Hydrolytic and Oxidative Mechanisms Involved in Cellulose Degradation

The enzymatic degradation of cellulose is an important process in nature. This thesis has focused on the degradation of cellulose by enzymes from two cellulose-degrading fungi, Hypocrea jecorina and Phanerochaete chrysosporium, including both the action of the individual enzymes and their synergistic interplay. The end-preference of cellobiohydrolases on crystalline cellulose was studied. Cellobiohydrolases belonging to glycosyl hydrolase (GH) family 7 were found to hydrolyse cellulose processively, starting from the reducing end of the cellulose chain. End-labelled cellulose can serve as a tool for functional classification of cellulases. The synergy mechanism between endoglucanases and cellobiohydrolases was studied using substrates with different physical properties derived from bacterial cellulose. A new mechanism for synergism between endo- and exoacting enzymes was proposed whereby endoglucanases, in addition to creating nicks in amorphous parts of cellulose, thereby making new starting-points for processively acting cellobiohydrolases, also “polish” the cellulose surface by removing shorter chains from cellulose surface. A new small endoglucanase belonging to the GH12 family was isolated and characterised. The proposed role of this enzyme is to make the cellulose in wood more accessible to other cellulases. Oxygen conversion by cellobiose dehydrogenase was studied. Hydrogen peroxide produced by cellobiose dehydrogenase can be decomposed even by traces of certain metal ions into a hydroxyl radical and a hydroxyl ion. As an example, reduced metal ions will be continuously regenerated by cellobiose dehydrogenase, which thus stimulates the degradation. Interactions between GH7 family cellobiohydrolases and o-nitrophenyl cellobioside were studied by fluorescence spectroscopy and kinetic tests. o-nitrophenyl cellobioside was used as indicator ligand to determine the dissociation constants for cellobiose binding to catalytically inactive Cel7A mutants by displacement binding experiments