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2.3 Glycosyl transferases

In contrast to synthases, glycosyl transferases are defined as enzymes that non-iteratively add glycosyl residues to backbone polymers. Whereas synthase activities are difficult to demonstrate in vitro, much less solubilize their activities for enzyme purification, several glycosyl transferases have been identified after solubilization and demonstration of specific transfer in vitro. Examples include the xyloglucan-specific glycosyl transferases for fucose and galactose residues that decorate xyloglucans of type I walls. These have been identified on the basis of mutants in which cell-wall fucosyl residues were underrepresented. A gene encoding a a-xylosyl transferase that adds the first xylosyl residue of xyloglucan has also been deduced based on the identification of an ß-galactosyl transferase involved in the decoration of galactomannans. Most of the cell-wall glycosyl transferases annotated thus far are type II membrane, Golgi-resident enzymes, with single membrane spans orienting the catalytic domains into the lumen of the Golgi membrane. The gene families represented here are based on motif structure and amino-acid similarities described by Henrissat and represented at the Carbohydrate Active Enzymes web site (CAZy). Readers should note that these classifications do not specify the monosaccharide transferred, as different monosaccharide transferases may belong to the same sub-class, and most of them have not been characterized.

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