Microbial xylanolytic carbohydrate esterases

This article reviews microbial esterases participating in the degradation of the major plant hemicellulose, xylan. The main chain of this polysaccharide built of beta-1,4-glycosidically linked xylopyranosyl residues is substituted by other sugars and also partially acetylated. Besides esters of acetic acid, there are two other types of ester linkages in plant xylans. L-Arabinofuranosyl side chains form esters with phenolic acids, predominantly with ferulic acid. The dimerization of ferulic acid residues leads to cross-links connecting the hemicel-lulose molecules. Ferulic acid cross-links were shown to serve as covalent linkage between lignin and hemicellulose. Another cross-linking between lignin and hemicellulose is provided by esters between the xylan side residues of glucuronic or 4-O-methyl-D-glucurononic acid and lignin alcohols. Regardless of the cross-linking, the side residues prevent xylan main chains from association that leads to crystallization similar to that of cellulose. Simulta-neously, xylan decorations hamper the action of enzymes acting on the main chain. The enzymatic breakdown of plant xylan, therefore, requires a concerted action of glycanases attacking the main chain and enzymes catalyzing debranching, called accessory xylanolytic enzymes including xylanolytic esterases. While acetylxylan esterases and feruloyl esterases participate directly in xylan degradation, glucuronoyl esterases catalyze its separation from lignin. The current state of knowledge of diversity, classification and structure-function re-lationship of these three types of xylanolytic carbohydrate esterases is discussed with em-phasis on important aspects of their future research relevant to their industrial applications.

Automated summary

This article reviews the microbial esterases involved in the degradation of xylan, a major plant hemicellulose. Xylan is a polysaccharide composed of beta-1,4-glycosidically linked xylopyranosyl residues with substitutions and acetylation. The ester linkages in xylan include esters of acetic acid and esters between L-arabinofuranosyl side chains and phenolic acids. These linkages contribute to the cross-linking between hemicellulose and lignin. The article discusses the diversity, classification, and structure-function relationship of xylanolytic esterases and their potential industrial applications.