ Ebringerová, Anna; Hromádková, Zdenka; Heinze, Thomas (2005-01-01). Heinze, Thomas (ed.). Hemicellulose. Advances in Polymer Science. Springer Berlin Heidelberg. pp. 1–67. doi:10.1007/b136816. ISBN 9783540261124.
  20. ^ "Xylan Glycoproducts for life sciences - Engineering and production". www.elicityl-oligotech.com. Retrieved 2016-04-20.
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  23. ^ Akpinar, O; Erdogan, K; Bostanci, S (2009). "Production of xylooligosaccharides by controlled acid hydrolysis of lignocellulosic materials". Carbohydrate Research. 344 (5): 660–666. doi:10.1016/j.carres.2009.01.015.
  24. ^ 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): 48–67. doi:10.2174/1389203717666160923155209. ISSN 1875-5550. PMC 5738707. PMID 27670134.
  25. ^ 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. PMID 19692229.
  26. ^ Johnson, Kim L.; Gidley, Michael J.; Bacic, Antony; Doblin, Monika S. (2018-02-01). "Cell wall biomechanics: a tractable challenge in manipulating plant cell walls 'fit for purpose'!". Current Opinion in Biotechnology. 49: 163–171. doi:10.1016/j.copbio.2017.08.013. ISSN 0958-1669. PMID 28915438.
  27. ^ Rennie, Emilie A.; Scheller, Henrik Vibe (2014-04-01). "Xylan biosynthesis". Current Opinion in Biotechnology. 26: 100–107. doi:10.1016/j.copbio.2013.11.013. ISSN 0958-1669. PMID 24679265.