VARIATION OF ENZYME INHIBITION WITH pH 677 



HoEI can exist, which in turn implies that the reactions E + I ;;:± EI 

 and H2E -f I ^ HgEI can occur. The dissociation constant for EI would 

 then, in his system, be given by Kj{K^"JK^/') and for HgEI a related 

 expression would hold. In a system such as assumed by Alberty, the ap- 

 parent inhibitor constant would be: 



K/ = Ki -^p-j^ (14-77) 



Jhei 



where K^ refers to the equilibrium constant for the reaction HE + I ^ HEI. 

 It is quite possible to assume generalized systems wherein all the pos- 

 sible reactions between the components occur. For example, when the 

 enzyme is monobasic and only HES forms the product, it may be useful 

 to set up some such system as the following if one is interested in deter- 

 mining the effects of one component on the interaction of another with 

 the enzyme: 



HE + P 



(14-78) 



where the equilibrium constants are related through factors expressing the 

 degree of interaction between the bound components. In fact, this system 

 is identical to Ec^. 3-57 with the activator replaced by the hydrogen ion, 

 and similar equations would be applicable. Even this system, omitting any 

 ionization of the inhibitor or substrate, presents difficulties in the interpre- 

 tation of experimental data. A comiDlete system in which the enzyme, 

 substrate, and inhibitor all are dibasic, and all combinations with the 

 enzyme are possible, would comprise forty-two forms of the enzyme and 

 its complexes. Such a system would be the inhibition by malonate of suc- 

 cinic dehydrogenase. However, only a relatively few complexes are actually 

 possible in most cases; in the inhibition of succinic dehydrogenase, it is 

 likely that the only significant complexes occur between HaE^^ and S^ 

 and I". For these reasons the generalized formulations are, at least at 

 present, impractical. 



The Determination of Ky 



One of the primary purposes of inhibition studies is the determination 

 of the true dissociation constant of the EI complex. It is by means of the 

 values of K^ that different inhibitors may be compared or the energies of 



