Submitted to: European Journal of Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 2, 1996
Publication Date: N/A
Interpretive Summary: There is a need for mild, specific methods which create modified carbohydrates for adding value to farm products, particularly processed grain fractions. Certain commercially available enzymes are useful (environmentally benign or "green" technology) for making some modifications. However, most of the previous work has dealt with the use of enzymes that 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. We found they act differently from previously used enzymes and present a new tool in the arsenal of methods for creating carbohydrate-based materials. This work will benefit specialty chemical producers, the paper industry, as well as chemists working on medically useful compounds. It will also help scientists 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.
Substrate specificity of a purified acetylxylan esterase from Schizophyllum commune was investigated on a variety of methyl per-O- acetyl glucopyranosides, methyl di-O-acetyl-beta-D-xylopyranosides and acetylated polysaccharides. The enzyme preferentially deacetylated the 3-position of methyl 2,3,4,-tri-O-acetyl-beta-D-xylopyranoside and 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranoside. Removal of the 3-acetyl group from the xylopyranoside was accompanied by a slower deactylation at positions 2 and 4. A similarly slower, accompanying deactylation occurred primarily at position 2 with the glucopyranoside. Such specificity corresponds well to the expected function of the esterase in acetylxylan degradation. Of the three possible diacetates of methyl beta-D-xylopyranoside, the 3,4-diacetate was found to be the most rapidly deactylated. Unexpectedly, products of its deacetylation were a mixture of 2- and 4-monoacetate. The formation of the methyl 2-O-acetyl-beta-D-xylopyranoside involved an enzyme-mediated acetyl group transfer because the rate of the enzyme-catalyzed reaction exceeded the rate of spontaneous migration of acetyl groups. This is the likely mechanism for acetyl removal from position 2 in the native substrate. The enzyme exhibited the highest regioselectivity with methyl 2,3,4,6-tetra-O-acetyl-beta-D-mannopyranoside. An 80% conversion of this substrate to methyl 4,6-di-O-acetyl-beta-D- mannopyranoside, a new mannose derivative, was achieved. The esterase was also active on soluble acetylated polysaccharides other than acetylxylan, but it did not show appreciable activity on insoluble acetylated polysaccharides.