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

Synthases of cellulose, chitin, hyaluronan, and all other polymers containing (1 -->4)beta -linked glucosyl, mannosyl and xylosyl units have overcome a substrate orientation problem in catalysis because the (1 -->4)beta -linkage requires that each of these sugar units be inverted nearly 180 degrees 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)beta -D-linkage structure is the mixed-linkage (1 -->3),(1 -->4)beta -D-glucan, a growth-specific cell wall polysaccharide found in grasses and cereals. beta -Glucan is composed primarily of cellotriosyl and cellotetraosyl units linked by single (1 -->3)beta -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)beta -linkages and not (1 -->4)beta-. In this short review of beta -glucan synthesis in maize, we show how such a model not only provides simple mechanisms of synthesis for all (1 -->4)beta -D-glycans but also explains how the synthesis of callose, or strictly (1 -->3)beta -D-glucans, occurs upon loss of the multiple modes or glycosyl transfer to a single one, (C) 2001 Published by Elsevier Science Ltd.