Substrate specificity and mode of action of acetylxylan esterase from Streptomyces lividans

The substrate specificity of purified acetylxylan esterase (AcXE) from Streptomyces lividans was investigated on partially and fully acetylated methyl glycopyranosides. The enzyme exhibited deacetylation regioselectivity on model compounds which provided insights pertaining to its function in acetylxylan degradation. The enzyme catalyzed double deacetylation of methyl 2,3,4-tri-O-acetyl-beta-D-xylopyranoside and methyl 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranoside at positions 2 and 3. Two methyl xylopyranoside diacetates, which had a free hydroxyl group at position 2 or 3, i.e. the derivatives that most closely mimic monoacetylated xylopyranosyl residues in acetylxylan, were deacetylated 1 to 2 orders of magnitude faster than methyl 2,3,4-tri-O-acetyl-beta-D-xylopyranoside and methyl 2,3-di-O-acetyl-beta-D-xylopyranoside. These observations explain the double deacetylation. The second acetyl group is released immediately after the first one is removed from the fully acetylated methyl beta-D-xylo- and -glucopyranoside. The results suggest that in acetylxylan degradation the enzyme rapidly deacetylates monoacetylated xylopyranosyl residues, but attacks doubly acetylated residues much more slowly. Evidence is also presented that the St. lividans enzyme could be the first real substrate-specific AcXE.