706 14. EFFECTS OF pH ON ENZYME INHIBITION 



EI complex would be either raised or lowered depending on whether the 

 change in enzyme charge tends to repel or attract the inhibitor. However, 

 many inhibitions do not involve such electrostatic mechanisms in the for- 

 mation of the EI complex, the reaction being frequently chemical, as in 

 the inhibitions produced by alkylating agents, arsenicals, and mercurials 

 on SH-dependent enzymes. It is, of course, possible that electrostatic in- 

 teractions occur even here if the inhibitor is charged, but the reaction once 

 the inhibitor has reached the surface of the enzyme may vary with pH 

 for quite different reasons. In these cases, it is possible that the inhibition 

 rate may be increased or decreased, but the final level of inhibition reached 

 remains unchanged by the pH if /{", is sufficiently small so that the inhibi- 

 tion is pseudoirreversible. No really quantitative work has been done on 

 the pH dependence of the inhibition rates of such reactions and what little 

 has been found will be discussed in the volumes on individual inhibitors. 



Variation of Inhibition with pH in Mutual Depletion Systems 



When the system is in zone C (inhiliitor almost entirely bound to the 

 enzyme), the inhibition will be independent of pH since i = (I^)/(E^) and 

 does not depend on /il,. If the inhibitor is not all in the active ionic form 

 that binds to the enzyme at the pH of the ext:)eriment, the strong binding 

 will shift the ionization equilibrium until all the inhibitor is combined 

 with the enzyme. In zone B (inhibitor partly bound and partly free), the 

 situation is more complex and one cannot rewrite Eq. 3-32 simply by 

 replacing K^ by fJ^K^, because the system will be intermediate between 

 zone A and zone C. In general we may say that zone B inhibition is less 

 sensitive to pH than a comparable system in zone A would be. 



One can also assume a system such as: 



^ ES ^ E + P 



E ^ (14-133) 



EI 



I -f H^ HI (14-134) 



which represents competitive inhibition by an ionizing inhibitor, the con- 

 servation equations for which are: 



(Ee) = (E) -t- (EI) + (ES) (14-135) 



(I,) = (I) + (HI) + (EI) (14-136) 



The concentration of ES is given by a quadratic equation, which may be 

 solved for different pH's to obtain the inhibitions. However, it is difficult 

 to use experimental data from such inhibitions to calculate Ki directly. 



