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The role of evolving interfacial substrate properties on heterogeneous cellulose hydrolysis kinetics

Interfacial enzyme reactions require formation of an enzyme-substrate complex at the surface of a heterogeneous substrate, but often multiple modes of enzyme binding and types of binding sites complicate analysis of their kinetics. Excess of heterogeneous substrate is often used as a justification to model the substrate as unchanging; but using the study of the enzymatic hydrolysis of insoluble cellulose as an example, we argue that reaction rates are dependent on evolving substrate interfacial properties. We hypothesize that the relative abundance of binding sites on cellulose where hydrolysis can occur (productive binding sites) and binding sites where hydrolysis cannot be initiated or is inhibited (non-productive binding sites) contribute to rate limitations. We show that the initial total number of productive binding sites (the productive binding capacity) determines the magnitude of the initial burst phase of cellulose hydrolysis, while productive binding sites depletion explains overall hydrolysis kinetics. Furthermore, we show that irreversibly bound surface enzymes contribute to the depletion of productive binding sites. We examine the relationship of the state of bound enzyme (productive or non-productive) with binding site evolution throughout hydrolysis. Our model shows that decreasing non-productive binding sites promotes productive binding and prevents the shortening of burst phase hydrolysis by lessening the effect of enzyme stalling.