Trapping and characterization of the reaction intermediate in cyclodextrin glycosyltransferase by use of activated substrates and a mutant enzyme

Cyclodextrin glycosyltransferases (CGTases) catalyze the degradation of starch into linear or cyclic oligosaccharides via a glycosyl transfer reaction occurring with retention of anomeric configuration. They are also shown to catalyze the coupling of maltooligosaccharyl fluorides. Reaction is thought to proceed via a double-displacement mechanism involving a covalent glycosyl-enzyme intermediate. This intermediate can be trapped by use of 4-deoxymaltotriosyl alpha-fluoride (4DG3 alpha F). This substrate contains a good leaving group, fluoride, thus facilitating formation of the intermediate, but cannot undergo the transglycosylation step since the nucleophilic hydroxyl group at the 4-position is missing. When 4DG3 alpha F was reacted with wild-type CGTase (Bacillus circulans 251), it was found to be a slow substrate (k(cat) = 2 s(-1)) compared with the parent glycosyl fluoride, maltotriosyl alpha-fluoride (k(cat) = 275 s(-1)). Unfortunately, a competing hydrolysis reaction reduces the lifetime of the intermediate precluding its trapping and identification. However, when 4DG3 alpha F was used in the presence of the presumed acid/base catalyst mutant Glu257Gln, the intermediate could be trapped and analyzed because the first step remained fast while the second step was further slowed (k(cat) = 0.6 s(-1)). Two glycosylated peptides were identified in a proteolytic digest of the inhibited enzyme by means of neutral loss tandem mass spectrometry. Edman sequencing of these labeled peptides allowed identification of Asp229 as the catalytic nucleophile and provided evidence for a covalent intermediate in CGTase. Asp229 is found to be conserved in all members of the family 13 glycosyl transferases.