180 



5. DETERMIXATIOX OF MECHANISMS AND CONSTANTS 



Product Inhibition 



When the product inhibits the forward reaction by combining with the 

 enzyme or an activator in a manner similar to that of an inhibitor by any 

 of the possible mechanisms, the kinetics and the plots derived from them 

 will be quite comparable to those exhibited by the inhibitions outlined 

 above. The only unique situation is where the back reaction is occurring 

 and the deviation from normal kinetics will depend on the over-all equili- 

 brium constant as indicated in Fig. 5-20. It mav be noticed that even when 



0.01 



-^{-B 



(S) 



'(S) 



Fig. 5-20. Product inhibition at different values of the over-all equilibrium constant 

 A'q. The dotted lines represent the uninhibited reaction. See Eq. 4-38. 



the equilibrium is strongly in favor of the product, the slope of the type 

 A plot is dependent on (P)/^^; in this case the product is behaving as a 

 competitive inhibitor and K^ in easily obtained. The K^ thus determined 

 will be a true dissociation constant and will not necessarily represent the 

 Michaehs constant for the back reaction. 



The inhibition of yeast pyruvate decarboxylase by acetaldehyde was 

 analyzed by Gruber and Wassenaar (1960) and designated as noncompet- 

 itive from a plot of l/v^- against (I). It must be emphasized that from a 

 single plot of this type (i.e., at one substrate concentration), the mecha- 

 nism of the inhibition cannot be immediately derived. However, from a 

 single curve it is possible to calculate Ki from either the slope or the in- 

 tercept on the (I) axis, assuming competitive or noncompetitive inhibition, 

 and these values for K^ should be identical if the assumed mechanism is 

 correct. Nevertheless, it is preferable to plot several curves for different 

 substrate concentrations and determine whether they do or do not meet 

 at one point on the (I) axis. 



