Arabinoxylan

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Arabinoxylan is a form of the hemicellulose [1] 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. [2] The term arabinoxylan usually refers to feruloyl-arabinoxylan from grasses and other commelinids containing moieties of the phenolic ferulic acid that can undergo oxidative coupling (in the same way as lignin units) forming crosslinks between arabinoxylan chains and with lignin. Whilst arabinose has been found linked to xylan in non-commelinid plants, ferulic acid has not been reported on these and unlike feruloyl-arabinoxylan these arabinoxylans are not monophyletic. The remainder of this article refers to feruloyl-arabinoxylan from cell walls of grasses and other commelinid species.

Contents

Structure

As with all xylan, the backbone of arabinoxylan chains is composed of a large number of 1,4-linked β-D-xylopyranosyl units. In arabinoxylan many of these xylose units are 3-linked with single α-L-arabinofuranosyl units and some of these arabinose in turn have ester-linked ferulic acid residues. [2] [3] These feruloyl units can undergo radical oxidative coupling forming ferulic acid dehydrodimers and possibly higher oligomers that covalently crosslink arabinoxylan chains. This mode of cross-linking is a key feature of both primary and secondary cell walls in grasses and other commelinid species.

Due to its importance in food, the structure of arabinoxylan from wheat grain and other cereals has been intensively studied. In particular, the arabinoxylan from wheat endosperm that gives rise to white flour has a simpler structure than that from most tissues. Apart from the structural features described above, it has xylose units di-substituted with 2 and 3-linked arabinose residues. [4] Due to a low degree of crosslinking into the cell wall, some of this endosperm arabinoxylan is extractable in water, giving rise to soluble dietary fiber.

Arabinoxylans from tissues other than endosperm are structurally more complex. The xylan backbone is often heavily substituted with glucuronic acid resides and other sugars and these arabinoxylan polymers are sometimes termed glucuronarabinoxylans or heteroxylans. [2] Both backbone and sidechain sugar residues can be acetylated. Arabinosyl residues can be acylated with phenolics other than ferulic acid, in particular p-coumaric acid but also sinapinic acid and others. These acylated arabinosyl residues are frequently additionally substituted with 2-linked sugars. In lignified cell walls, there is strong evidence that feruloyl residues on arabinoxylan can be crosslinked into the lignin polymer, increasing recalcitrance to digestion.

Functions

Arabinoxylans chiefly serve a structural role in the plant cells. [5] They are also the reservoirs of large amounts of ferulic acid and other phenolic acids which are covalently linked to them. Phenolic acids may also be involved in defense including protection against fungal pathogens.

Arabinoxylans are one of the main components of soluble and insoluble dietary fibers which are shown to exert various health benefits. [6] In addition, arabinoxylans, owing to their bound phenolic acids, are shown to have antioxidant activity. [7] Their ion exchange capacity and viscosity are also partly responsible for their beneficial metabolic effects. [8]

Related Research Articles

<span class="mw-page-title-main">Hemicellulose</span> Class of plant cell wall polysaccharides

A hemicellulose is one of a number of heteropolymers, such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls. Cellulose is crystalline, strong, and resistant to hydrolysis. Hemicelluloses are branched, shorter in length than cellulose, and also show a propensity to crystallize. They can be hydrolyzed by dilute acid or base as well as a myriad of hemicellulase enzymes.

<span class="mw-page-title-main">Polysaccharide</span> Long carbohydrate polymers comprising starch, glycogen, cellulose, and chitin

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 water (hydrolysis) using amylase enzymes as catalyst, which produces constituent sugars. They range in structure from linear to highly branched. Examples include storage polysaccharides such as starch, glycogen and galactogen and structural polysaccharides such as cellulose and chitin.

<span class="mw-page-title-main">Dietary fiber</span> Portion of plant-derived food that cannot be completely digested

Dietary fiber or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition, and can be grouped generally by their solubility, viscosity, and fermentability, which affect how fibers are processed in the body. Dietary fiber has two main components: soluble fiber and insoluble fiber, which are components of plant-based foods, such as legumes, whole grains and cereals, vegetables, fruits, and nuts or seeds. A diet high in regular fiber consumption is generally associated with supporting health and lowering the risk of several diseases. Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins, pectins, beta-glucans, and oligosaccharides.

<span class="mw-page-title-main">Pectin</span> Structural carbohydrate in the cell walls of land plants and some algae

Pectin is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal chemical component of pectin is galacturonic acid which was isolated and described by Henri Braconnot in 1825. Commercially produced pectin is a white-to-light-brown powder, produced from citrus fruits for use as an edible gelling agent, especially in jams and jellies, dessert fillings, medications, and sweets; and as a food stabiliser in fruit juices and milk drinks, and as a source of dietary fiber.

<span class="mw-page-title-main">Lignin</span> Structural phenolic polymer in plant cell walls

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors.

<span class="mw-page-title-main">Bran</span> Hard outer layers of cereal grain

Bran, also known as miller's bran, is the hard layers of cereal grain surrounding the endosperm. It consists of the combined aleurone and pericarp. Corn (maize) bran also includes the pedicel. Along with the germ, it is an integral part of whole grains, and is often produced as a byproduct of milling in the production of refined grains.

Cellulosic ethanol is ethanol produced from cellulose rather than from the plant's seeds or fruit. It can be produced from grasses, wood, algae, or other plants. It is generally discussed for use as a biofuel. The carbon dioxide that plants absorb as they grow offsets some of the carbon dioxide emitted when ethanol made from them is burned, so cellulosic ethanol fuel has the potential to have a lower carbon footprint than fossil fuels.

<span class="mw-page-title-main">Ferulic acid</span> Chemical compound

Ferulic acid is a hydroxycinnamic acid; it is an organic compound with the formula (CH3O)HOC6H3CH=CHCO2H. The name is derived from the genus Ferula, referring to the giant fennel (Ferula communis). Classified as a phenolic phytochemical or polyphenol, ferulic acid is an amber colored solid. Esters of ferulic acid are found in plant cell walls, covalently bonded to hemicellulose such as arabinoxylans. Salts and esters derived from ferulic acid are called ferulates.

<span class="mw-page-title-main">Phenylpropanoid</span>

The phenylpropanoids are a diverse family of organic compounds that are biosynthesized by plants from the amino acids phenylalanine and tyrosine in the shikimic acid pathway. Their name is derived from the six-carbon, aromatic phenyl group and the three-carbon propene tail of coumaric acid, which is the central intermediate in phenylpropanoid biosynthesis. From 4-coumaroyl-CoA emanates the biosynthesis of myriad natural products including lignols, flavonoids, isoflavonoids, coumarins, aurones, stilbenes, catechin, and phenylpropanoids. The coumaroyl component is produced from cinnamic acid.

<span class="mw-page-title-main">Xylan</span> A plant cell wall polysaccharide

Xylan is a type of hemicellulose, a polysaccharide consisting mainly of xylose residues. It is found in plants, in the secondary cell walls of dicots and all cell walls of grasses. Xylan is the third most abundant biopolymer on Earth, after cellulose and chitin.

<span class="mw-page-title-main">Lignocellulosic biomass</span> Plant dry matter

Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of biofuels. It is composed of two kinds of carbohydrate polymers, cellulose and hemicellulose, and an aromatic-rich polymer called lignin. Any biomass rich in cellulose, hemicelluloses, and lignin are commonly referred to as lignocellulosic biomass. Each component has a distinct chemical behavior. Being a composite of three very different components makes the processing of lignocellulose challenging. The evolved resistance to degradation or even separation is referred to as recalcitrance. Overcoming this recalcitrance to produce useful, high value products requires a combination of heat, chemicals, enzymes, and microorganisms. These carbohydrate-containing polymers contain different sugar monomers and they are covalently bound to lignin.

Laccases are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of phenolic substrates, performing one-electron oxidations, leading to crosslinking. For example, laccases play a role in the formation of lignin by promoting the oxidative coupling of monolignols, a family of naturally occurring phenols. Other laccases, such as those produced by the fungus Pleurotus ostreatus, play a role in the degradation of lignin, and can therefore be classed as lignin-modifying enzymes. Other laccases produced by fungi can facilitate the biosynthesis of melanin pigments. Laccases catalyze ring cleavage of aromatic compounds.

<span class="mw-page-title-main">Diferulic acids</span>

Diferulic acids (also known as dehydrodiferulic acids) are organic compounds that have the general chemical formula C20H18O8, they are formed by dimerisation of ferulic acid. Curcumin and curcuminoids, though having a structure resembling diferulic acids', are not formed that way but through a condensation process. Just as ferulic acid is not the proper IUPAC name, the diferulic acids also tend to have trivial names that are more commonly used than the correct IUPAC name. Diferulic acids are found in plant cell walls, particularly those of grasses.

Arabinogalactan, also known as galactoarabinan, larch arabinogalactan, and larch gum, is a biopolymer consisting of arabinose and galactose monosaccharides. Two classes of arabinogalactans are found in nature: plant arabinogalactan and microbial arabinogalactan. In plants, it is a major component of many gums, including gum arabic and gum ghatti. It is often found attached to proteins, and the resulting arabinogalactan protein (AGP) functions as both an intercellular signaling molecule and a glue to seal plant wounds.

<span class="mw-page-title-main">Rhamnogalacturonan-II</span>

Rhamnogalacturonan-II (RG-II) is a complex polysaccharide component of pectin that is found in the primary cell walls of dicotyledonous and monocotyledonous plants and gymnosperms. It is supposed to be crucial for the plant cell wall integrity. RG-II is also likely to be present in the walls of some lower plants. Its global structure is conserved across vascular plants, albeit a number of variations within the RGII side chains have been observed between different plants. RG-II is composed of 12 different glycosyl residues including D-rhamnose, D-apiose, D-galactose, L-galactose, Kdo, D-galacturonic acid, L-arabinose, D-xylose, and L-aceric acid, linked together by at least 21 distinct glycosidic linkages. Some resides are further modified via methylation and acetylation. It moreover supports borate mediated cross-linking between different RGII side-chain apiosyl residues. The backbone consists of a linear polymer of alpha-1,4-linked D-galactopyranosiduronic acid. RG-II can be isolated from different sources, such as apple juice and red wine.

Glucuronoxylans are the primary components of hemicellulose as found in hardwood trees, for example birch. They are hemicellulosic plant cell wall polysaccharides, containing glucuronic acid and xylose as its main constituents. They are linear polymers of β-D-xylopyranosyl units linked by (1→4) glycosidic bonds, with many of the xylose units substituted with 2, 3 or 2,3-linked glucuronate residue, which are often methylated at position 4. Most of the glucuronoxylans have single 4-O-methyl-α-D-glucopyranosyl uronate residues (MeGlcA) attached at position 2. This structural type is usually named as 4-O-methyl-D-glucurono-D-xylan (MGX).

<span class="mw-page-title-main">Xylose isomerase</span> Class of enzymes

In enzymology, a xylose isomerase is an enzyme that catalyzes the interconversion of D-xylose and D-xylulose. This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases interconverting aldoses and ketoses. The isomerase has now been observed in nearly a hundred species of bacteria. Xylose-isomerases are also commonly called fructose-isomerases due to their ability to interconvert glucose and fructose. The systematic name of this enzyme class is D-xylose aldose-ketose-isomerase. Other names in common use include D-xylose isomerase, D-xylose ketoisomerase, and D-xylose ketol-isomerase.

Root mucilage is made of plant-specific polysaccharides or long chains of sugar molecules. This polysaccharide secretion of root exudate forms a gelatinous substance that sticks to the caps of roots. Root mucilage is known to play a role in forming relationships with soil-dwelling life forms. Just how this root mucilage is secreted is debated, but there is growing evidence that mucilage derives from ruptured cells. As roots penetrate through the soil, many of the cells surrounding the caps of roots are continually shed and replaced. These ruptured or lysed cells release their component parts, which include the polysaccharides that form root mucilage. These polysaccharides come from the Golgi apparatus and plant cell wall, which are rich in plant-specific polysaccharides. Unlike animal cells, plant cells have a cell wall that acts as a barrier surrounding the cell providing strength, which supports plants just like a skeleton.

The term "lignin characterization" refers to a group of activities within lignin research aiming at describing the characteristics of a lignin by determination of its most important properties. Most often, this term is used to describe the characterization of technical lignins by means of chemical or thermo-chemical analysis. Technical lignins are lignins isolated from various biomasses during various kinds of technical processes such as wood pulping. The most common technical lignins include lignosulphonates, kraft lignins, organosolv lignins, soda lignins and lignin residue after enzymatic treatment of biomass.

Breznakibacter xylanolyticus is a freshwater gliding bacterium that degrades insoluble particulate xylans and dominates xylan fermentation, particularly in sulfur and methane-rich environments. It is the only species in the genus Breznakibacter. At the time of isolation it was classified in the order Cytophagales on the basis of phenotypic characteristics such as polymer degradation and gliding motility. It has since been reclassified to the order Bacteroidales due to 16s rRNA genome sequence analysis.

References

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