Divergent modes of action on xyloglucan of two isoenzymes of xyloglucan endo-transglycosylase from Tropaeolum majus

Two isoenzymes of xyloglucan endo-transglycosylase (XET, EC 2.4.1.207) were identified in nasturtium (Tropaeolum majus L.), so far. One is located in seeds (sXET) and is expressed during germination. The other enzyme (eXET) is confined to epicotyls and other growing regions. In this work, we examined catalytic properties of the two XETs and tried to find a correlation with their presumed functions. The two enzymes had similar isoelectric points at about pH 6.5 but had different pH-activity profiles and differed in Km values for xyloglucan-derived oligosaccharides (XGOS) as acceptor substrates. Moreover, they showed clearly distinct preferences in selecting the site of attack on xyloglucan (XG) molecules. While sXET selected the site of cleavage on XG molecules stochastically along the length of their polyglucose main chain and preferred low-molecular mass (MM) XGOS as glycosyl acceptors, eXET attacked the substrate molecule predominantly near the reducing end and showed no preference as to the size of XGOS acceptors. These properties corroborate well with the proposed functions of the two isoenzymes: the sXET plays a role in degrading XG reserves in seeds during germination, whereas the eXET is engaged in cell wall rearrangement and integration of new XG molecules into the preexisting cell wall structure during growth.