A kinetic model for the burst phase of processive cellulases
Article first published online: 28 MAR 2011
© 2011 The Authors Journal compilation © 2011 FEBS
Volume 278, Issue 9, pages 1547–1560, May 2011
How to Cite
Praestgaard, E., Elmerdahl, J., Murphy, L., Nymand, S., McFarland, K. C., Borch, K. and Westh, P. (2011), A kinetic model for the burst phase of processive cellulases. FEBS Journal, 278: 1547–1560. doi: 10.1111/j.1742-4658.2011.08078.x
- Issue published online: 19 APR 2011
- Article first published online: 28 MAR 2011
- Accepted manuscript online: 3 MAR 2011 08:00PM EST
- (Received 30 October 2010, revised 21 February 2011, accepted 25 February 2011)
- burst phase;
- kinetic equations;
- slowdown of cellulolysis
Cellobiohydrolases (exocellulases) hydrolyze cellulose processively, i.e. by sequential cleaving of soluble sugars from one end of a cellulose strand. Their activity generally shows an initial burst, followed by a pronounced slowdown, even when substrate is abundant and product accumulation is negligible. Here, we propose an explicit kinetic model for this behavior, which uses classical burst phase theory as the starting point. The model is tested against calorimetric measurements of the activity of the cellobiohydrolase Cel7A from Trichoderma reesei on amorphous cellulose. A simple version of the model, which can be solved analytically, shows that the burst and slowdown can be explained by the relative rates of the sequential reactions in the hydrolysis process and the occurrence of obstacles for the processive movement along the cellulose strand. More specifically, the maximum enzyme activity reflects a balance between a rapid processive movement, on the one hand, and a slow release of enzyme which is stalled by obstacles, on the other. This model only partially accounts for the experimental data, and we therefore also test a modified version that takes into account random enzyme inactivation. This approach generally accounts well for the initial time course (approximately 1 h) of the hydrolysis. We suggest that the models will be useful in attempts to rationalize the initial kinetics of processive cellulases, and demonstrate their application to some open questions, including the effect of repeated enzyme dosages and the ‘double exponential decay’ in the rate of cellulolysis.
Database The mathematical model described here has been submitted to the Online Cellular Systems Modelling Database and can be accessed at http://jjj.biochem.sun.ac.za/database/Praestgaard/index.html free of charge.