Rhamnogalacturonan-I: A Structurally Puzzling and Functionally Versatile Polysaccharide from Plant Cell Walls and Mucilages

Rhamnogalacturonans-I (RGs-I) are complex structural components of the (primary) cell wall. Their backbone, composed of the repeating diglycosyl [-> 2)-alpha-L-Rhap-(1 -> 4)-alpha-D-GalpA-(1 ->], is believed to be branched at O-4/O-3 positions by 4 different side chain types, viz. (1 -> 5)-alpha-L-arabinan, (1 -> 4)-beta-D-galactan, arabinogalactan-I, and sometimes with arabinogalactan-II. However, the fine structure of RGs-I remains somewhat enigmatic, as shown by various continuous findings, such as branches of galactoarabinan, arabinogalactan with a (1 -> 6)-beta-D-galactan core, and possibly (1 -> 3)-rhamnan. This timely review is the first of its kind, highlighting the freak structural diversity and the functional versatility of RG-I, one of the two major structural domains of complex pectins.