86 



3. KINETICS OF ENZYME INHIBITION 



(S) 



V[(Ie) - (S.) + K,,Y + 4(S,)Z,, 



[(I,) - (S,) + Z,,] (3-72) 



This value can then be used in the Michaelis-Menten equation v^ = F,„(S)/ 

 [(S) + Kg] and compared with the rate in the absence of inhibitor, given 

 by the equation v = 7,„(S^)/[(S,) + K,], to determine the inhibition. The 

 inhibition curves, plotted in Fig. 3-17 for two substrate concentrations, 

 show an effect of substrate on the inhibition that superficially may resemble 

 competitive inhibition. The inhibition curves rise more steeply in the region 

 where (I^) is comparable to (S^) than for competitive inhibition; in fact, if 



pl- 



FiG. 3-17. Variation of the fractional inhibition with the inhibitor concentration 



when the inhibitor reacts with the substrate. Kg = \ vaM and K^i = 0.1 xq.M. 



Curve A: (S<) = 5 mi/; curve B: (Sj) = 0.5 xaM. 



the inflection is near this region it is often an indication of reaction with 

 substrate although, of course, this is not necessary. A plot of the dependence 

 of inhibition on substrate concentration (Fig. 3-18) shows that raising (S^) 

 will more readily overcome the inhibition caused by reaction with the 

 substrate than competitive inhibition. It may be noticed that (I/)9o/(I/)io 

 for competitive inhibition is usually around 100 while for reaction with 

 substrate it is usually between 5 and 25. 



If inhibition by reaction with the substrate is established, it is possible 

 to determine K^^ from the kinetic data. The concentration of free substrate 

 may be estimated from the degree of inhibition produced at a certain (I^): 



(S) = 



(S,)Z,(1 - I) 

 Ks + i(S,) 



(3-73) 



