pectin

A structural polysaccharide found in plant cell walls. It is rich in a (1-4) linked galacturonic acid residues, though other sugars, e.g. rhamnose, are present in small quantities. The carboxyl groups of the galacturonic acid residues in pectin may be methylated. Methyl es-terification takes place after polymerization, the methyl donor for the reaction being S-adenosyl methionine (see methionine).