Abstract
Thermal deactivation is a major constraint to the use of enzymes in biotechnological processes. Enzymes are complex heterogeneous systems. Most contemporary models of enzyme deactivation do not account for heterogeneity in deactivation parameters. A simple model for the effect of microheterogeneity on first-order thermal enzyme deactivation was developed, assuming continuous distribution of enzyme substrates. This model was used to study the general effect of heterogeneity, especially microheterogeneity, on enzyme deactivation using simple physical parameters. It was observed that microheterogeneity can substantially influence activity-time trajectories for deactivation, even when the degree of heterogeneity is not large. Microheterogeneity was also seen to increase the apparent order of deactivation reactions, and give higher stability to the enzyme samples. Role of the distribution of enzyme substrates was examined. It was seen that continuous distributions representative of microheterogeneity can also be extended to heterogeneity in general. Several symmetric and skewed distributions were examined, and it was seen that the normal distribution is a good approximation for microheterogeneity. The behavior of thermally deactivating heterogeneous enzyme systems in fixed-bed and fluid-bed reactors were examined. Fixed-bed reactors were seen to give higher conversions than fluid-bed reactors for heterogeneous systems. The deactivation behavior of heterogeneous immobilized enzyme systems with severe diffusion limitation was also examined. Both internal and external diffusion limitations were considered. Diffusion was seen to mask the effect of heterogeneity on the deactivation behavior of enzymes. The significance of microheterogeneity in enzyme deactivation is discussed, and a brief experimental methodology for the study of such systems is also included.