Plants homogalacturonan

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.

Willats WGT, Gilmartin PM, Mikkelsen JD, Knox JP. Cell wall antibodies without immunization generation and use of de-esterified homogalacturonan block-specific antibodies from a naive phage display library. Plant J. 1999 18 57-65. 

GOUBET, F., COUNCIL, L. N., MOHNEN, D., Identification and partial characterization of the pectin methyltransferase "homogalacturonan-methyltransferase from membranes of tobacco cell suspensions., Plant Physiol., 1998,116,337-347. 

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. 

Vincken J-P, Schols HA, Oomen RJFJ et al (2003) If homogalacturonan were a side chain of rhamnogalacturonan I. Implications for cell wall architecture. Plant Physiol 132 1781-1789... 

Figure 1. (A). A diagrammatic representation of a plant cell. The cytoplasm is bounded by the plasma membrane which is itself surrounded by the primary wall. The region where the primary walls of two cells abut one another is called the middle lamella. Air spaces are often present particularly at the junction of cell comers. Abbreviations used—Pla = plastid Chi = chloroplast Mit = mitochondrion Nuc = nucleus Vac = vacuole Cyt = cytoplasm Pdm = plasmodesmata. (B). A simplified model of a possible arrangement of the major wall components. Wall proteins have been omitted for clarity. Abbreviations used HG = homogalacturonan RG-I = rhamnogalacturonan I RG-II = rhamnogalacturonan II B-ester = borate ester cross-link of RG-II XG xyloglucan...

Homogalacturonan is a linear chain of 1,4-linked a-D-GalpA residues in which many of the carboxyl groups are methyl esterified (Figure 4A). Homogalacturonan may, depending on the plant source, also be partially 0-acetylated [10],... 

A large number (>50) of glycosyltransferases and other types of transferase are required to synthesize pectic polysaccharides [20]. The activities of several glycosyltransferases have been identified in cell-free membrane preparations from plants. However, only a few of these enzymes have been partially characterized and shown to be involved in pectin biosynthesis. These include D-galacturonosyl transferase, galactosyl transferase, arabinosyl transferase, and apiosyl transferase [20]. An enzyme that catalyzes the methylesterification of homogalacturonan (homogalatur-onan methyltransferase) has been partially characterized [20]. There are also reports that plants contain methyltransferases that catalyze the methylesterification of RG-I and RG-II [25]. No 0-acetyl transferase involved in pectin biosynthesis has been characterized nor have the enzymes that catalyze the addition of phenolic acids to pectins [20]. 

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