Anomeric Selectivity of Trehalose Transferase with Rare l-Sugars.

Retaining LeLoir glycosyltransferases catalyze the formation of glycosidic bonds between nucleotide sugar donors and carbohydrate acceptors. The anomeric selectivity of trehalose transferase from Thermoproteus uzoniensis was investigated for both D- and L-glycopyranose acceptors. The enzyme couples a wide range of carbohydrates, yielding trehalose analogues with conversion and enantioselectivity of >98%. The anomeric selectivity inverts from alpha,alpha-(1 -> 1)-glycosidic bonds for D- glycopyranose acceptors to alpha,beta-(1 -> 1)-glycosidic bonds for L-glycopyranose acceptors, while (S)-selectivity was retained for both types of sugar acceptors. Comparison of protein crystal structures of trehalose transferase in complex with alpha,alpha-trehalose and an unnatural alpha,beta-trehalose analogue highlighted the mechanistic rationale for the observed inversion of anomeric selectivity.