Ping-pong character of nasturtium-seed xyloglucan endotransglycosylase (XET) reaction

Plant xyloglucan endotransglycosylase (XET, EC 2.4.1.207) degrades its subs trate by a transglycosylation mechanism while endo-cleaving xyloglucan (XG) molecules at their beta -1,4-linked polyglucosyl main chain and transferri ng the newly generated reducing chain ends to hydroxyls at C-4 of non-reduc ing glucosyl ends of the main chains of other XG molecules or of low-M, XG- fragments (OS). Kinetic data obtained with purified nasturtium seed (Tropae olum majus, L.) XET while using high-M, xyloglucan and H-3-labeled XGOS ald itols (DP 7-9) as substrates could be best fitted to the model for Ping-Pon g Bi Bi reaction mechanism. Such mechanism is typical for transglycosylases operating with retention of the anomeric configuration of the formed glyco sidic bond and involving the formation of a covalent glycosyl-enzyme reacti on intermediate.