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.
Highlights
OCCURRENCE OF O-ACETYLATION IN LIGNOCELLULOSE O-acetyl and methyl esterification are the most common substitutions in different cell wall matrix polysaccharides (Figure 1)
O-acetylation may occur on the backbones or branches of many cell wall polymers, but the nature of acetylated polymer and the extent of acetylation differ between species, tissues and types of cell walls (Figure 1; Table 1)
The largest pool of acetyl residues in lignocellulose, comes from the secondary cell walls since they constitute the bulk of biomass
Summary
Sumi (1964) Timell (1967); Sjöström (1993) Cetinkol et al (2010) Timell (1967) Timell (1967) Laffend (1967); Timell (1967) Timell (1967) Timell (1967). The acetylation is initially introduced to the secondary wall layers where hydroxyl groups of hemicelluloses are likely the main reactants, whereas prolonged treatment introduces acetyl to the middle lamella where pectins and lignin are the main targets (Rowell, 2009) Most likely it is the acetylation of xylan and mannan in secondary wall layers that is responsible for the increased stiffness, possibly by allowing more hydrophobic interactions with lignin. Rhamnogalacturonan acetyl esterase (EC 3.1.1.86) deacetylates RGI at GalA O-2 and O-3 positions and belongs to CE12 (Molgaard et al, 2000) This activity has been shown in Aspergillus aculeatus (Schols et al, 1990), and in bacteria where it has broad substrate specificity including acetylated xylan and cephalosporin C (Martinez-Martinez et al, 2008; Navarro-Fernandez et al, 2008).
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