Secondary Cell-Wall Polysaccharide

In addition to the anthrax carbohydrate present on the surface of spores described above, the structure of the secondary cell wall polysaccharide of B. anthracis vegetative cells consists of hexasaccharide repeating units having unusual (3-linked N-acetylmannosamine 45. The two corresponding synthetic trisaccharides were recognized by the sera from rabbits exposed to spores of B. anthracis.The first total synthesis of the hexasaccharide repeating unit was accomplished recently. ... 

Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls constituting about 20-30% of the biomass of dicotyl plants (hardwoods and herbaceous plants). In some tissues of monocotyl plants (grasses and cereals) xylans occur up to 50% [6j. Xylans are thus available in huge and replenishable amoimts as by-products from forestry, the agriculture, wood, and pulp and paper industries. Nowadays, xylans of some seaweed represent a novel biopolymer resource [4j. The diversity and complexity of xylans suggest that many useful by-products can be potentially produced and, therefore, these polysaccharides are considered as possible biopolymer raw materials for various exploitations. As a renewable resource, xylans are... 

Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

CelIulose.-Most biologists know that cellulose (l->4-linked P-D-glucan) is a polysaccharide component of all primary and secondary cell walls. Indeed, plant cell walls are... 

Hemicelluloses and Related Polysaccharides. Hemicelluloses [9034-32-6] are a large group of polysaccharides that are associated with cellulose in the primary and secondary cell walls of all higher plants, but otherwise have no relationship to cellulose (2). They are also present in some other plants. 

Some monocot, primary cell-wall polysaccharides may be cross-linked by esters of ferulic acid (4-hydroxy-3-methoxycinnamic acid). There is evidence for the existence of such cross-links in monocot tissues containing secondary walls, including Italia ryegrass stem276 and wheat endosperm.277 Ferulic acid is also present in barley cell-wall,278 and it has been reported that treatment with base releases ferulic acid from the cell walls of several Graminae,276 supporting the idea that ferulic acid is bound to the wall as an ester. However, ferulic acid has not, so far, been reported to be present specifically in primary cell-walls in either monocots or dicots. 

The standard procedure by Saeman et al. (I) involves manual stirring of the polysaccharide with 72% H2S04, standing at 30°C, and secondary hydrolysis at 100° or 120°C in a steam autoclave. While certain resistant polysaccharides are still incompletely depolymerized, decomposition of the more sensitive monosaccharides formed cannot be avoided. An alternative method by using trifluoroacetic acid was applied successfully for plant cell wall polysaccharides by Albersheim et al. (2) and for dissolving pulps and hemicelluloses by Fengel et al. (3). Highly crystalline cellulose was not well dissolved and not completely hydrolyzed by CFsCOOH. 

Phenolic acids, and especially ferulic acid, which is abundantly present in cereals, is found esterified to the polysaccharides present in primary and secondary cell walls of plants. Ferulic acid is the major phenolic acid occurring in the cell walls of monocotyledons and appears as cis and the more abundant trans isomers (reviewed in [Klepacka and Forna, 2006]). Ferulic acid is found in wheat, maize, rye, barley [Sun et al., 2001], oats, spinach, sugar beet, and water chesnuts [Clifford, 1999], generally esterified, and rarely as free form, such as in barley [Yu et al., 2001]. It is esterified in primary cell walls to arabinoxylans (Fig. 2.4) in the aleurone layer and pericarp [Clifford, 1999], as in spinach [Fry, 1982] or in wheat bran [Smith and Hartley, 1983], Ferulic acid can also be found esterified to other hydroxycinnamic acids such as in Mongolian medicinal plants where it is found as feruloylpodospermic acid, which is... 

Fractionation and Chemistry of Citrus Pectic Polysaccharides. Pectic polysaccharides, commonly known as pectin, appear early in plant cell-wall formation. A series of complex biochemical steps results in the formation of cell plates followed first by its growth in area (primary cell wall) then in thickness (secondary cell wa.ll). Exclusive of randomly oriented cellulose fibrils, primary cell wall is composed mainly of pectic polysaccharides (34). These pectic polysaccharides are rich in D-galacturonic acid, D-galactose and L-arabinose residues. With growth in thickness of cell wall (secondary cell wall),there appears to be a replacement of pectic polysaccharide deposition with polysaccharides rich in D-glucuronic acid or 4-0-methyl-D-glucuronic acid,... 

Molecular weight measurements have shown that cotton cellulose in its native state consists of about 15,000 and wood cellulose of about 10,000 glucose residues. Some polydispersity data on cellulose derivatives and polysaccharides are shown in Table 3-1. There are indications that the native cellulose present in the secondary cell wall of plants is monodisperse, that is, contains only molecules of one size. In such a case, number and weight average molecular weights ought to be identical. The cellulose in the primary cell wall, on the other hand, which has a lower average molecular... 

The secondary cell wall, produced after the cell has completed growing, also contains polysaccharides and is strengthened by polymeric lignin covalently crosslinking to hemicellulose.6... 

In comparison with mammals, plants contain considerably more GTs because, in addition to the reactions carried out by mammalian GTs, they are required to convert the products of photosynthesis into diverse cell carbohydrates. For example, these GTs synthesize cell wall polysaccharides as well as secondary metabolites and xenobiotics. Plant GTs differ from mammalian GTs even more by their diversity of nucleotide donors. They use not only eight of the nine mammalian nucleotide donors, but numerous others such as UDP-L-rhamnose, GDP-L-glucose, GDP-L-galactose, UDP-L-arabinose, UDP-D-galacturonic acid, UDP-D-apiose, and so on. (29). 

In both primary and secondary cell walls, cellulose is mixed with other polysaccharides (O Table 1) and lignin [20,21,22,23,24,25,26]. Historically, the other polysaccharides have been called hemicelluloses. The term hemicellulose indicates a polysaccharide closely associated with cellulose in cell walls. The relationship is physical, not stmctural. These polysaccharides are almost always heteroglycans 8-glucans and some polysaccharides that are composed almost exclusively of D-xylose are among the exceptions. Stmctures of hemicelluloses vary from linear to highly branched and bushlike. 

The predominant hemicelluloses in secondary cell walls of both monocotyledons and dicotyledons are xylans. These polysaccharides have a xylan main chain that is decorated with 4-0-methyl-D-glucopyranosyluronic acid, D-glucopyranosyluronic acid, L-arabinofuranosyl, and/or L-arabinopyranosyl units. These related polysaccharides are usually acetylated to various degrees. Gymnosperms contain glucomannans, also acetylated, as the principal secondary cell-wall hemicellulose [26]. 

These data confirm the role of Golgi bodies in the synthesis of cell-wall polysaccharides, especially the pectins and hemicelluloses that form the main bulk of the middle lamella and the primary wall. Gellulose, which is the major component of the secondary wall, seems, however, to be generally produced by a different system. 

Xylan is a polysaccharide consisting mainly of D-xylopyranose residues and a small proportion of L-arabinofuranose residues. The polysaccharide exists, mainly in association with cellulose, in the secondary cell-walls of practically all higher plants. 

Hemicelluloses form a group of polysaccharides that are found in the primary and secondary cell walls of all land... 

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