ABSTRACT
In this chapter we introduce and discuss the biocatalytic potential of microbial carbohydrate esterases (CEs). These esterases operate on highly hydrated substrates such as partially acylated polysaccharides occurring in plant cell walls. This is a special group of carboxylic acid esterases (EC 3.1.1.1) discovered relatively recently and therefore less well known than lipases and other esterases, particularly in view of their biocatalytic potential. Microbial hemicellulolytic systems involved in degradation of naturally occurring hemicelluloses include three types of esterases: 1. acetylxylan esterases (AcXEs, EC 3.1.1.72), which deacetylate partially acetylated 4-O-methylglucuronoxylan, the major hardwood hemicellulose (acetylated xylan also occurs in annual plants);1,2 2. cinnamoyl esterases, feruloyl esterases (FeEs), and related aryl esterases, which liberate phenolic acids and their dimers (ferulic acid and p-coumaric acid) from plant cell walls, where they mainly occur as esters with L-arabinofuranose-containing polysaccharides, such as L-arabino-D-xylans and L-arabinans;3-5 3. acetylgalactoglucomannan esterases, which deacetylate O-acetyl galactoglucomannans, the major softwood hemicellulose.6,7 In addition to these three basic classes, esterases that deacetylate pectin have also been described.8,9 In comparison with lipases and some plant esterases (wheat germ, orange peel) that have been used for decades as powerful tools for hydroxyl group protection and deprotection in carbohydrate chemistry,10 the catalytic properties of hemicellulolytic esterases are relatively unknown beyond their physiological functions in plant cell wall degradation, and therefore remain underutilized as practical biocatalysts.
