Increased Enzyme Binding to Substrates Does Not Always Increase Catalytic Activity

Substrate binding affinity is typically thought to be one of the primary rate-limiting steps preceding enzyme catalytic action during most homogeneous reactions. However, interfacial-based enzyme catalysis on insoluble crystalline substrates, like cellulose, has an additional step comprising of individual biopolymer chain decrystallization from the substrate interface. This additional decrystallization step has ramifications on the role of enzyme-substrate binding and its relationship to overall catalytic efficiency. We find that altering the crystalline structure of cellulose from its native allomorphic state called cellulose I to cellulose III results in reduced overall cellulase binding but enhanced hydrolytic activity on the latter substrate. We have constructed a comprehensive kinetic model for processive enzymes acting on insoluble substrates to explain this anomalous finding. Our study indicates that this phenomenon can be explained by the enhanced decrystallization kinetics of individual cellulose chains from the surface of cellulose III that despite reduced overall binding affinity results in enhanced catalytic activity. These findings have implications on our understanding of interfacial enzyme catalysis for biopolymers that incorporate analogous decrystallization events.