Kinetic models of cellulase-lignin interaction: structural contributions to adsorption kinetics

The kinetics and mechanism of protein interaction with solid surfaces is a broadly relevant phenomena, important to fields as diverse as industrial biocatalysis, biomedical applications, food science, cell biology, and medicine. The nonproductive interaction of cellulase enzymes with lignin hinders the large-scale conversion of biomass to soluble sugars; for this reason the kinetics of these interactions is of interest. Cellulase has been shown to interact with lignin, but heterogeneity of lignin surfaces, challenges in measuring irreversible components of these interactions, and fast reaction kinetics have made quantifying the kinetics of these reactions difficult. 

We used quartz crystal microgravimetry with dissipation monitoring (QCM-D) to take real-time measurements of adsorbed mass on a flat lignin surface. We have developed a method for casting homogeneous lignin films that are chemically similar to lignin found in pretreated biomass, and use QCM-D to compare three models of reversible-irreversible binding behavior: a transition model, a a two-state model with changing adsorbate footprint, and a multiple-site transition model. 

We have applied this model to the four main cellulases produced by T. reesei, and their catalytic domains. We have found surprising diversity in the adsorption, desorption, and irreversible adsorption kinetics of both the full-length enzymes and their catalytic domains. In addition, we find that while the carbohydrate binding module confers lignin affinity to cellulases, their propensity to become irreversibly bound to the surface may be controlled by features of the catalytic domain. This has implications in enzyme recycle and in development of new industrial enzymes.