Pectic homogalacturonan

Willats, W. G., Orfila, C., Limberg, G., Buchholt, H. C., van Alebeek, G. J., et al., Modulation of the degree and pattern of methyl-esterification of pectic homogalacturonan in plant cell walls. Implications for pectin methyl esterase action, matrix properties, and cell adhesion. J Biol Chem 2001, 276 (22), 19404-13. 

There are three pectic polysaccharides in all primary cell walls that have been studied these are rhamnogalacturonan n, rhamnogalacturonan I, and homogalacturonan. 

The pectin network revisited.--The importance of the interconnections of the pectic polysaccharides to the integrity of the pectin network has been highlighted by the recent discovery that RG-II is present in primary walls as a mixture of monomers and dimers [54]. The dimers are covalently cross-linked by borate diesters [55,56]. If single molecules of homogalacturonan are covalently attached to both RG-I and RG-II, the covalently cross-linked RG-n dimers would explain how the network of the three types of pectic polysaccharides is covalently connected and covalently cross-linked. 

Cell free synthesis of the pectic polysaccharide homogalacturonan... 

Mohnen, D., Doong, R.L., Liljebjelke, K., FraUsh, G., and Chan, J. (1996) Cell free synthesis of the pectic polysaccharide homogalacturonan. In Pectins and Pectinases -proceedings of a conference. Anonymous Elsevier Science B.V. Amsterdam. 

Extrapolated to the pectic backbone, the present findings support previous descriptions concerning the stuctural role that may be ascribed to the presence of rhamnosyl units. When smooth homogalacturonan regions are interspersed with hairy rhamnogalacturonan regions there is no major effect on the extension and the orientation of the pectic polysaccharide. 

Pectic polysaccharides have very complex structures and are usually composed of four commonly occurring domains or blocks homogalacturonan (HG), rhamnogalacturonan I (RG-I), rhamnogalacturonan II (RG-II) and xylogalacturonan (XGA) (Ridley et al., 2001 Willats et al., 2001a Willats et al., 2006 Mohnen et al., 2008). The backbones of the different domains are... 

Figure 3.2 Structures of the pectic polysaccharides homogalacturonan (HG) and rhamnogalacturonan I (RG-I). (1 — 5)-a-Arabinan, (1 — 4)-P-galactan and arabino-4-galactan are side chains of RG-I.

Dufour and Bayonove (1999a) reported two criteria for polysaccharide discrimination acidity and protein content. Neutral peptic substances (type II arabinogalac-tans and arabinogalactans-proteins) represent 40% of the polysaccharides in wine and acidic pectic polysaccharides, (e.g. homogalacturonans and rhamnogalacturo-nans) account for 20% of them. Because of the difficulty in purifying wine polysaccharides, most of the studies on interactions between wine polysaccharides and aroma compounds have been carried out with exocellular and cell wall mannoproteins (thus mainly glycoproteins) of Saccharomyces (see effect of yeast and derivatives in the next section). 

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