Deconstructing RG-II, the Most Structurally Complex Glycan

Walls of growing plants are extremely complex and sophisticated composite materials incorporating a dynamic assembly of polysaccharides, proteins and phenolics. Among the polysaccharides, the pectins encompass a group of acidic heteropolysaccharides ; they offer a repertoire of structural complexity associated with the occurrence of, at least, three specific domains. Whereas most of these domains are notable for their structural heterogeneity, one of these, the so-called rhamnogalacturonan II (RG-II) exhibits a remarkable conservation throughout the plant kingdom. RG-II is thought to be the most complex plant polysaccharide on Earth (MW 5–10 kDa) ; its occurrence and strong conservation may indicate that it plays a major role in the structure and growth of higher plant.

The authors of the work report that a single gut bacteria from the Bacteroides phylum metabolizes this complex substrate by cleaving all but one of the 21 glycosidic linkages. This degradation process involves seven newly discovered families of glycoside hydrolases, as well as novel catalytic functions of several known enzymes. This work delves deep into the mechanisms of glycan degradation by bacteria in the human gut microbiota and how they have evolved with this degradation to utilize rare sugars. The work establishes new structural features, such as the occurrence of a previously unknown lateral branch of RG-II, and show that some two glycosidic linkages have a different spatial arrangement of atoms from that previously thoughts.