VARIATION OF ENZYME ACTIVITY WITH pH 653 



I. Direct alteration of the state of the ionizable groups at the active center 



A. Effects on the binding of substrates, activators, or coenzymes to the 

 active center. 



B. Effects on the rate of dissociation of the ES complex. 



C. Effects on the catalytic reaction when this involves H"*" donated or 

 accepted by groups at the active center. 



II. Indirect effects on the ionizable groups at the active center 



A. Arising from ionizable groups vicinal to the active center. 



B. Arising from changes in the total charge on the enzyme protein. 



III. Alteration of the states of ionization of the nonenzyme components of 

 the system 



A. Components directly involved in the reaction (substrates, activators, 

 or coenzymes). 



B. Modifiers indirectly affecting the reaction rate (buffers and possible 

 impurities in the enzyme preparation). 



IV. Alteration of enzyme protein structure 



A. Local changes in structure modifying the configuration of the ac- 

 tive center. 



B. Denaturation processes involving the total protein structure. 



C. Association-dissociation reactions of enzyme subunits. 



All of these effects relate fundamentally to changes in the ionization states 

 of groups on the components of the total enzyme system. Hence, the pri- 

 mary reactions of the hydrogen ion may be expressed in terms of the classic 

 acid-base dissociation concepts. The relations between these primary reac- 

 tions and the over-all enzymic rate, however, are often not simple. 



The classification of the mechanisms involving enzyme ionization groups 

 according to the location of these groups relative to the active center is 

 similar to that of Alberty (1956 a) who divided the ionizable groups into 

 those that (1) occur at the active center and which must exist in a par- 

 ticular ionized state for catalytic activity, (2) occur in the neighborhood 

 of the active site and have a definite effect on the catalytic activity, and 

 (3) occur over the rest of the surface of the enzyme. The major effects ob- 

 served upon changing the pH will depend on the ionizable groups at the 

 active center; the quantitative kinetic treatments have dealt almost ex- 

 clusively with such groups. However, the vicinal groups and the total 

 enzyme charge can exert effects which are appreciable and they must be 

 considered in more refined theories. Some of the observed discrepancies 



