Chemistry and biochemistry of hemicelluloses: Relationship between hemicellulose structure and enzymes required for hydrolysis

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Abstract
The lignified plant cell wall is a composite material in which cellulose, hemicelluloses and lignin are in tight association. Xylans constitute the predominant hemicellulose of hardwoods and straw, wherease galactomannans represent the largest hemicellulose fraction in softwoods. Naturally occuring hemicelluloses, commercially available hemicelluloses and xylans and mannans from technical processes have distinct differences with respect to degree of polymerization and extent of branching. In principle, naturally occuring hemicelluloses carry more side chains compared to those which have been extracted with alkali or under pressure and heat. The presence or absence of substituents is also reflected by the enzymes required for complete hydrolysis. The action of the depolymerizing xylanases and mannanases may be partly dependent on debranching enzymes. Special empasis is therefore given on the substrate requirements of the so‐called accessory enzymes (i.e. α‐glucuronidase, α‐arabinosidase, α‐galactosidase, acetyl‐ and phenolic acid esterase) as well as on synergy effects with the depolymerizing enzymes.