Unique mode of acetylation of oligosaccharides in aqueous two-phase system by Trichoderma reesei acetyl esterase

Trichoderma reesei RUT C-30 acetyl esterase, known to catalyze transacetylation reactions in water/vinyl acetate two-phase mixtures, was studied with respect to regioselectivity of acetylation of oligosaccharides in aqueous environment. Using series of oligosaccharides and their methyl glycosides, it was found that the enzyme catalyzes an efficient acetylation at O-3 position of the non-reducing terminal units of gluco-, xylo- and manno-oligosaccharides and a less efficient acetylation of O-2 position of the reducing end units of gluco- and xylo-oligosaccharides. The axial hydroxyl group at O-2 position of the reducing end mannose in mannooligosaccharides was not recognized by the enzyme and its acetylation was not observed. The structure of isolated transacetylation products was established by NMR, ESI-MS analysis and on the basis on their resistance towards action of glycosidases acting from the non-reducing end of oligosaccharides. The position of acetylation allowed deduce on some of the structural requirements of the enzyme for the acetyl group acceptors. T. reesei RUT C-30 acetyl esterase was also found to be capable of liberation of acetyl groups from terminal units of oligosaccharides, which speaks for its classification as an exo-acting acetyl esterase.