Enzymes in bioenergy

Abstract

Cellulose (∼40%), hemicelluloses (∼30%), and lignin (∼20%) are the major components of lignocellulosic biomass. Cellulose, specifically, can be degraded by a synergistic action of several enzymes, including the endoglucanases (EC 3.2.1.4), which cleave randomly internal b-1-4 linkages; the exoglucanases/ cellobiohydrolases (EC 3.2.1.91), that act at the cellulose chains termini to release cellobiose, and the β-glucosidases (EC 3.2.1.21), which catalyze hydrolysis of cellobiose into glucose. Most cellulases have a two-domain organization with a large catalytic core domain (CCD) connected to a small cellulose-binding module (CBM) via long, heavily O-glycosylated linker peptide More recently, the use of cellulases for the industrial conversion of cellulose-containing biomass to fermentable sugars as an alternative to fossil fuels has attracted increasing attention. Hemicellulose can be decomposed by a diverse group of enzymes in which xylanases play an important role. Xylan is the second most abundant polysaccharide in Nature. The main enzymes involved in xylan hydrolysis are endo-xylanase (EC 3.2.1.8), which show a preference for internal xylan bonds and exo-xylanase (EC 3.2.1.37), which act mostly at the termini of xylan chains. The modular structures of xylanases are similar to cellulases, however, in addition to enzymes that contain domains that bind specifically to xylan (xylan-binding modules or XBMs), there are several xylanases which contain domains that bind specifically to cellulose rather than xylan. Lignin is a heterogeneous hydrophobic phenolic polymer and one of its main physiological functions is to cement the cellulose fibers in plants. Lignin is degraded by different classes of enzymes, which are produced by several microorganisms and in different combinations. The main extracellular enzymes involved in lignin degradation are lignin peroxidases (LiPs - EC 1.11.1.14), manganese peroxidases (MnPs -EC 1.11.1.13) and laccase (EC 1.10.3.2). Although a number of microorganisms, including fungi and bacteria, have the capacity to degrade plant biomass, most commercially available enzymes are currently produced by genetically engineered strains of filamentous fungi Trichoderma reesei (Hypocrea jecorina) and Aspergillus niger. Cost of the cellulolytic enzymes represents significant fraction of the second generation ethanol, therefore the hydrolytic activities of individual enzymes should be enhanced and the industrial cellulase cocktails must be balanced to guarantee improved efficiency of pretreated biomass conversion. Knowledge of the molecular mechanisms of the enzymatic hydrolysis of lignocellulosic feedstocks is one of the important parts of developing cost-effective processes for biomass to bioenergy conversion. © 2011 Springer Science+Business Media, LLC.