100 3. KINETICS OF ENZYME INHIBITION 



K^ can be tested and deviation from 50% inhibition may indicate enzyme 

 inactivation. Similar procedures may be undertaken for competitive in- 

 hibition. 



The Inhibitor Is Progressively Destroyed during the Reaction 



The situation here is relatively simple since the rate of destruction of 

 the inhibitor can usually be determined, whether it is spontaneous or due 

 to the enzyme preparation, and from this rate of destruction the inhibition 

 at any time or over an experimental interval may be calculated. However, 

 if a change in inhibitor concentration is not recognized, the kinetic analy- 

 sis may be in error. In a plot of \ji\ against (I), for example, the slope will 

 be changed (because initial inhibitor concentrations will be plotted where- 

 as the actual concentrations will be lower than these) but the intercept 

 will not, and therefore the calculated K^ will be incorrect. If the inhibitor 

 concentration decreases at an approximately linear rate, the inhibition 

 over the experimental interval is given by: 



^ ^ (3-103) 



(F) + [2/(1 + r)] 



where r = {V)jJ{1')q, {V)q and (I')/ being the initial and final inhibitor 

 concentrations. (I)(,5 is equal to [2/(1 + ^)]jK^i for noncompetitive inhi- 

 bition. 



Inhibition of Forward and Backward Reactions 



It might appear, on the basis of simple chemical reactions, that inhibi- 

 tion would be produced equally on the forward and reverse reactions, since 

 both forward and reverse rate constants must be changed equally if the 

 over-all equilibrium is to be unaffected. However, in enzyme reactions the 

 over-all equilibrium depends often on many rate constants and these may 

 be changed in a great variety of ways without altering the equilibrium. If 

 either mechanism 



E+S^EX^E+P (3-104) 



or 



E +S;=±ES^EP^E -fP (3-105) 



k_i A'_2 A'_3 



applies, the forward and reverse rates are given by: 



^ F.(S) ^ F,(P) 



""' (S) + K, ""' (?) + K, 



