REACTION WITH SUBSTRATE, COENZYME, OR ACTIVATOR 85 



and the rate equation is: 



"' = ^" (S'XA') + (.V) + (D + 1 '^-««' 



whicli compared to Eq. 2-69 gives the inhibition as: 



(!') 



(D + (A') + 1 + (S')(A') 



(3-69) 



The unexpected dependence of the inhibition on substrate concentration, 

 although the mechanism is completely competitive with activator, is due 

 to the fact that increasing (S) and (A) forms more EAS complex and reduces 

 the amount of enzyme available to combine with inhibitor. Reciprocal plots 

 will remain linear but an error may be introduced into the calculation of K^. 



INHIBITION BY REACTION WITH SUBSTRATE, COENZYME, 



OR ACTIVATOR 



The inhibitions previously discussed have all resulted from reaction of 

 the inhibitor with the apoenzyme at various regions relative to the binding 

 sites for the substrate or other components of the system. The situation in 

 which the inhibitor reacts directly with the substrate, or with other disso- 

 ciable components, will now be considered. The types of inhibition produced 

 are frequently difficult to distinguish from those where the apoenzyme is 

 attacked if the usual kinetic analysis is applied uncritically. 



Reaction of Inhibitor with Substrate 



When reaction of an inhibitor with the substrate prevents the binding 

 of the substrate to the enzyme, the effective concentration of free substrate 

 is reduced; only that fraction of the substrate that is uncombined with the 

 inhibitor is free to undergo the enzymic reaction. From the dissociation 

 constant for the substrate-inhibitor compound it is possible to calculate 

 the concentration of free substrate (S) and use this value, instead of the 

 total concentration (S^). in the previous equations. 



s + i — SI ^--^-^^iT" ^^-^^^ 



(S,) = (S) + (SI) 

 (I,) = (I) + (SI) 

 (S)^ -f [(I,) - (S,) + ^,,](S) - (S,)Z,, = (3-71) 



