Acetylation of woody lignocellulose: significance and regulation

Abstract

Non-cellulosic cell wall polysaccharides constitute approximately one quarter of usable biomass for human exploitation. In contrast to cellulose, these components are usually substituted by O-acetyl groups, which affect their properties and interactions with other polymers, thus affecting their solubility and extractability. However, details of these interactions are still largely obscure. Moreover, polysaccharide hydrolysis to constituent monosaccharides is hampered by the presence of O-acetyl groups, necessitating either enzymatic (esterase) or chemical de-acetylation, increasing the costs and chemical consumption. Reduction of polysaccharide acetyl content in planta is a way to modify lignocellulose toward improved saccharification. In this review we: (1) summarize literature on lignocellulose acetylation in different tree species, (2) present data and current hypotheses concerning the role of O-acetylation in determining woody lignocellulose properties, (3) describe plant proteins involved in lignocellulose O-acetylation, (4) give examples of microbial enzymes capable to de-acetylate lignocellulose, and (5) discuss prospects for exploiting these enzymes in planta to modify xylan acetylation.

Phylogeny, classification and metagenomic bioprospecting of microbial acetyl xylan esterases

“…Such mechanisms include acetylation and 'feruloyation' of plant cell polymers (xylan, mannan, pectin, peptidoglycan and chitin), thereby reducing the efficiency of polysaccharide hydrolases [4,5]. Acetylation occurs mainly in hardwood (acetyl glucuronoxylan), typically at the C2 or C3 or both positions, depending on whether it is a mono-or di-O-acetylated xylopyranosyl unit ( Figure 3).…”

“…Acetylation occurs mainly in hardwood (acetyl glucuronoxylan), typically at the C2 or C3 or both positions, depending on whether it is a mono-or di-O-acetylated xylopyranosyl unit ( Figure 3). On non-reducing xylopyranosyl residues of oligosaccharides or xylopyranosides, acetyl groups can migrate to position C4 [5,16]. The migration is accelerated by increased pH and temperature [17].…”

“…It has been established [3,5,19] that acetylation of xylans generally increases the recalcitrance of plant polymers to depolymerisation by interfering with access of endoxylanases to glycosidic linkages on the xylopyranose main chain and resulting in incomplete hydrolysis of polymeric xylan (hemicellulose). This is a significant factor in reducing the efficiency of saccharification of lignocelluosic biomass for biofuel production [3,5,19].…”

“…They were first described by Biely, Puls [1,2] and have since attracted steadily increasing attention from researchers ( Figure 1A), with a number of recent reviews focusing principally on the chemistry and specificity of AcXE catalysis [3][4][5][6]. A number of AcXE-producing microorganisms have been isolated from various environments (further discussed below) and AcXE activity was recently reported for esterases from some plants; e.g., Populus-secreted pectin acetyl esterase [7] and Arabidopsis thaliana [8].…”

Understanding the structural and functional properties of carbohydrate esterases with a special focus on hemicellulose deacetylating acetyl xylan esterases

“…Compared to O -acetylation, methyl esterification of plant polysaccharides was highly studied. O -acetylation is observed in the branches and backbone structures of various cell wall polymers with the nature, and extent of the acetylation significantly varies based on the species, type of tissues and cell walls (Gille and Pauly 2007; Pawar et al 2013). The sources of O -acetyl groups in primary and secondary cell walls of woody plants such as softwood and hardwood are observed to be as homogalacturonan, rhamnogalacturonan I, rhamnogalacturonan II (Pectin), xyloglucan, glucuronoarabinoxylan (type II primary cell walls of grasses), glucuronoxylan and glucomannans (Table 1).…”