NMR Study of Kinetics of Protein / Cellulose Interactions Junji Iwahara, Department of Biochemistry & Molecular Biology, University of Texas Medical Branch Our focus in this project is on the kinetics of interactions between cellolose and the cellulose-binding domains (CBDs) of the cellulase enzymes. We are studying CBDs from Clostridium cellulovorrans cellulase 5A (Cel5A) and Cellulomonas fimi cellulase 9B (Cel9B). These CBDs bind noncrystalline cellulose and play important role to keep the enzymes in close proximity from the substrate. Using NMR spectroscopy, we have successfully determined the kinetic rate constants for dissociation and association processes for Cel9B CBD. The dissociation rate constant koff was found to be 4.2 x 103 s-1 for a 5-ring oligomer of cellulose. The relatively fast dissociation is likely to crucial for rapid turnover in the enzymatic process. The association process is extremely fast with kon = 2.0 x 108 M-1 s-1. Considering that there are no electrically charged groups in the cellulose molecules, the mechanism for the surprisingly fast association of the CBD to cellulose is an interesting subject for further investigations.