Hemicellulases and Auxiliary Activities for Biomass Hydrolysis

Hemicelluloses are heteropolymers present along with cellulose in all terrestrial plant cell walls. Lignocellulosic biomass consists of 20–30% hemicellulose which can be effectively valorized for value-added products and biofuels. Xylan, arabinoxylan, glucuronoxylan, xyloglucan, mannan, galactan, arabinogalactan, glucomannan, galactomannan, arabinogalactan, and arabinan are the major hemicelluloses present in plant biomass of which xylan is the most abundant hemicellulose. Amorphous, branched structure and lower molecular weight of hemicellulose make it relatively easy to hydrolyze. However, due to its complex structure, hemicellulose requires different enzymes for effective hydrolysis. Hemicellulases can be divided into two groups: core enzymes and ancillary or auxiliary enzymes. The core and ancillary enzymes that hydrolyze the complex xylan structure require the synergistic action of an array of enzymes such as endo-β-xylanase, β-xylosidase, α-arabinofuranosidase, acetyl xylan esterase, ferulic acid esterase, p-coumaric acid esterase, and α-glucuronidase for effective and complete hydrolysis of xylan. Recently, several studies have reported on the advancement of enzymatic hydrolysis for industrially relevant pretreated lignocellulosic substrates such as rice straw, sugarcane bagasse, corn stover, finger millet, and wheat straw for the production of bioethanol.