Cellulose hydrolysis in evolving substrate morphologies II: Numerical results and analysis

Numerical simulation results are presented for a cellulose hydrolysis model which incorporates both the enzymatic glucan chain fragmentation kinetics and the hydrolytic substrate morphology evolution within the general framework of our companion article I. To test the local Poisson (LP) approximation employed in the site number formalism of I, we numerically compare it to the corresponding exact chain number formalism of I. The LP results agree to very high accuracy with the exact chain number kinetics, assuming realistic parameters. From simulations of different types of random and non-random model morphologies, we then show that the details of the random substrate morphology distribution, and its hydrolytic time evolution, profoundly affect the hydrolysis kinetics. Essential, likely very general, experimentally testable features of such morphology-based hydrolysis models are (i) the existence of two distinct time scales, associated with the hydrolysis of the outermost surface-exposed cellulose chains and, respectively, of the entire substrate; (ii) a strongly morphology-dependent hydrolysis slow-down effect, which has also been observed in previous experimental work. Our results also suggest that previously proposed non-morphologic chain fragmentation models can only be applied to describe the hydrolytic short-time behavior in the low enzyme limit. Further experiments to test our modeling framework and its potential applications to the optimization of the hydrolytic conversion process are discussed. Biotechnol. Bioeng. 2009; 104: 275-289