Wednesday, 18 September, 2013
Christina M. Payne
Assistant Professor, Department of Chemical and Materials Engineering, University of Kentucky
Cell wall polysaccharides represent a vast source of organic carbon on Earth. To access these carbohydrate polymers as food sources, micro-organisms have evolved cocktails of glycoside hydrolase enzymes to depolymerize individual chains from insoluble substrates employing a host of different mechanisms to locate and hydrolyze glycosidic bonds. These enzymes are key components of many commercial biomass conversion processes and are thus primary targets for protein engineering. Our group uses molecular simulation to understand the fundamental mechanisms driving deconstruction of recalcitrant polysaccharides by carbohydrate-active enzymes.
Here, we will focus on two distinct vignettes related to how cellulases degrade cellulose. First, we will focus on our recent discovery of additional linker domain functionality. Many cellulases are multi-modular consisting of a carbohydrate binding module (CBM) connected to a catalytic domain by a flexible, glycosylated linker. These linkers have long been thought to simply serve as a tether between structured domains or to act in an inchworm-like fashion during catalytic action. With molecular dynamics simulations of Trichoderma reesei Family 6 and Family 7 cellulases bound to cellulose, we predicted that glycosylated linkers directly bind to cellulose. Our prediction was experimentally examined by measuring binding affinity of the isolated CBM and the glycosylated CBM-linker construct; an order of magnitude enhancement was observed as a result of the linker. Together, these results suggest glycosylated linkers in carbohydrate-active enzymes, which are intrinsically disordered proteins in solution, aid in dynamic binding during enzymatic deconstruction of plant cell walls. Secondly, we will focus on the processive mechanism of cellulases whereby an individual carbohydrate polymer chain is decrystallized and hydrolyzed along the chain without substrate dissociation. We developed a theoretical relationship to describe glycoside hydrolase processivity in terms of experimentally measureable parameters and apply enhanced sampling free energy calculations to examine our model in Family 7 cellulases. Overall, our approaches represent significant steps toward the development of structure-activity relationships in these industrially and environmentally important enzymes.