KE ACTION WITH SUBSTRATE. COENZYME, OR ACTIVATOR 



and this value may be substituted in: 



(S) [(I,) - (S,) + (S)] 



Ksi = 



(S,) - (S) 



87 



(3-74) 



which is obtained from Eq. 3-70. 



Occasionally the inhibition may not be due to the simple depletion of 

 free substrate but to the combination of the SI complex with the enzyme. 

 In other words, the substance added and called the inhibitor actually does 

 not interact with the enzyme but by complexing with the substrate pro- 



FiG. 3-18. Variation of the fractional inhibition with substrate concentration for 

 competitive inhibition and for the situation where the inhibitor reacts with the sub- 

 strate. K, = 1 mM. Curve A: K,i = 0.\ mM, (1^) = 1 mi/; curve B: A', = 0.1 mM, 



(It) = 1 mM. 



duces an inhibitory compound. If the free substrate concentration is not 

 significantly depressed and the inhibition is entirely due to the SI com- 

 plex according to the following scheme: 



E+S— ES-^E+P K, 



S + I^SI K,i 



E + SI — ESI Ki 



the expression for the concentration of SI complex: 



(I.)(S,) 



(3-75) 



(SI) = 



(S,) + K,, 



