Structures, Biosynthesis, and Physiological Functions of (1,3;1,4)-β-D-Glucans

Abstract

(1,3;1,4)-β-D-Glucans, also named as mixed-linkage glucans, are unbranched non-cellulosic polysaccharides containing both (1,3)- and (1,4)-β-linkages. The linkage ratio varies depending upon species origin and has a significant impact on the physicochemical properties of the (1,3;1,4)-β-D-glucans. (1,3;1,4)-β-D-Glucans were thought to be unique in the grasses family (Poaceae); however, evidence has shown that (1,3;1,4)-β-D-glucans are also synthesized in other taxa, including horsetail fern Equisetum, algae, lichens, and fungi, and more recently, bacteria. The enzyme involved in (1,3;1,4)-β-D-glucan biosynthesis has been well studied in grasses and cereal. However, how this enzyme is able to assemble the two different linkages remains a matter of debate. Additionally, the presence of (1,3;1,4)-β-D-glucan across the species evolutionarily distant from Poaceae but absence in some evolutionarily closely related species suggest that the synthesis is either highly conserved or has arisen twice as a result of convergent evolution. Here, we compare the structure of (1,3;1,4)-β-D-glucans present across various taxonomic groups and provide up-to-date information on how (1,3;1,4)-β-D-glucans are synthesized and their functions.

Keywords: (1,3; 1,4)-β-D-glucan; cell wall; mixed-linkage glucan; polysaccharide.

Figure 1

Structures of 1,3-β-d-glucan and (1,3;1,4)-β-d-glucans. Lichenase is an endo-hydrolase that hydrolyses the (1,3;1,4)-β-d-glucans into smaller oligomers, such as degree of polymerization 2 (DP2), DP3 and DP4 shown above. The blue dash lines represent the enzymatic cleavage sites. Oligosaccharide profiles in DPs vary depending on the taxonomic origin of (1,3;1,4)-β-d-glucans.

Figure 2

Cell wall architecture of Gram-negative bacteria Sarcina ventriculi and Gram-negative bacteria Sinohizobium meliloti. Both species have unique capsular polysaccharide (1,3;1,4)-β-d-glucans deposited on the surface of bacteria.