No abstract

A general model of enzyme deactivations involving unimolecular processes is introduced. For most mechanisms of this type, the parameters of the general model can be expressed in terms of actual physical parameters. The number of physical parameters that can be determined from the deactivation data cannot exceed the number of independent constants in the general model. When there is an excess of physical parameters, then some parameters must be determined from independent methods of analysis. If this is not possible, then some parameters must be left as lumped parameters or global parameters. The general form of the model can be useful in determining the number of independent, potentially active forms of the enzyme present during deactivation. Some exceptions to the general model are due to higher-order processes such as dissociation, autolysis, and biological contamination.

A two-parameter deactivation model is proposed to describe the kinetics of activity stabilization for some enzymes. The single-step unimolecular mechanism exhibits non-first-order deactivation kinetics since the final enzyme state, E(1) is not completely inactivated. The usefulness of the model is demonstrated by applying it to the inactivation of different enzymes. The influence of the concentration of active ester, ionic strength, and pH on the model parameters is examined during the inactivation of electric eel acetylcholinesterase.(25) In general, inactivators would decrease the level of activity stabilization, alpha(1), and increase the first-order inactivation rate constant, k(1). The effect of protecting agents would be to increase alpha(1) and to decrease k(1).

A series-type enzyme deactivation model is utilized to theoretically analyze and to quantify the effect of chemical modifier concentration on the eventual level of enzyme activity stabilization, alpha(2). An increase in the concentration of phosphate ion and NADP increases alpha(2) for the enzymes studied. One example of each enzyme deactivation is given wherein the introduction of chemical modifiers changes the deactivation mechanism from a single-step to a series-type mechanism, and from a series-type to a single-step mechanism. Simple empirical equations are proposed to quantify the effect of chemical modifier concentration on alpha(2).

No abstract