Xylan (; ) ( CAS number: 9014-63-5) is a type of hemicellulose, 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 w ...

consisting mainly of

xylose Xylose ( grc, ξύλον, , "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 g ...

residues. It is found in

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

s, in the secondary

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

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

. Xylan is the third most abundant

biopolymer Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, ...

on Earth, after

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

and

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

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, fructose, and galactose. Compound sugars, also called disaccharides or do ...

) 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 (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 dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood (which comes from ...

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 s ...

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, acetyl is a functional group with 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, acetyl is called ethanoyl, ...

groups and the presence of

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

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

Algae

Some macrophytic green algae contain xylan (specifically homoxylan) especially those within the '' Codium'' 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 (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

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. Xylooligosaccharides 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 glycosyltransferases 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 flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter ...

'' 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 (EC 3.2.1.8, 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 cel ...

convert xylan into

. Given that plants contain a up to 30% xylan, xylanase is important to the nutrient cycle. 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, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration ...

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

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