Xylans arabinoxylans

The noncellulose (1-n-glucans in hemicellulose are generally 1,3- and 1,4-bonded arabinans, xylans, arabinoxylans, arabinogalactans, etc. Their aqueous dispersions are quite viscous to the detriment of filters used to clarify cereal-based fermented beverages. Oat (3-glucans are Theologically... 

The D-xylanases of rumen microbial origin are all of the endo type, and degrade D-xylan, arabinoxylan, hemicellulose B, and xylo-oligosaccharides of d.p. 2=3, mainly to X2 and D-xylose X2 remains unattacked. The D-xylanases were found38,42 to have a higher affinity for linear D-xylans than for the branched hemicelluloses, namely, arabinoxylan and a branched B fraction from Trifolium pratense (red clover). The presence of L-arabinose, D-galactose, and uronic acid substituents on the D-xylan backbone thus appears to inhibit the action of these D-xylanases. The D-xylan was not effectively hydrolyzed until the side-group substituents had mostly been removed, for example, by arabinofuranosidase action on arabinoxylan.44... 

Several xylo-oligosaccharides of d.p. 2-7 have been isolated from enzymic hydrolyzates of terrestrial-plant D-xylans, arabinoxylans, arabino-4-O-methylglucuronoxylans, and 4-O-methylglucuronoxylans by the action of endo-xylanases,5,6,38-40,42-44,47,181,215-217,222,228 236,247-250, 254,256,258-261 an(j have been shown to constitute a series of /3-D-(l — 4)-linked D-xylo-oligosaccharides. 

Hemicellulases are groups of enz5mies belonging to glycosidases families, whose common feature is hydrolysis of hemicellulose. This polysaccharide has a complex structure consisting of linear and branched chains made up of xylane, arabinoxylan, xyloglucan. 

FAE was specific for esterified xylan-oligomers, but did not show selectivity towards a specific ester. This enzyme could release ferulic acid as well as acetyl groups from esterified arabinoxylans in the presence of an endoxylanase. 

The only major accessory enzyme purified thus far for the xylanase was an a-(l,3)-L-arabinosidase with a molecular weight of 66,(XX) and isoelectric point of 4.1 (publication in preparation). The enzyme was not capable of acting on intact xylans. However, it acted on the end-products of arabinoxylan (but not glucuronoxylan) degradation by the xylanase, doubling the total extent of degradation. 

Both D-glucuronic acid and/or its 4-O-methyl ether and arabinose are also present in cereal xylans (4). Endospermic arabinoxylans of annual plants, often referred to as pentosans, are because of their branched structures more soluble in water and dilute alkali than xylans of lignocellulosic materials. They also have at least one, or even two, substituents per xylose residue (5). 

The presence of feruloyl and p-coumaroyl acids linked via L-arabinose residues has been verified in several studies of xylan (6-8). The amounts of these components, however, are rather small. Every 15th arabinose unit in barley straw arabinoxylan is estimated to be esterified with ferulic acid, and every 31st with p-coumaric acid (7). These hydroxycinnamic acids are bound to C-5 of the arabinose residue (7, 8). It has been suggested that oxidative dimerization of ferulic acid residues crosslinks the arabinoxylan chains and renders them insoluble as a result of the diferuloyl bridges (9). 

In addition to enzymes hydrolyzing the glycosidic linkages in xylans, the requirement of esterases to remove esterified acids from xylans has recently been discovered. Acetyl xylan esterases remove O-acetyl groups from the C-2 and C-3 positions of xylose residues in both xylan and g looligomers. Feruioyl esterases liberate ferulic acid from arabinoxylans of monocotyledons. 

Amylopectin Amylose Chitin Glucogen Inulin Mannan Cellulose Xylan Lichenan Galactan Arabinoxylan... 

In describing and interpreting some of the more important properties of plant galactomannans, comparisons will be made with structurally similar polysaccharides, including the closely related glucomannans and galactoglucomannans, and those based on (1 — 4)-/3-D-xylan main-chains (for example, the arabinoxylans) and (1 — 4)-/3-D-glucan main-chains [for example, the amyloids and sodium 0-(carboxymethyl)cellulose]. 

There have been few studies of polymer interconnections within the primary wall of monocots. An arabinoxylan from the primary wall of cultured, barley-aleurone cells,61 and a glucuronoarabinoxylan from maize-coleoptile primary-wall,200 have been shown to bind reversibly to cellulose in vitro, Because xylans are, quantitatively, the major component of monocot primary cell-walls, this interconnection is an important finding it is very likely to occur through multiple hydrogen-bonds, analogous to the interconnection between xyloglucan and cellulose in dicot cell-walls.56,57,59 It is also possible that heteroxylans participate in binding other cell-wall polymers to cellulose. 

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