Rhamnogalacturonan borate ester

M. A. O Neill, D. Warrenfeltz, K. Kates, P. Pellerin, T. Doco, A. G. Darvill, and P. Alber-sheim, Rhamnogalacturonan-II, a pectic polysaccharide in the walls of growing plant cell, forms a dimer that is covalently cross-linked by a borate ester. In vitro conditions for the formation and hydrolysis of the dimer, J. Biol. Chem., 271 (1996) 22923-22930. 

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...

Borate diol ester linkage between apiose rings in oligosaccharide clusters of rhamnogalacturonan II in two pectin chains. 

Diatoms also require boron, which is incorporated into the silicon-rich cell walls.3 Some strains of Streptomyces griseus produce boron-containing macrolide antibiotics such as aplasmomycin (right).0 Recently a function in plant cell walls has been identified (see also main text) as crosslinking of rhamnogalacturonan portions of pectin chains by borate diol ester linkages as illustrated. 

Plants contain signiFcant concentrations of polysaccharides of which the potentially negatively charged oxygen functions can bind cations electrostatically or chelate them via polyhydroxy groups [89]. Particular attention was attracted by a structurally complex pectic polysaccharide rhamnogalacturonan-II (RG-II) [90]. This ubiquitous component of primary plant cell walls forms dimers cross-linked by 1 2 borate diol esters (dRG-II) that were found to complex in vitro sped be divalent cations and the majority of Ba, Pb, Sr, and rare earth elements (REEs) in fruit and vegetables [45, 91]. 

Figure 5. (A). A partial structure of rhamnogalacturonan II. The backbone of RG-II, released by endopolygalacturonase treatment of primary walls, is composed of between 8 and 15 4-linked a-D GalpA residues. Four side chains (I IV) are known to be attached to the backbone, although their locations relative to one another are not known. (B). The 1 2 borate-diol cross-link of RG-II. A single 1 2 borate-diol ester is believed to cross-link two RG-II monomers. The cross-link is located in each monomer between the apiosyl residues of the 2-0-Me xylose-containing side chain (side chain I in A).

M. Kobayashi, T. Match, J. Azuma. Two chains of rhamnogalacturonan-II are cross-linked by borate-diol ester bonds in higher plant cell-walls. Plant Physiol, 1996,110, 1017 1020... 

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