Plant primary cell walls are hydrated extracellular complexes composed largely of
polysaccharides: cellulose, hemicellulose and pectin. Cell wall constituents and
composition vary in cell-, environment-, and species-dependent manners. For
example, within land plant hemicelluloses xyloglucan is ubiquitous while mixedlinkage
(1→3),(1→4)-β-D-glucan (MLG) is found only in the Poales and Equisetum.
Glycosyl hydrolase 16 (GH16) enzyme family members include numerous enzymes
with pertinence to the understanding of the ‘lives’ of cell wall hemicelluloses.
However, despite this, the details of the interactions between GH16 enzymes and their
substrates have often not been elucidated. Likewise, the true preferences of many of
these enzymes and the range of substrates which they can utilise remain to be fully
explored. By providing a greater wealth of information for the correlation of enzyme
structure with reaction catalysed, such an understanding would enable better
predictions of the activities of novel enzymes. Crucially, this would also allow better
identification of roles performed by these enzymes in planta as well as of the potential
applications of these enzymes.
This work sought to further our understanding of the interactions between GH16
enzymes and their substrates by the study of five activities exhibited by GH16
enzymes – xyloglucan endotransglucosylase (XET), xyloglucan
endoglucanase/hydrolase (XEG/XEH), mixed-linkage glucan : xyloglucan
endotransglucosylase (MXE), lichenase and cellulose : xyloglucan
endotransglucosylase (CXE). All of the analysed activities act on xyloglucan and/or
MLG. Of particular focus is the novel enzyme MXE from the evolutionarily isolated
genus Equisetum (horsetail), which acts on both. Notable findings include:
identification of MXE/CXE gene; determination of the substrate specificity of MXE;
defining of the sites of attack of lichenase, XEG, XET and MXE; discovery of novel
xyloglucan structures and discrepancies between the xyloglucan present in different
barley organs