Degradation of wood polysaccharides by fungal glycoside hydrolases
Sammanfattning: The enzymatic degradation of wood polysaccharides such as cellulose and hemicellulose is an important process in nature. In addition, cellulases and hemicellulases can be used in industrial applications. Fuel ethanol can potentially be produced from wood by enzymatic hydrolysis of cellulose followed by yeast fermentation of the formed sugars. In this thesis, fungal glycoside hydrolases, cellulases and hemicellulases were studied with the aim of increasing our knowledge of the mechanisms involved in the enzymatic hydrolysis of cellulose and lignocellulose. The focus was mainly on cellulases from the filamentous fungus Trichoderma reesei. However, lignocellulose also contains hemicellulose and studies of hemicellulases are included In Paper I-IV the mechanisms involved in cellulose degradation were investigated. Features of enzymatic cellulose hydrolysis such as synergism, decreasing hydrolysis rate during hydrolysis and the effect of adding surfactants to the hydrolysis mixture were studied. The observed decrease in hydrolysis rate by T. reesei Cel7A could be explained by unproductive binding of enzymes to the substrate. Furthermore, an extension of the existing model explaining end-exo synergism is presented and discussed. The use of surfactant to increase efficiency of the enzymatic cellulose hydrolysis is evaluated and a mechanism explaining the effect of surfactant is discussed. In addition, the major components of the cellulase system from the filamentous fungus Penicillium brasilianum was purified and characterised. This thesis elucidates the need to use complex polysaccharide substrate in order to understand the mechanisms of degradation of natural complex substrates. O-acetyl-galactoglucomannan, the major softwood hemicellulose, isolated from spruce wood was used to study enzyme specificity and other biochemical properties (Paper V). Moreover, the function of the carbohydrate-binding module of the T. reesei b-mannanase Man5A was investigated (Paper VI). It was concluded that the carbohydrate-binding module of Man5A bound to cellulose but had an important function in potentiate the mannan hydrolysis of substrates containing both mannan and cellulose. The majority of fungal glycoside hydrolases are glycoproteins. In Paper VII the N- glycosylation of T. reesei Cel7B was investigated using mass-spectrometry. N-glycosylation sites on the catalytic module were identified and it was shown that large parts of the observed heterogeneity of the enzyme preparation was caused by heterogeneity in N-glycan structures.
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