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Enzymology of hemicellulose hydrolysis

Hemicellulose polysaccharides, located in plant cell walls, are among the most abundant renewable resources on earth. We are studying the enzymatic hydrolysis of galacto(gluco)mannan, the major soft-wood hemicellulose polysaccharide.

The aim is to understand the hydrolysis, which include several specific enzymes. We have been focusing on the structural and functional features of endo-mannanase, the major depolymerising enzyme, produced by the soil-fungus Trichoderma reesei.

On the basis of the X-ray crystallographic 3D structure, an approach including site-directed and random mutagenesis is taken to determine functional important residues and regions.

We are interested in the substrate - enzyme interaction, and especially in the interactions that are likely to direct the specificity of the enzyme. The active site is situated in a cleft. Within the cleft, at least five different subsites interact with the substrate, each subsite binding one monomeric unit of the polysaccharide.

The endo-mannanase from T. reesei has a small polysaccharide binding domain, separated from the catalytic domain by a highly glycosylated linker peptide.

We are also studying the function of the binding domain, the interaction between the binding domain and the catalytic domain and the flexibility of the linker.

In addition to endo-mannanase, the exo-acting enzymes beta-mannosidase and alfa-galactosidase are needed for complete hydrolysis of galacto(gluco)mannan. The synergistic action of these enzymes and endo-mannanase are investigated. Enzymes from Aspergillus niger and Cellulomonas fimi are also included in the studies. Furthermore, galactomannans are very interesting substrates due to their gel-forming properties. The studied enzymes have several important applications in the food/feed and pulp/paper industries.