132 



4. SUBSTRATE INHIBITION AND PRODUCT INHIBITION 



Competition of Substrate with Substrate-Activator Complex (Type D) 



When the true substrate for the enzyme is a complex of the added sub- 

 strate and an activator (as in the first reaction scheme in the previous sec- 

 tion) it is possible that the reaction: 



E -f S ;;:± ES 



may also occur and block the AS complex from the enzyme. An example of 

 this mechanism may be the inhibition of inorganic pyrophosphatase by 

 excess substrate, first demonstrated by Bailey and Webb (1944). This 

 inhibition was initially believed to be type A, later type C with pyrophos- 

 phate complexing with and removing the activator Mg++ (Heppel and 

 Hilmoe, 1951), and most recently type D since evidence was obtained that 

 the true substrate is magnesium pyrophosphate (Bloch-Frankenthal, 1954). 

 One is unable to distinguish between types C and D inhibition by varying 

 the concentrations of substrate and activator, since type D is also charac- 

 terized by a symmetrical rate-pS curve most commonly and a rate equation 

 identical to 4-16 but with the following values for the constants: 



K. 



(A,) 



K„ 



B = 



C = 



(A<) 



^a.(A,) 



■** Aas 



( E)(AS) 

 (EAS) 



Indeed, this type of inhibition is indistinguishable from types A and B if 

 the activator is unrecognized and its concentration not varied. The con- 

 formity of most types of substrate inhibition to the same kinetic pattern 

 has made it very easy for theorists to prove their particular mechanism. 



The Substrate Interferes with the Binding of an Acceptor (Type E) 



This represents the situation in transfer reactions where one substrate 

 interferes with the binding of the second substrate. In reactions involving 

 a dye as an electron acceptor, the reduced substrate may hinder the reac- 

 tion of the dye with the enzyme. The inhibition of xanthine dehydrogenase, 

 using methylene blue as an acceptor, by high concentrations of xanthine 

 or hypoxanthine has been attributed to such a mechanism (Dixon and 



