Homogalacturonans

For the homogalacturonan, the activity coefficient of sodium is 0.54 but that of calcium 0.12, in very dilute solution, indicating a dimer formation. The activity coefficient Y is directly imposed by X, with ... 

With different pectins, one found that the activity coefficient of calcium has a value half that of magnesium this is interpreted as the basis of a dimer formation in presence of calcium. The specific interaction of calcium was described as the egg-box model first proposed for polyguluronate in which oxygen atoms coordinated to calcium [46]. Recently, the comparative behaviour of Mg and Ca with homogalacturonan was reexamined [47]. 

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. 

Other polysaccharides of primary cell walls.-A complex mixture of enzymes including endopolygalacturonase, pectin methylesterase, and/or pectin lyase solubilizes a mixture of polysaccharides from the primary cell walls of fruits [57-64]. Food scientists have referred for some 15 years to this mixture of polysaccharides as the hairy region to describe the highly branched character of the polysaccharides in the fraction and to emphasize the contrast to unbranched homogalacturonan. The recent discovery of rhamnogalacturonan hydrolase [65,66], which selectively cleaves the backbone of RG-I, led to the realization that the hairy... 

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... 

Figure 1. The model structure of plant RG-11 [1, 13] with four oligoglycosidic side-chains linked to the homogalacturonan backbone.

Our two network model of the primary wall receives support from a variety of indirect observations. For example it has been shown Aat when a cell wall is regenerated by a carrot protoplast a homogalacturonan/ rhamnogalacturonan shell is laid down first, through which the cellulose/ hemicellulose network is later intercalated (8). Further evidence that pectin may form an independent network is seen in the fact that walls from suspension-cultured cells of tofnato Lycopersicon esculentum VF 36),... 

Cell free synthesis of the pectic polysaccharide homogalacturonan... 

This paper begins with a brief description of pectin structure and an overview of the general mechanism of cell wall polysaccharide biosynthesis. This is followed by a summary of previous research on PGA-GalAT and a description of a facile method to synthesize UDP-[ Cj-galacturonic acid. Finally, the paper ends with a summary of our work on the identificadon, partial characterization, and initial solubilization of the homogalacturonan biosynthetic enzyme PGA-GalAT. 

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