Arabinoxylans, structure

Psyllium mucilage exhibits a highly branched acidic arabinoxylan structure. The molecular chain backbone contains D-Xylose residues bonded mainly via (3-( 1 4) and glycosidic linkages. Branching takes place at 0-2 and 0-3 of the xylose units [24]. 

P., and Mills, E. (2012) Spectroscopic analysis of diversity in the spatial distribution of arabinoxylan structures in endosperm cell walls of cereal species in the HEALTHGRAIN diversity collection. J. Cereal Sci, 56 (2), 134-141. 

This essay was written in an attempt to explain our overview of primary cell walls and to reach consensus on the nomenclature of primary cell wall polysaccharides. We present evidence supporting the hypothesis that cellulose, xyloglucan, arabinoxylan, homogalacturonan, RG-I, and RG-II are the six polysaccharides common to all primary cell walls of higher plants. In many cells, these six polysaccharides account for all or nearly all of the primary wall polysaccharides. Like the physically interacting proteins that constitute the electron transport machinery of mitochondria, the structures of the six patently ubiquitous polysaccharides of primary cell walls have been conserved during evolution. Indeed, we hypothesize that the common set of six structural polysaccharides of primary cell walls have been structurally... 

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). 

Based upon these studies, Table III summarizes the partially deduced structure of the mono-, di-, and trisaccharides, obtained by the partial degradation of the original arabinoxylan. These structures may not be completely accurate, because of some uncertainty in the theory used to speculate the mode of linkage. Consequently, each one needs to be isolated to unambiguously verify its structure. These results do, however, provide some indication of the complicated structure of the side-chains of bran 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]. 

Similar structural problems exist for the distribution of side chains in galactoglucomannans, xyloglucans, arabinoxylans, and 4-0-methylglucuronoxylans, and even for the distribution of acetyl groups in glucomannans and for related features in many other polysaccharides. 

Of the related polysaccharides mentioned earlier (see p. 267), only arabinoxylans have been examined in any detail by enzymic methods. Goldschmid and Perlin153 studied the fine structure of wheat arabinoxylan by using the /3-D-xylanase from Streptomyces QMB 814. Their results indicated that arabinoxylan molecules are mainly constituted of highly branched regions in which isolated and paired L-arabinosyl (A) branches are separated by single D-xylosyl (X) residues, as shown in 12, but that, at unequal intervals (averaging... 

Further information on the fine structure of arabinoxylan should be forthcoming when hydrolysis is performed with a purified (l- 4)-/S-D-xylanase, such as that from Cryptococcus albidus,60 62 whose subsite-... 

Rao and Muralikrishna (2004b) isolated a fraction of the arabinoxylan from ragi and determined its structure using a combination of methyla-tion, enzyme digestion, NMR, and MALDI-TOF-MS. They determined that the arabinoxylan was a (l-4)-l inked xylose backbone carrying arabinose at the C-3 position and an occasional glucuronic acid at the (l-3)-position. Xylose was also found in the side chain (Fig. 6.6). 

All the arabinoxylans, so far examined, are of this general type, and, where these structures have been distinguished, the evidence points to direct attachment of the L-arabinofuranose units to the backbone as in I. In one case, direct proof of this mode of linkage has been obtained by the isolation... 

In other cases, the evidence in favor of structure I is based upon the examination of a degraded polysaccharide isolated after controlled acid hydrolysis has resulted in removal of some or all of the acid-labile L-arabinofuranose residues without significant cleavage of D-xylopyranosyl linkages. Thus, arabinoxylans I and II would give rise to the corresponding degraded polysaccharides IV and V. Structures IV and V may be differentiated by... 

Cleemput, G., Roels, S. P., van Oort, M., Grobet, P. J., and Delcour, J. A. (1993). Heterogeneity in the structure of water-soluble arabinoxylans in European wheat flours of variable bread-making quality. Cereal Chem. 70 324-329. 

Lee, R. C., Burton, R. A., Hrmova, M., and Fincher, G. B., Barley arabinoxylan arabinofuranohydrolases purification, characterization and determination of primary structures from cDNA clones. Biochem 72001, 356 (Pt 1), 181-9. 

Arabinogalactans are also polysaccharide components of wheat flour. Arabinogalactans in wheat flour are attached to protein molecules and thus are part of the structures of arabino-galactan-protein molecules. Like the arabinoxylans, there are variations in both amounts and structures of water-extractable arabinogalactan-protein molecules in wheat flours. 

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