Xylan (; ) ( CAS number: 9014-63-5) is a type of

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls. Cellulose is crystalline, strong, an ...

, a

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

consisting mainly of

xylose Xylose ( , , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is deriv ...

residues. It is found in

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

s, in the secondary

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

s of

dicot The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, ...

s and all cell walls of

grasses Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in ...

. Xylan is the third most abundant

on Earth, after

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

and

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

Composition

Xylans are

s made up of β-1,4-linked

pentose In chemistry, a pentose is a monosaccharide (simple sugar) with five carbon atoms. The chemical formula of many pentoses is , and their molecular weight is 150.13 g/mol.sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

) residues with side branches of α-arabinofuranose and/or α-glucuronic acids. On the basis of substituted groups xylan can be categorized into three classes i) glucuronoxylan (GX) ii) neutral

arabinoxylan Arabinoxylan is a form of the hemicellulose xylan found in both the primary and secondary cell walls of plants which in addition to xylose contains substantial amounts of another pentose sugar, arabinose.Scheller, H. V., & Ulvskov, P. (2010). Hem ...

(AX) and iii) glucuronoarabinoxylan (GAX). In some cases contribute to cross-linking of cellulose microfibrils and lignin through ferulic acid residues.

Occurrence

Plant cell structure

Xylans play an important role in the integrity of the plant

and increase cell wall recalcitrance to enzymatic digestion; 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

hardwood Hardwood is wood from Flowering plant, angiosperm trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal ecosystem, boreal latitudes they are mostly deciduous, but in tropics and subtropics mostl ...

s is 10-35%, whereas they are 10-15% in

softwood Scots pine, a typical and well-known softwood Softwood is wood from gymnosperm trees such as conifers. The term is opposed to hardwood, which is the wood from angiosperm trees. The main differences between hardwoods and softwoods is that the sof ...

s. 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 In organic chemistry, an acetyl group is a functional group denoted by the chemical formula and the structure . It is sometimes represented by the symbol Ac (not to be confused with the element actinium). In IUPAC nomenclature, an acetyl grou ...

groups and the presence of

arabinose Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. Properties For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, o ...

units linked by α-(1,3)-glycosidic bonds to the xylan backbone.

Algae

Some macrophytic

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

contain xylan (specifically homoxylan) especially those within the ''

Codium ''Codium'' is a genus of edible green macroalgae (or seaweed) under the order Bryopsidales. The genus name is derived from a Greek word that pertains to the soft texture of its thallus. One of the foremost experts on ''Codium'' taxonomy was P ...

'' and '' Bryopsis'' genera where it replaces cellulose in the

matrix. Similarly, it replaces the inner fibrillar cell-wall layer of cellulose in some

red algae Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...

Food science

The quality of cereal flours and the hardness of dough are affected by their xylan content, 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. Poultry feed has a high percentage of xylan. Xylan is one of the foremost anti-nutritional factors in common use feedstuff raw materials.

Xylooligosaccharide Xylooligosaccharides (XOS) are polymers of the sugar xylose. They are produced from the xylan fraction in plant fiber. Their C5 (where C is a quantity of carbon atoms in each monomer) structure is fundamentally different from other prebiotics, whi ...

s produced from xylan are considered as "functional food" or dietary fibers due their potential prebiotic properties.

Crystallinity

The regular branching patterns of xylans may facilitate their co-crystallization with cellulose in the plant cell wall. Xylan also tends to crystallize from aqueous solution. Additional polymorphs of (1→4)-β-D-xylan have been obtained by crystallization from non-aqueous environments.

Biosynthesis

Several

glycosyltransferase Glycosyltransferases (GTFs, Gtfs) are enzymes ( EC 2.4) that establish natural glycosidic linkages. They catalyze the transfer of saccharide moieties from an activated nucleotide sugar (also known as the "glycosyl donor") to a nucleophilic gl ...

s are involved in the biosynthesis of xylans. In eukaryotes, GTs represent about 1% to 2% of gene products. 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 ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...

'' 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. ''Arabidopsis'' mutant ''irx9'' (At2g37090), ''irx14'' (At4g36890), ''irx10/gut2'' (At1g27440), ''irx10-L/gut1'' (At5g61840) showed defect in xylan backbone biosynthesis. ''Arabidopsis'' mutants ''irx7'', ''irx8'', and ''parvus'' are thought to be related to the reducing end oligosaccharide biosynthesis. 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. 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.''

Breakdown

Xylanase Endo-1,4-β-xylanase (, systematic name 4-β-D-xylan xylanohydrolase) is any of a class of enzymes that degrade the linear polysaccharide xylan into xylose, thus breaking down hemicellulose, one of the major components of plant cell walls: : En ...

converts xylan into

. Given that plants contain up to 30% xylan, xylanase is important to the

nutrient cycle A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...

. The degradation of xylan and other hemicelluloses is relevant to the production of

biofuel Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...

s. Being less crystalline and more highly branched, these hemicelluloses are particularly susceptible to

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

Research

As a major component of plants, xylan is potentially a significant source of renewable energy especially for second generation biofuels. 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.

References

{{Reflist Polysaccharides Cell biology