Insight into multi-site mechanisms of glycosyl transfer in (1 -> 4)beta-D-glycans provided by the cereal mixed-linkage (1 -> 3),(1 -> 4)beta-D-glucansynthase

Synthases of cellulose, chitin, hyaluronan, and all other polymers containi ng (1 -->4)beta -linked glucosyl, mannosyl and xylosyl units have overcome a substrate orientation problem in catalysis because the (1 -->4)beta -link age requires that each of these sugar units be inverted nearly 180 degrees with respect to its neighbors. We and others have proposed that this proble m is solved by two modes of glycosyl transfer within a single catalytic sub unit to generate disaccharide units, which, when linked processively, maint ain the proper orientation without rotation or re-orientation of the synthe tic 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 g rowth-specific cell wall polysaccharide found in grasses and cereals. beta -Glucan is composed primarily of cellotriosyl and cellotetraosyl units link ed by single (1 -->3)beta -D-linkages. In reactions in vitro at high substr ate concentration, a polymer composed of almost entirely cellotriosyl and c ellopentosyl units is made. These results support a model in which three mo des of glycosyl transfer occur within the synthase complex instead of just two. The generation of odd numbered units demands that they are connected b y (1 -->3)beta -linkages and not (1 -->4)beta-. In this short review of bet a -glucan synthesis in maize, we show how such a model not only provides si mple mechanisms of synthesis for all (1 -->4)beta -D-glycans but also expla ins how the synthesis of callose, or strictly (1 -->3)beta -D-glucans, occu rs upon loss of the multiple modes or glycosyl transfer to a single one, (C ) 2001 Published by Elsevier Science Ltd.