Conformation change enzyme reactions: ionization

To illustrate the complexity, we will take a simple case in which the enzymes typically follow the Michaelis-Menten kinetic mechanism. We will also make the following assumptions (a) only one ionic form of the enzyme is active (mono-protic), (b) no change occurs in RDS due to ionization, and (c) the enzyme maintains active conformation within the experimental pH range. For this type of enzyme, an observable quantity, e.g., rate constant (k), depends on the pH of the system as follows (i) the plot of k against pH resembles a titration curve, yielding at the inflection point the pK of the acid involved, (ii) the follows the ionization of the enzyme—substrate complex, (iii) the fC follows the ionizations of both the free enzyme and the enzyme—substrate complex, and (iv) the follows the ionization of the free enzyme only, even when there are multiple intermediates on the reaction pathway. 

The H" -ion concentration has myriad functions within the cell. It acts, for example, as a substrate or product of dehydrogenases and as a factor affecting the ionization of groups which comprise the active site or stabilize its conformation. Substrate ionization may also be responsive to changes in A discussion of the manner in which one may analyze the effect of pH on both the apparent Km and the apparent I max is available in Dixon and Webb (1964). Recently, Knowles (1976) has evaluated the nature of the experiments that have hitherto been used to assess the relationship of pH optima to putative sites involved in the catalytic activity of a particular enzyme. His review defines limits to overambitious interpretations and extrapolations regarding participation of particular groups in the reaction mechanism. 

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