Mechanisms of enzymatic glycoside hydrolysis

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“…Taken together with NMR and mass-spectrometric evidence for a covalent intermediate in the a-amylase family (see ref. 11 for an overview), this result definitively shows that the intermediate in the a-retaining mechanism is not a charged oxocarbenium ion 4 . Previously, one of Fig.…”

“…Ever since the structure of lysozyme was determined 1 , the enzyme mechanisms of glycosyl hydrolases and transferases have aroused wide interest [2][3][4] . Using similar chemical principles, these enzymes process a wide variety of carbohydrate polymers, many of which are important as nutrients, cell wall components or signal transmitters 5 .…”

“…Using similar chemical principles, these enzymes process a wide variety of carbohydrate polymers, many of which are important as nutrients, cell wall components or signal transmitters 5 . Glycosyl hydrolases and transferases employ four basic mechanisms, named according to the anomeric configuration of the substrate (a or b-glycosidic bonds) and the stereochemical outcome of the reaction (retention or inversion) 4 . The b-retaining mechanism of hen egg white lysozyme is a wellknown example 6,7 .…”

“…1) 4,10 . In the first step, CGTase cleaves an a(1-4) glycosidic bond in its substrate, starch, and forms a covalent b(1-4)-linked glycosyl-enzyme intermediate 11 .…”

“…CGTase is unique in its ability to use the free hydroxyl at the non-reducing end of the intermediate letters sugar chain as an acceptor, thus forming a cyclic oligoglucoside, a cyclodextrin. Both steps of the mechanism go through highenergy oxocarbenium-like transition states 4 , in which the reaction center of the sugar is planar and positively charged 12 . Two active site amino acids play distinct roles in catalysis.…”