VARIATION OF EXZYME ACTIVITY WITH pH 655 



likely that in'oteins are fictile enough to respond structurally to changes 

 in local electrical fields. The distance between two binding or reactive groups 

 at the active center might be varied slightly as the charged groups in the 

 neighborhood appear or disappear. Likewise, little is known of the imiDor- 

 tance of the dissociation of enzymes into subunits and the effects this would 

 have on activity. It is known that there is little association of the units 

 of «-chymotry]3sin at pH 8, but that association occurs to some extent at 

 pH 7 and presumably increases at lower pH's (Booman and Niemann, 1957). 

 The enzymic activity seems to be greater in the dissociated form. Since the 

 degree of association depends on the enzyme concentration, the likelihood 

 of significant association, and thus the possibility of modifying it, would 

 be greater in mitochondria or cells where the concentrations of enzymes 

 are often much greater than in studies of isolated enzymes. 



General Dependence of Enzymic Rate on pH: Occurrence of pH Optima 



A characteristic feature of the rate-pH curves for most enzymes is that 

 the rate is maximal at a certain pH (or over a range of pH) and falls off 

 on either side. The pH at which the activity is maximal is called the optimal 

 pH. The optimal jdH for a iiarticular enzyme will usually vary with the 

 substrate used, the temperature, the ionic strength, and other factors. The 

 concentration of the substrate may also be important: the pH optimum 

 of the alkaline phosphatase of rat kidney shifts to progressively higher 

 values as the substrate concentration is increased (Neumann, 1949). The 

 presence of an inhibitor may modify the pH optimum and certain informa- 

 tion on the mechanism of the inhibition can occasionally be obtained from 

 such an effect. Possible explanations for pH optima will be presented in 

 the following sections but it is well to emphasize immediately that these 

 optima do not depend on irreversible or denaturation changes, inasmuch 

 as they occur within pH ranges in which the changes in activity are com- 

 pletely reversible. When denaturation takes place, its effect on activity 

 adds to the reversible changes, and for the purposes of kinetic evaluation 

 this should be corrected for. Finally, a common misconception in the in- 

 terpretation of pH optima should be pointed out. It is frequently stated 

 that the existence of a pH optimum implies two ionizing groups on the en- 

 zyme; this is sometimes extended to state that these groups are actively 

 engaged in the catalytic reaction (e.g., acting as proton donors and accept- 

 ors). In the first place, a single dibasic group on the enzyme can account 

 for this phenomenon. In the second place, when two groups are involved, 

 it is not necessary that they both be on the enzyme; the interaction of E~ 

 and SH+, for example, would occur significantly only in the pH range be- 

 tween the pK^'s of the enzyme and substrate. All that pH optima imply 

 is that the system involves two or more dissociable protons. 



