Matrix Polysaccharides: Further Polysaccharides of the Primary Wall; Glycoproteins

Both in the primary and in the secondary cell wall are the cellulose microfibrils embedded in an amorphous ground substance, the matrix. Matrix materials are heteropolymers that are traditionally grouped into two families of polysaccharides: pectins and hemicelluloses. Pectins are mainly polygalacturonic acids with differing degrees of D-galactosyl, L-arabinosyl, or L-rhamnosyl residues.

Pectins are predominant in the middle lamella, the layer between neighbouring cells. Hemicelluloses have short chains and are therefore partially soluble polymers, consisting of xylosyl-, glucosyl-, galactosyl-, arabinosyl- or mannosyl-residues. Depending on the dominant sugar is it spoken of xylans, galactanes, or of arabinogalactans, for example, if both sugars occur in nearly equal amounts. The delimination between the two classes is fluid. During the last years has the analysis of the composition and structure of matrix polysaccharides (and other components) produced a number of new results. They were based both on the choice of suitable test objects and on the use of sensitive analytical techniques.

The studies of maple cell cultures (Acer platanoides) proved to be especially favourable since the cells develop only primary but no secondary cell walls under certain culture conditions. Moreover do they secrete polysaccharides into the media, the composition of which resembles that of wall polysaccharides. A little time ago was it proven that certain oligosaccharides set free by the wall have specific regulatory functions. They influence growth and differentiation of other cells and tissues and they participate in defensive reactions against fungi and other micro-organisms. Such oligosaccharides are called oligosaccharins. For their analysis seemed gas chromatography and the analysis of enzymatically partially degraded polymers, later also mass spectroscopy to suggest themselves.

The studies were (and are) mainly performed in the laboratory of P. ALBERSHEIM at the Complex Carbohydrate Research Center, Athens, Georgia. After hydrolyses of polysaccharides and the thus released monomers were the latter separated by gas chromatography and identified.

For the elucidation of the binding types were specifically degrading enzymes used. The studies lead to the result that the primary wall contains the polymers arabinogalactan, rhamnogalacturonan, xyloglucan and a hydroxyproline-rich protein besides cellulose.

What does the structure of the heteropolysaccharides look like? K. W. TALMAGE et al. (1976) proposed a structure for rhamnogalacturonan in which the molecule has a zigzag structure because the rhamnosyl residue is linked via 1>2 glycosidic bonds to the linearly linked 1>4 galacturonan chain. The rhamnosyl residues are often linked to additional (arabino) galactan chains via their C₄-atoms, so that they represent Y-shaped branching points.

Arabinogalactan is composed of units that consist on average of six galactosyl- and seven arabinosyl residues and that may carry short lateral chains. These chains are nearly without exception covalently bound to the just mentioned rhamnogalacturonan and to xyloglucan, so that the latter form a kind of bridge.

Xyloglucans belong to the components that are secreted into the nutrient media in large quantities. It is distinguished between three fractions:

free xyloglucan,
xyloglucan that is bound to rhamnogalacturonan and
xyloglucan that is bound to arabinogalactan.

Besides the sugars xylose and glucose contain xyloglucans galactose and fucose, too. Their quantities in the different xyloglucans may vary.

Besides the polysaccharides of the hemicellulose fractions have glycoproteins rich in hydroxyproline (extensins) an important part in wall formation (D. T. A. LAMPORT and D. H. NORTHCOTE, University of Cambridge, 1960). They are ubiquitous in the plant kingdom. Usually are they isolated from the wall of Chlamydomonas as well as of that of several higher plants. Most of their hydroxyproline residues are glycosylated (usually always linked to L-arabinose). In lower plants are 26 - 67 percent of all hydroxyproline residues glycosylated, in gymnosperms 79 to 86 percent. In angiosperms exists a fundamental difference between monocots (25 - 34 %) and dicots (87 - 97%).

The biological significance of these differences is unknown. Numerous serine residues of the protein carry galactosyl residues. Extensins are essential and integral components of the macromolecular cell wall complex. They seem to have an influence on the wall's flexibility. Hydrogen bonds between the molecules of the different classes help stabilizing the complex.

© Peter v. Sengbusch - Impressum