Complex pectins: Structure elucidation using enzymes

A pectic fraction, retained by ultrafiltration of the juice from enzyme treated apple tissue and resistant to further enzymic degradation, was isolated and characterized using chemical and enzymic methods. The fraction was termed MHR (modified hairy regions) and this fraction was characterized by a high arabinose content, next to a high rhamnose to galacturonic acid ratio and a high acetyl content and smaller proportions of xylose and galactose. Rhamnogalacturonase (RGase), an enzyme able to hydrolyze galacturonic acid-(1-->2)-rhamnosyl linkages within the rhamnogalacturonan backbone of MHR was used to obtain both oligomeric and polymeric degradation products. These RGase-oligomers consist of a tetrameric or hexameric backbone of alternating rhamnose and galacturonic acid residues with a galactose residue substituted at C-4 of part of the rhamnose moieties. Next to the subunit from which these oligomers were released, two other subunits were recognized: a highly methyl esterified xylogalacturonan segment and residual stubs of the backbone rich in branched arabinose side chains. Comparison of the MHR with non-modified pectic hairy regions of apple cell wall, isolated in a mild and defined way, revealed great resemblance indicating that the modifications of the MHR during enzymic liquefaction were only minor. Analogous MHR fractions could be isolated from potato fibre, pear, carrot, leek, and onion tissue. Finally, an adapted model is presented for the prevailing population of apple MHR having the highest molecular weight. The universal validity of this model for pectic hairy regions from other plant sources is discussed.