Analysis of Pectin Structure by HPAEC-PAD

Abstract

Pectins are complex, branched, acidic polysaccharides that are located throughout the plant primary cell wall matrix. Pectin structure in cell walls has been described as block-like (Jarvis 1984) in which smooth and hairy regions exist (de Vries et al. 1982). The smooth pectin regions consist of homogalacturonan (McNeil et al. 1984). Variation in the degree of methyl-esterification exists within pectin, with calcium mediating the cooperative cross-linking of pectin chains containing seven adjacent nonesterified galacturonic acid residues (Powell et al. 1982). When pectin is treated with endo-polygalacturonase (endo-PG¹), the homogalacturonan degrades to oligogalacturonic acids leaving the hairy, branched pectic regions unhydrolyzed (reviewed by O’Neill et al. 1990). The hairy regions of pectin consist of rhamnogalacturonan I (RG-I), rhamnogalacturonan II (RG-II), xylogalacturonan, and RG with arabinan side chains that can be separated based on size following endo-polygalacturonase (O’Neill et al. 1990) or rhamnogalacturonase (Voragen et al. 1993) hydrolysis. RG-I consists of alternating rhamnose and galacturonic acid residues in the backbone. Arabinose- and galactose-containing side chains are attached to the rhamnose residues. RG-II, from the cell walls of suspension-cultured sycamore cells, consists of a not fully characterized structure that contains unusual sugars such as apiose, 3-deoxy-d-manno-octulosonic acid (KDO), aceric acid, methyl-fucose, and methyl-xylose. Xylogalacturonan, present in the modified hairy regions (MHR, residue remaining after enzymatic liquefaction of pectin) of apple pectin, consists of a galacturonan backbone with each galacturonic acid residue substituted at the 3-position with a single β-linked xylose residue (Voragen et al. 1993).