ABSTRACT
Hemicellulases are a group of diverse enzymes that hydrolyze hemicelluloses. These enzymes have many biotechnological applications and their structure/function relationships are the subject of intensive researches.
They include endoxylanases acting on the backbone of xylans, and glucuronidases, arabinofuranosidases, feruloyl esterases, and galactosidases acting on the branches; in addition, β-xylosidases hydrolyzing short xylooligomers.
Their structure contains catalytic and non-catalytic domains. The catalytic modules of hemicellulases are either glycoside hydrolases (GHs) that hydrolyze glycosidic bonds, or carbohydrate esterases (CEs), which hydrolyze ester linkages of acetate or ferulic acid side groups. Based on their sequence homology, GHs and CEs are classified into different families, each of them having preferential substrate.
The biodegradation of xyloglucans (XyG) involves an initial attack by specific endoglucanases (endoxyloglucanases or xyloglucan endohydrolases) that depolymerize high molecular mass XyG into XyG oligosaccharides. Afterwards, exo-acting enzymes (specific exoglycosidases) trim the branches of the oligosaccharides, so as to typically form xylosylated oligosaccharides that are then degraded to monosaccharides by α-xylosidases and β-glucosidases.
The biodegradation of plant cell wall xylans requires the concerted action of a diverse enzymatic syndicate like endoxylanases, oligosaccharide reducing-end xylanases, xylan β-1,4-xylosidases, α-arabinofuranosidases, acetyl xylan esterases, feruloyl esterases, and α-glucuronidases.
The complete bioconversion of galactomannan into galactose and mannose requires the activity of three types of microbial enzymes, namely mannanases or endo-1,4-β-mannanases, β-mannosidases and α-galactosidases.
Arabinans, galactans, and arabinogalactans constitute together the so-called neutral pectic substances. The enzymatic degradation of arabinans and arabinogalactans requires the action of several enzymes, among which endo-1,5-α-arabinanases, endo-1,4-β-galactanases, endo-1,3-β-galactanases, galactan 1,3-β-galactosidases or exo-β-1,3-galactanases, galactan-endo-1,6-β-galactosidases or endo-β-1,6-galactanases and β-galactosidases.
