(574d) Identification of Molecular-Level Contributions to Processivity in Glycoside Hydrolases From Computational and Experimental Studies of Serratia Marcescens Chitinases

Processive enzymes conduct much of the cellulose and chitin turnover in the biosphere making them important contributors to the global carbon and nitrogen cycles. As a result, these enzymes are traditionally major components in industrial cocktails for biomass conversion, though enormous potential remains to be gained through improved performance. Processive enzymes are typically multi-modular with at least one carbohydrate-binding module (CBM) connected via a linker region to a large catalytic domain (CD) with a tunnel or cleft in which a single carbohydrate chain can be threaded and hydrolyzed to a soluble product. Processive enzymes are believed to acquire single carbohydrate chains from polymer crystals and hydrolyze chains without detaching from the crystalline surface between catalytic events. The nature of this processive action from a molecular perspective remains generally uncharacterized. Here, we investigate processivity, both through computational and experimental studies, in the recently completed suite of chitinase enzymes from the bacterium Serratia marcescens. This model system consists of two well-characterized processive chitinases from glycoside hydrolase family 18, ChiA and ChiB, as well as a non-processive family 18 chitinase, ChiC.  Using molecular dynamics simulations, thermodynamic integration, and insights from structural and biochemical studies, we define hallmarks of processive and non-processive glycoside hydrolases and the roles of aromatic residues in the action of processive enzymes.