Polysaccharide-Modifying Enzymes in the Plant Cell Wall

Enzymes found in the cell walls of higher plants are surveyed briefly, especially those in the primary cell walls of growing tissue. Attention is focused on hydrolases and transglycosylases that attack the structural carbohydrates of the wall. The reactions catalyzed by these enzymes in vitro are described. Exo-O-glycosylhydrolases are divided into two categories based on specificity for the aglycone group of the substrate: Those with low specificity are in the majority over those with high specificity. Endo-O-glycosylhydrolase-catalyzed reactions are discussed, especially the ability of cellulase to hydrolyze cellulose, xyloglucan, and mixed-linkage glucan. Transglycosylation is defined, and the distinction is drawn between exo- and endo-transglycosylation. Enzymes that catalyze only exo-transglycosylation have not been found in plant cell walls, although some exo-O-glycosylhydrolases can catalyze limited exo-transglycosylation under suitable conditions. In contrast, xyloglucan endo-transglycosylase (XET) catalyzes endo-transglycosylation in the absence of hydrolysis. The evidence that these enzymes act in the walls of living cells is evaluated critically, and the natural and biotechnological regulation of wall enzyme action is discussed. Many enzymes occur in cell walls, and many biological roles have been proposed for them, but we still have far to go in investigating the reactions catalyzed by wall enzymes in vivo and in testing the proposed roles of these enzymes.