68 



3. KINETICS OF ENZYME INHIBITION 



tions are given in Eq. 3-32. We have retained the use of fractional inhibition 

 i, rather than fractional activity a as in the equations of Straus and Gold- 

 stein, to be in accord with our usual symbolism and because we shall be 

 particularly concerned with the degree of inhibition. 



When n molecules of inhibitor react for each active center of the enzyme, 

 Eq. 3-33 is modified to: 



(I/) 



1 



m{E/) 



(3-37) 



where (I/) = (I^)/(K()^/". The zones are defined similarly but the boundaries 

 between the zones depend on the value of n (Straus and Goldstein, 1943; 

 Goldstein, 1944). 



The variation of inhibition with inhibitor concentration depends upon 

 the zone in which the system lies, as shown in Fig. 3-9, where the deviation 



Fig. 3-9. Variation of the fractional inhibition with the inhibitor concentration 



at various concentrations of the enzyme in a mutual depletion system in different 



zones. Ki = 10-^ mil/. Curve I: (E/) = 0.01, zone A; curve II: (E/) = 1, zone B; 



curve III: (E/) = 10, zone B; curve IV: (E/) = 100, zone C. 



from zone A behavior as enzyme concentration rises manifests itself as a shift 

 of the curves to the right and a change in shai)e. The decrease of inhibition 

 at higher enzyme concentrations is due, of course, to the depletion of free 

 inhibitor. In Fig. 3-10 the dependence of the inhibition on enzyme concen- 

 tration is illustrated. It is clear that the lower (I,), the more readily will 

 increasing (E,) reduce the inhibition; thus a system in which the inhibition 

 is high will tend to remain in zone A longer as (E^) increases, compared to 

 one in which the inhibition is low. 



