Structural features of the plant cell-wall polysaccharide rhamnogalacturonan-II

Abstract

Rhamnogalacturonan-II (RG-II), isolated from the cell walls of suspension-cultured sycamore cells, has been further characterized. End-group analysis of RG-II showed that the polysaccharide contains about 30 glycosyl residues. Some 28 residues have been found as constituents of well characterized oligosaccharide fragments of RG-II. RG-II was treated with lithium metal dissolved in ethylenediamine to degrade the glycosyluronic acid residues. The major product was isolated, characterized, and shown to be the triglycosylalditol α-Xyl-(1→3)-α-Fuc-(1→4)-β-Rha-(1→3 1)-apiitol. This tetrasaccharide fragment of RG-II has three residues in common with a previously characterized heptasaccharide that had been derived from RG-II by partial hydrolysis with acid. RG-II was found to contain a large number of branched galactosyluronic acid residues that have not yet been identified as components of oligosaccharide fragments, although they are undoubtedly part of an octa(galactosyluronic acid) fragment generated by partial acid hydrolysis. The results of sequential partial acid hydrolysis provided evidence that, in RG-II, the extremely acid-labile 3-deoxy- d- manno-2-octulosonic-acid and 3-deoxy- d- lyxo-2-heptulosaric acid residues are attached to O-3 of 3,4-linked galactosyluronic acid residues, and that the mildly acid-labile apiofuranosyl residues are attached to O-2 of 2,4-linked galactosyluronic acid residues. These and previously published data suggested that RG-II has a highly branched structure, arranged around an α-(1→4)-linked galactosyluronic acid backbone.