Pectic polysaccharide rhamnogalacturonan II is covalently linked to homogalacturonan.
A borate-containing pectin was solubilized from sugar beet (Beta vulgaris L. ) cell walls by treatment with 0.5 M imidazole, pH 7. The molecular weight of the pectin was reduced when the borate ester was hydrolyzed by treatment with 1 N HCl. Treatment of the acid-treated pectin with boric acid in the presence of Pb(2+) gave a product whose molecular weight distribution was similar to the imidazole-soluble pectin. The imidazole-soluble pectin was saponified and then digested with endo- and exo-polygalacturonases. These treatments shifted the boron peak at the high molecular weight region to the low molecular weight (10 kDa), which corresponds to rhamnogalacturonan II-borate ester cross-linked dimer (dRG-II-B). The treatment also generated rhamnogalacturonan I (RG-I), dRG-II-B, monomeric rhamnogalacturonan II and galacturonic acid. These results show that imidazole solubilizes a high molecular weight borate-containing pectic complex composed of homogalacturonan-rhamnogalacturonan II and RG-I. Our data suggest that borate esters formed between rhamnogalacturonan II molecules cross-link the macromolecular pectin.
Ishii
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Forestry and Forest Products Research Institute, PO Box 16, Tsukuba Norin Kenkyu, Danchi-nai, 305-8687, Ibaraki, Japan. tishii@ffpri.affrc.go.jp
Matsunaga
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D001881:Borates; D002473:Cell Wall / Q000737:chemistry; D019663:Chenopodiaceae / Q000737:chemistry*; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D019281:Dimerization; D006868:Hydrolysis; D008970:Molecular Weight; D009005:Monosaccharides / Q000032:analysis; D010368:Pectins / Q000737:chemistry* / Q000302:isolation & purification; D011096:Polygalacturonase
D001881:Borates; D009005:Monosaccharides; C042492:rhamnogalacturonan II; D010368:Pectins; D011096:Polygalacturonase; C003181:polygalacturonic acid