ARS | Publication request: DIFFERENCES IN CATALYTIC PROPERTIES OF ACETYLXYLAN ESTERASES AND NON- HEMICELLULOLYTIC ACETYLESTERASES

Submitted to: Carbohydrate International Symposium
Publication Acceptance Date: June 30, 1999
Publication Date: N/A

Interpretive Summary: New markets for agricultural commodities are needed. This work addresses a particular need for mild, specific methods to make sugar derivatives from agriculturally-derived carbohydrates. Certain commercially available enzymes are useful for making some types of derivatives. However, most of the work has dealt with the use of enzymes that normally act on fats and proteins, rather than on carbohydrates. Taking advantage of a newly described class of enzymes that act specifically on carbohydrates, we have described their action on certain carbohydrate derivatives. In this comparative study, we found that they act differently from the previously used enzymes, and present chemists with a new tool in the arsenal of methods for creating carbohydrate-based materials. This work will especially benefit chemists working on medically useful carbohydrate analogs, as well as specialty chemicals such as detergents. It will also help us to understand the manner in which plant materials are broken down in nature, since the enzymes are involved in the breakdown of agricultural residues and wood pulp.

Technical Abstract: The substrate specificities of acetylxylanesterases (AcXEs) from Schizophyllum commune, Trichoderma reesei and Streptomyces lividans were compared with the specificities of wheat germ lipase, orange peel esterase, and Candida cylindracea lipase. Investigated substrates included aryl acylates, acetylated methyl glycosides, and acetylxylan. The latter three enzymes were unable to deacetylate xylan to any significant degree. Only AcXE from Streptomyces lividans did not hydrolyze aryl acylates. With the exception of C. cylindracea lipase, the relative activities of the enzymes on aryl acylates were 4- nitrophenyl acetate > propionate > butyrate. The AcXEs showed preference for deacetylation at 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 regiospecificity corresponded to the function of the AcXEs in acetylxylan degradation and was found to be complementary to that exhibited by the non-hemicellulolytic enzymes. This complementation offers new possibilities for chemoenzymic synthetic carbohydrate chemistry. Based on the kinetics of deacetylation of diacetates of methyl beta-D-xylopyranoside, it has been hypothesized that the mechanism of deacetylation by AcXEs at positions 2 and 3, when the neighboring positions (O-3 and O-2) are not acetylated, involves enzyme-catalyzed formation of a five-membered transition state from which the acetyl group is subsequently released.