Relaxations of proton equilibria

We have already emphasized in chapter 1 that the study of electrolytes has been a bridge between physical chemists and physiologists formed by common interests. Many of the latter researchers have made considerable contributions to the subject and some of the former owe their introduction to and interest in biological systems to the important role of ionic equilibria in physiological phenomena. In the historical introduction (section 6.1) to 

Alcohol dehydrogenase serves as an example of an enzyme reaction which involves proton uptake and release during a substrate induced conformation change, and also as part of the stoichiometry of the reaction catalysed. The present discussion is only intended to deal with certain elementary steps without going into the detail of the whole reaction mechanism (see section 5.1). The reaction catalysed by this enzyme 

Binding NAD to the enzyme, E, involves a conformation change which can be monitored by concomitant fluorescence quenching  

The pH dependence of the binding equilibrium shows that the p of a group adjacent to the binding site shifts from 9.8 to 7.6 as the complex between enzyme and NAD is formed. Consequently the pH independent equation for the relaxation time (equation (6.3.7)) for the conformation change should be expanded to 

Clearly, equation (6.4.1) has to be further expanded at pH significantly above neutrality when the dissociation of EH to E + H has to be taken into account. This is a typical example of a relaxation linked to a protonic equilibrium. Such systems can be studied with pH jumps coupled to pressure or temperature dependent buflFers (see pp. 205-6). In the particular case of the reaction of alcohol dehydrogenase the cycle must be completed during the catalytic reaction when a proton is taken up by the enzyme during product dissociation. 

---