Itineraries of enzymatically and non-enzymatically catalyzed substitutions at O-glycopyranosidic bonds

Several crystal structures of glycoside hydrolases in complex with a substrate analog, or of inactive mutants complexed with a substrate, reveal non-ground state carbohydrate conformations within subsite -1 of the active site. These "frozen" local minima as preferred by the enzyme represent pre-transition-state situations along the reaction itinerary. Substantiated by theoretical considerations, this leads to the proposal that substitutions on beta-equatorial as well as alpha-axial D-O-glycopyranosidic bonds may always follow an itinerary that is predetermined by the original configuration at the anomeric center, where the formation of a transition state that is similar to a half-chair is always preceded by a conformational change away from the ground state.