Computational Evaluation of the Dynamic Fluctuations of Peripheral Loops Enclosing the Catalytic Tunnel of a Family 7 Cellobiohydrolase

• Published in: The journal of physical chemistry. B 2014-05, Vol.118 (20), p.5340-5349
• Main Authors: Granum, David M., Schutt, Timothy C., Maupin, C. Mark
• Format: Article
• Language: English
• Subjects: Binding Sites; Catalysis; Catalysts; Catalytic Domain; Cellulose 1,4-beta-Cellobiosidase - chemistry; Cellulose 1,4-beta-Cellobiosidase - metabolism; Dynamic tests; Dynamics; Endoglucanase; Enzymes; Fluctuation; Fungal Proteins - chemistry; Fungal Proteins - metabolism; Hydrogen-Ion Concentration; Kinetics; Molecular Docking Simulation; Protein Structure, Secondary; Sordariales - enzymology; Substrate Specificity; Thermodynamics; Tunnels (transportation)
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp5011555

The size and character of the peripheral loops enclosing the active site for cellulase enzymes is believed to play a major role in dictating many critical enzymatic properties. For many cellulases it is observed that fully enclosed active sites forming a tunnel are more conducive to cellobiohydrolase activity and the ability to processively move along the substrate. Conversely, a more open active site groove is indicative of endoglucanase activity. For both cellobiohydrolases and endoglucanases, the loop regions have been implicated in the ability of the enzyme to bind substrate, influence the pKa of active site residues, modulate the catalytic activity, and influence thermal stability. Reported here are constant pH molecular dynamics (CpHMD) simulations that investigate the role of dynamic fluctuations, substrate interactions, and residue pK a values for the peripheral loops enclosing the active site of the cellobiohydrolase Melanocarpus albomyces Cel7B. Two highly flexible loop regions in the free enzyme have been identified, which impact the overall dynamical motions of the enzyme. Charge interactions between Asp198 and Asp367, which reside on two adjacent loops, were found to influence the overall loop conformations and dynamics. In the presence of a substrate the protonation of Asp367, Asp198, and Tyr370 were found to stabilize substrate binding and control the movement of two peripheral loops onto the active site containing the substrate (i.e., clamping down). The substrate-induced response of the loop regions secures the cellulose polymer in the catalytic tunnel and creates an environment that is conducive to hydrolysis of the glycosidic bond.