Mechanism and Catalytic Site Atlas

Xyloglucan Endotransglycosylase (XET) is sourced from Populus tremula. It cleaves and religates xyloglucan (a soluble hemicellulose with a backbone composed of beta(1->4)-linked glucose residues similar to cellulose) in plant cell walls via a transglycosylation mechanism. XET is therefore a key enzyme in plant cell processes requiring cell wall remodelling, such as germination, growth, fruit ripening, organ abscission and vascular differentiation. Specifically, XET breaks a beta-(1->4) bond in the backbone of a xyloglucan and transfers the xyloglucanyl segment onto the O-4 of the non-reducing terminal glucose residue of an acceptor. This acceptor can be a xyloglucan or an oligosaccharide of xyloglucan. During processes such as fruit ripening, hydrolytic enzymes can be produced to degrade the cell wall. After cell wall expansion and elongation, cell wall loosening is a temporary requirement that must be followed by reinforcement of the cell wall. In the first step XET catalyses endolytic cleavage of a cross-linking xyloglucan polymer, which permits cellulose microfibres to separate allowing cell expansion. In the second step XET transfers the newly generated end to another sugar polymer, restoring stable cell wall structure. XET requires sugar residues in both the donor and acceptor sites before it is able to catalyse the reaction.