Hemicellulosic Polysaccharides

Hemicelluloses are defined as those plant cell wall polysaccharides that are not solubilzed by water or chelating agents but are solubilized by aqueous alkali (Selvendran and O'Neill, 1985). With this definition hemicelluloses include xyloglucan, glucomannan, mannan, xylan, arabinoxylan, and arabinogalactan. Hemicelluloses are also defined chemically as cell wall polysaccharides that are structurally homolgous to cellulose because they have a backbone composed of 1,4-linked β-D-pyranosyl residues (O'Neill and York, 2003). Xyloglucan, glucomannan, mannan, xylan, arabinoxylan, but not arabinogalactan are included using this chemical definition of hemicellulose.

Xyloglucan

Xyloglucan is the most abundant hemicellulose in the primary walls of non-graminaceous plants, often comprising 20% of the dry mass of the wall (O'Neill and York, 2003). Xyloglucan has a backbone composed of 1,4-linked β-D-Glcp residues. Up to 75% of the backbone residues are substituted at C6 with mono, di, or trisaccharide side chains. Xyloglucans are classified as XXXG-type or XXGG-type depending on the number of backbone residues that are branched. XXXG-type xyloglucans are present in the walls of numerous higher plants whereas XXGG-type xyloglucans occur in the walls of solanaceous plants (see figure below).

There is variation in the structures of the side chains of XXXG-type xyloglucans. For example, many plants synthesize xyloglucans with side chains that contain a terminal α-L-fucosyl residue. The fucosyl residue is absent in xyloglucan isolated from Tamarind seeds whereas the xyloglucan isolated form jojoba seeds contain a α-L-Fuc and a α-L-Gal residue (see structures below):

To date only one monoclonal anbtibody (CCRC-M1) that recognizes fucosylated xyloglucan has been described. No antibodies that recognize solanaceous or non-fucosylated xyloglucans are available.

Back to Top

Galactoglucomannan

Mannose-containing polysaccharides including mannans, galactomannans, and galactoglucomannans are present in the walls of many plants (Stephen, 1982). These polysaccharides all have a backbone containing 1,4-linked β-D-Man residues. Galactoglucomannans, which are particularly abundant in the walls of solanaceous plants, have a backbone composed of both 1,4-linked β-D-Man and β-D-Glc residues. Some of the mannose residues are substituted at C6 with mono- and disaccharides (see structure below). A monoclonal antibody that recognizes 1,4-linked β-D-mannan is commercially available from BioSupplies.

Xylans and Substituted Xylans

Xylans, including arabinoxylan, glucuronoxylans, and glucuronoarabinoxyalans, are quantitatively minor components of the primary walls of dicots and non-graminaceous monocots but are abundant in the walls of the Poaceae and secondary walls of woody plants (Darvill et al. 1980; Ebringerova and Heinze, 2000). These polysaccharides all have a backbone composed of 1,4-linked β-D-Xyl residues. Many of the Xyl residues are substituted at O-2 and/or O-3 with α-Ara, β-GlcA, 4-O-Me β-GlcA, and β-Xyl residues. The backbone Xyl residue may also be O-acetylated. The side chain Ara residues may be esterified with ferulic or coumaric acid (see figure below). No monoclonal antibodies that recognize this family of hemicelluloses have been described.

Mixed-linkage β-Glucans

Mixed linkage β-glucans occur in the cell walls of the gramineae. β-glucans are linear polysaccharides that contain β-1,4 and β-1,3-linked Glc residues (Carpita, 1996). The 4-linked Glc residues occur in cellotriosyl and cellotetraosyl units that are linked together by a single β-1,3-Glc residue. β-glucans rarely if ever contain a single 1,4-linked Glc or consecutive 1,3-linked Glc. A monoclonal antibody that recognizes mixed-linkage β-glucan is commercially available from BioSupplies.

References

Carpita (1996). Structure and biogenesis of the cell walls of grasses. Ann. Rev. Plant Physiol. Plant Mol. Biol. 47: 445-476.

Darvill et al (1980). Structure of plant cell walls. XI. Glucuronoarabinoxylan, a second hemicellulose in the primary cell walls of suspension-cultured sycamore cells. Plant Physiol. 66:1135-1139.

Ebringerova and Hienze (2000). Xylan and xylan derivatives - biopolymers with valuable properties. 1. Naturally occurring xylans, isolation procedures and properties. Macromol. Rapid Commun. 21: 542-556.

O'Neill and York (2003) The composition and structure of plant primary walls. In The Plant Cell Wall (JKC Rose ed), Blackwell, pp. 1-54.

Selvendran and O'Neill (1985). Isolation and analysis of cell walls from plant material. In Methods of Biochemical Analysis, Vol 32 (D Glick ed), John Wiley & Sons, pp. 25-123.

Stephen (1982). Other plant polysaccharides. In The Polysaccharides Vol 2 (GO Aspinall ed), Academic press, pp. 97-123.

Back to Top

[Home]

[People]

[Links]

A National Science Foundation-funded (Grant No DBI-0421683) research project at
The Complex Carbohydrate Research Center of The University of Georgia