Synthesis of rhamnogalacturonan U (RG-H)

RG-II is the most structurally complicated polysaccharide in the cell wall. It is present in the walls of all plants and its structure is highly conserved. RG-II makes up approximately 4% of the cell wall in dicotyledonous plants and less than 1% of the wall in monocots (157). It contains 12 different types of glycosyl residues in at least 20 different linkages (159) including both methyl etherified (e. g., 2-0 Me-xylose and 2-O Me-fucose) and O — acetylated glycosyl residues (e. g., 3-O-, or 4-0-Ac-fucose). RG-II also contains unusual sugars such as aceric acid (3-C-carboxy-5-deoxy-L-xylose) (346), KDO (2-keto-3-deoxy-D- manno-octulopyranosylonicacid (345), andDHA (3-deoxy-D-lyxo-2-heptulopyranosylaric acid) (346). RG-II has a backbone of a-1,4-linked D-galactosyluronic acid with structurally complex side chains attached to C-2 and/or C-3 (2, 157, 159, 269, 345-352). RG-II in the wall exists largely complexed with borate as an RG-II dimer that is cross-linked by a borate diester (159,352-357). At least 24 transferase activities are expected to be required for RG-II synthesis. There have been very few systematic studies of RG-II synthesis, although progress is beginning to be made on several enzymes.