Insight into multi-site mechanisms of glycosyl transfer in (1→4)β- d-glycans provided by the cereal mixed-linkage (1→3),(1→4)β- d-glucan synthase

Abstract

Synthases of cellulose, chitin, hyaluronan, and all other polymers containing (1→4)β-linked glucosyl, mannosyl and xylosyl units have overcome a substrate orientation problem in catalysis because the (1→4)β-linkage requires that each of these sugar units be inverted nearly 180° with respect to its neighbors. We and others have proposed that this problem is solved by two modes of glycosyl transfer within a single catalytic subunit to generate disaccharide units, which, when linked processively, maintain the proper orientation without rotation or re-orientation of the synthetic machinery in 3-dimensional space. A variant of the strict (1→4)β- d-linkage structure is the mixed-linkage (1→3),(1→4)β- d-glucan, a growth-specific cell wall polysaccharide found in grasses and cereals. β-Glucan is composed primarily of cellotriosyl and cellotetraosyl units linked by single (1→3)β- d-linkages. In reactions in vitro at high substrate concentration, a polymer composed of almost entirely cellotriosyl and cellopentosyl units is made. These results support a model in which three modes of glycosyl transfer occur within the synthase complex instead of just two. The generation of odd numbered units demands that they are connected by (1→3)β-linkages and not (1→4)β-. In this short review of β-glucan synthesis in maize, we show how such a model not only provides simple mechanisms of synthesis for all (1→4)β- d-glycans but also explains how the synthesis of callose, or strictly (1→3)β- d-glucans, occurs upon loss of the multiple modes of glycosyl transfer to a single one.