Cellulase Adsorption and Reactivity on a Cellulose Surface from Flow Ellipsometry

Abstract

Enzymatic deconstruction of cellulose occurs at the aqueous/cellulose interface. Most assays to explore cellulase activity, however, are performed in bulk solution and, hence, fail to elucidate surface-reaction kinetics. We use flow ellipsometry to quantify the adsorption and surface reactivity of aqueous cellulase on a model cellulose film substrate. The rate of cellulose digestion at the aqueous/solid interface increases with increasing bulk concentration of enzyme, but only up to a plateau corresponding to the maximum adsorption density of cellulase. Kinetic data are analyzed according to a modified Langmuir–Michaelis–Menten framework including both reversible adsorption of cellulase to the cellulose surface and complexation of surface cellulose chains with adsorbed cellulase. At ambient temperature, the molar turnover number is 0.57 ± 0.08 s–1, commensurate with literature values, and the Langmuir adsorption equilibrium constant, characterizing the binding strength of the cellulase, is 0.086 ± 0.026 ppm–1. The rate-determining step in the surface-reaction sequence is complexation of adsorbed cellulase with the solid-cellulose surface. Simultaneous knowledge of sorption and digestion kinetics is necessary to quantify cellulose deconstruction.