Monday, April 30, 2007 - 11:10 AM

Family 1 carbohydrate binding modules show induced fit interaction with the cellulose substrate

Mark R. Nimlos1, William S. Adney2, Michael E. Himmel2, John W. Brady3, James F. Matthews3, Michael F. Crowley4, Joseph M. Cleary5, Ross C. Walker5, and Linghao Zhong6. (1) National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80403, (2) Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80403, (3) Department of Food Science, Cornell University, Ithaca, NY 14853, (4) The Scripps Research Institute, 10550 N. Torrey Pines Rd, La Jolla, CA 92037, (5) San Diego Supercomputer Center, 9500 Gilman Dr., La Jolla, CA 92093, (6) Sciene, Pennsylvania State University, Mont Alto, Mont Alto, PA 17237

Family 1 carbohydrate binding modules (CBMs) are known to enable the activities of exoglucanases acting on crystalline cellulose by providing a substrate binding platform.   In addition, these CBMs may play a role in the disruption of crystalline cellulose inter-chain hydrogen bonding; a key property that contributes to its recalcitrance.  It has long been assumed that binding of this family of CBMs to crystalline cellulose results from the interaction of the hydrophobic (1,0,0) surface of crystalline cellulose 1β with the hydrophobic surfaces of the CBM created by three aromatic residues (Y5, Y30, and Y31 in T. reesei CBH I).  However, this binding has never been explicitly demonstrated, nor have there been studies that show how these CBMs recognize ends of cellulose chains, a step necessary for processive exoglucanases.  Such detailed molecular information is necessary to develop competent strategies to improve the performance of these enzymes by site directed mutagenesis. We present the results of computational molecular dynamics (MD) simulations, which were used to investigate critical aspects of CBM function on crystalline cellulose.  Our simulations suggest that the cellulose surface can induce a conformational change in the CBM structure, where a fourth aromatic residue moves to establish additional van der Waals interactions with the surface.  This fourth aromatic residue is highly conserved throughout the Family 1 CBMs and we are also studying this conformational change in other family members.  The results of this work suggest new mutational approaches to experimentally investigate the relationship between protein structure and biocatalytic activity.