ALBERT

Description: Background Monoacetylated xylosyl residues of the main hardwood hemicellulose acetylglucuronoxylan undergo acetyl group migration between positions 2 and 3, and predominantly to position 4 of the non-reducing end xylopyranosyl (NRE-Xyl p ) residues which are amplified by saccharifying enzymes. On monoacetylated non-reducing end xylopyranosyl (NRE-Xyl p ) residues of xylooligosaccharides the acetyl group migrates predominantly to position 4 and hinders their hydrolysis by β-xylosidase. Methods Acetyl migration on the NRE-Xyl p residues and their enzymatic deacetylation by various xylan deacetylases was followed by 1 H-NMR spectroscopy and TLC. Results A 5-min heat treatment of 4-nitrophenyl 3- O -acetyl-β-D-xylopyranoside was sufficient to establish equilibrium between monoacetate derivatives acetylated at positions 2, 3 and 4. Rapid acetyl migration along the NRE-Xyl p ring at elevated temperature was confirmed in derivatives of methyl β-1,4-xylotrioside (Xyl 3 Me) monoacetylated solely on the NRE-Xyl p residue, the analogues of naturally occurring acetylated xylooligosaccharides. The Xyl 3 Me monoacetates were used as substrates to study regioselectivity of the NRE-Xyl p residue deacetylation by various acetylxylan esterases (AcXEs) of distinct carbohydrate esterase (CE) families. CE1, CE4 and CE6 AcXEs hydrolyzed considerably faster the 2″- O -acetyl derivative than the 3″- O -acetyl derivative. In contrast, the CE16 acetyl esterase preferred to attack the ester bond at position 3 followed by position 4. Conclusions Redistribution of acetyl group on the NRE-Xyl p residues is extremely rapid at elevated temperature and includes the formation of 4-acetate. Regioselectivity of AcXEs and CE16 acetyl esterase on these monoacetates is complementary. General significance The formation of all isomers of acetylated xylosyl residues must be taken into account after a long-term incubation of acetylxylan and acetylated xylooligosaccharides solutions or upon their treatment at elevated temperatures. This phenomenon emphasizes requirement of both types of xylan deacetylases to enable a rapid saccharification of xylooligosaccharides by glycoside hydrolases.