Submitted to: European Carbohydrate Symposium
Publication Acceptance Date: July 9, 1999
Publication Date: N/A
Abstract only

Technical Abstract: The substrate specificities of acetylxylanesterases (AcXEs) from Schizophyllum commune, Trichoderma reesei, and Streptomyces lividans were compared with those of wheat germ lipase, orange peel esterase, and Candida cylindracea lipase. Aryl acylates, acetylated methyl glycosides, and acetylxylan were used as substrates. The last three enzymes were unable to deacetylate xylan to a significant degree. AcXE from Streptomyces lividans was the only enzyme which did not attack aryl acylates. With the exception of C. cylindracea lipase, the relative activities of other tested enzymes decreased in the order 4- nitrophenyl acetate, propionate, and butyrate. AcXEs showed a preference for deacetylation of positions 2 and 3 of methyl 2,3,4-tri- O-acetyl beta-D-xylopyranoside and methyl 2,3,4,6-tetra-O-acetyl beta- D-glucopyranoside. This regioselectivity corresponds to the function of AcXEs in acetylxylan degradation and was found to be complementary to that exhibited by non-hemicellulolytic enzymes. This offers new possibilities for chemoenzymatic synthetic carbohydrate chemistry. AcXEs may utilize a different mechanism of deacetylation than other esterases. Based on kinetics of deacetylation of diacetates of methyl beta-D-xylopyranoside, it has been hypothesized that the mechanism of deacetylation by AcXEs of positions 2 and 3 when the neighboring positions (O-3 or O-2) are not acetylated might involve an enzyme- catalyzed formation of a five-membered transition state, followed by release of the acetyl group.