Recent Progress in Understanding the Mode of Action of Acetylxylan Esterases

Abstract

The major obstacle of efficient and economically feasible bioconversion of renewable plant biomass is its recalcitrance as a result of complexity of the plant cell wall structure. The structural complexity is, in part, the result of plant evolution under strong pressure to enhance the resistance to microbial invaders. With the exception of insoluble cellulose, almost all other cell wall polysaccharide components, mainly hemicelluloses and pectin, are to some degree branched and partially esterified with acetic acid. 1,2) Esterification of the side chain sugar residues of polysaccharides with acetyl groups is apparently critical in determining the physical properties of the cell wall, although other polymer modifications are also important; for example, esterification with phenolic acids plays a role in cross-linking. 3,4) Here we review recent progress in understanding of enzymatic degradation of native acetylated hemicellulose using NMR and MS approaches. Particular attention is paid to the breakdown of partially acetylated hardwood glucuronox-ylan, as studies of the action of deacetylating carbohydrate esterases have principally focused on this polysaccharide. ROLE OF ACETYLATION OF PLANT CELL WALL POLYSACCHARIDES Acetylation has been clearly shown to hinder the action of microbial glycanases. 5) However, acetylation also dramatically changes the physico-chemical properties of polysac-charides. While full acetylation makes polysaccharides water-insoluble, partial acetylation, as observed in plant xylans, increases their solubility in water. Partial acetylation increases the bulk volume of the polysaccharide due to a higher degree of hydration. This is likely to have important consequences for transport of water and nutrients through the plant cell walls and also increase their susceptibility to enzymatic hydrolysis. This idea finds support in recent reports that the knockout of acetyl transferase genes in Arabidopsis resulted in a lower degree of hemicellulose acetylation, enhanced the recalcitrance of the cell wall and the plant resistance to infection by the necrotrophic fungal pathogen Botrytis cinerea. 68) Acetyl groups in the main xylan chain obviously prevent closer association of xylan with cellulose, thus increasing the plant resistance to pathogen attack. It is not known at which stage of plant evolution the acetylation of polysaccharides was introduced, however, it is likely that deacetylating enzymes evolved in parallel with microorganisms invading plants or proliferating on plant residues. The substrate specificity of some carbohydrate esterases for elements of the polysaccharide main chain suggests that some of these enzymes evolved from glycoside hydrolase ancestors. 5,911) Abstract: Acetylation is one of the main obstacles to the effective enzymatic conversion of hemicelluloses to fermentable sugars. In nature, the microbial degradation of hemicellulose involves the action of deacet-ylating esterases that act synergistically with glycoside hydrolases. In the industrial processing of lignocel-luloses biomass, alkaline pretreatments remove acetyl groups by saponification, but other non-alkaline pretreatment methods generate acetylated hemicelluloses. Complete saccharification of plant hemicellu-loses cant be achieved without the deacetylating enzymes. Recent years have witnessed considerale progress in our understanding of the mode of acetylation of hemicellulose and mode of action of microbial polysaccharide deacetylases. In this article we focus on the diversity and role of acetylxylan esterases in the breakdown of acetylxylan, the most abundant hemicellulose in nature.