Mihhail Kurašin will defend his doctoral thesis titled „Processivity of cellulases and chitinases” on 24 March 2017 at 10:15.
Senior Research Fellow Priit Väljamäe, University of Tartu
Associate Professor Jerry Ståhlberg, Sveriges Lantbruksuniversitet (SLU), Uppsala, Sweden
Dwindling oil reserves and increasing world population necessitate the search for alternative sources of energy. Thus, cellulose and chitin, the net annual production of which are estimated in billions of tons, have recently become potential candidates as a renewable sources for the production of biofuel for gasoline working vehicles as well as essential chemical compounds for food and pharmaceutical industries. The main idea lies in degradation of cellulose and chitin into monomers (i.e. glucose and N- acetylglucosamine respectively). The latter can be easily converted into ethanol and other chemicals. In nature cellulose is degraded by enzymes with common name cellulases and chitin is degraded by enzymes called chitinases. Cellulases and chitinases are also employed for industrial uses since they enable to avoid the use of high temperatures and concentrated mineral acids. However, natural enzymes are slow and their contribution to the cost of the end products remains high. Efficient degradation of polysaccharides is achieved by concerted action of different types of enzymes. The main contribution to this process generally comes from processive enzymes that constitute the major part of cellulolytic/chitinolytic systems. Processivity is defined as the ability of an enzyme to conduct sequential catalytic acts without dissociating from its substrate. It is therefore an important characteristic of cellulose/chitin degrading enzymes, and its investigation is likely to provide a good clue to the improvement of the process of saccharification of recalcitrant polysaccharides. The present study was aimed at biochemical description of processive cellulases and chitinases. New methods for the measurement of processivity and other kinetic parameters were developed. It was previously know that the limiting step in the catalytic cycle of enzyme acting in isolation is the dissociation from the substrate. On the contrary we found that the limiting step in the catalytic cycle of enzyme acting in the synergistic mixture is the processive movement along the polymer chain. We also demonstrated that for the efficient degradation of recalcitrant polysaccharides strong interactions in the substrate binding site and in the product binding site are required. The loss of one interaction leads to reduced activity on crystalline substrates but concomitantly increases activity on amorphous substrates.