SUBSTRATE-SPECIFICITY AND MODE OF ACTION OF ACETYLXYLAN ESTERASE FROMSTREPTOMYCES-LIVIDANS

The substrate specificity of purified acetylxylan esterase (AcXE) from Streptomyces lividans was investigated on partially and fully acetyla ted methyl glycopyranosides. The enzyme exhibited deacetylation regios electivity on model compounds which provided insights pertaining to it s function in acetylxylan degradation, The enzyme catalyzed double dea cetylation of methyl 2,3,4-tri-O-acetyl-beta-D-xylopyranoside and meth yl 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 mono acetylated 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 grou p is released immediately after the first one is removed from the full y acetylated methyl beta-D-xylo- and -glucopyranoside. The results sug gest that in acetylxylan degradation the enzyme rapidly deacetylates m onoacetylated xylopyranosyl residues, but attacks doubly acetylated re sidues much more slowly, Evidence is also presented that the St, livid ans enzyme could be the first real substrate-specific AcXE.