Cellulase-lignin interactions in the enzymatic hydrolysis of lignocellulose: Dissertation

Lignin, a major non-carbohydrate polymer in lignocellulosic plant biomass, restricts the action of hydrolytic enzymes in the enzymatic hydrolysis of lignocellulosic feedstocks. Non-productive enzyme adsorption onto lignin is a major inhibitory mechanism, which results in decreased hydrolysis rates and yields and difficulties in enzyme recycling. The mechanisms of non-productive binding are poorly understood; therefore, in this thesis, enzyme-lignin interactions were studied using isolated lignins from steam pretreated and non-treated spruce and wheat straw as well as monocomponent cellulases with different modular structures and temperature stabilities. The origin of the isolated lignin had an undisputable effect on non-productive binding. Ultrathin lignin films, prepared from steam pretreated and non-treated lignin preparations, were employed in QCM adsorption studies in which Trichoderma reesei Cel7A (TrCel7A) was found to bind more onto lignin isolated from steam pretreated biomass than onto lignin isolated from non-treated lignocellulosic biomass. Botanical differences in lignin chemistry had only a minor effect on non-productive binding when enzyme binding to non-treated wheat straw and spruce lignin was compared. Increase in temperature was found to increase the inhibitory effect arising from non-productive enzyme binding to lignin. Different enzymes were shown to have a characteristic temperature at which the inhibition emerged. Thermostable enzymes were the most lignin-tolerant at high temperatures, suggesting that in addition to the surface properties of an enzyme, non-productive binding onto lignin may be influenced by stability of the enzyme structure. In addition, for lignin-bound T. reesei cellulases, increase in temperature resulted in loss of catalytic activity and tighter binding, suggesting that at high temperature enzyme binding to lignin was probably coupled to conformational changes in the protein folding. With TrCel7A, carbohydrate-binding module (CBM) was found to increase non-productive adsorption to lignin. The Talaromyces emersonii Cel7A catalytic module was linked to a CBM from TrCel7A, giving rise to a fusion enzyme TeCel7A-CBM1. Despite a similar CBM, TeCel7A-CBM adsorbed significantly less to lignin than TrCel7A, indicating that the catalytic module (TeCel7A) had a strong contribution to the low binding. Probably, the contribution of CBM or catalytic core module in non-productive binding varies between different enzymes, and the role of the CBM is not always dominant. To date, very little attention has been paid to the role of electrostatic interactions in lignin-binding. In this work, binding of Melanocarpus albomyces Cel45A endoglucanase onto lignin was found to be very dependent on pH, suggesting that electrostatic interactions were involved in the binding. At high pH, significantly less non-productive binding occurred, probably due to increasing electrostatic repulsion between negatively charged enzymes and lignin. Modification of the charged chemical groups in enzymes or lignin may be a viable strategy to reduce non-productive enzyme binding in the hydrolysis of lignocellulosic substrates.