SUBSTRATE INHIBITION 



125 



Rise in pH thus reduces affinity for both pyruvate molecules but has rela- 

 tively less effect on the inhibitory binding. If the inhibition by pyruvate 

 occurs when two pyruvate molecules are bound to two sites with which only 

 one pyruvate molecule is bound actively, increase in aK^^ with pH prob- 

 ably reflects in part the same decrease in affinity for pyruvate in the ac- 

 tive complex. An imidazolium ring of histidine with a p^^ of about 7 may 

 be involved in the binding and proton transfer, since there is evidence that 



5 



( PYRUVATE) 



Fig. 4-10. Inhibition of the reverse reaction of lactate de- 

 hydrogenase by pyruvate at different pH's. (From Winer and 

 Schwert, 1958.) 



the hydrogen ion does not come directly from the solution. Inhibition by 

 higher concentrations of DPNH is usually not observed (Hakala et ah, 

 1956; Nygaard, 1956; Winer and Schwert, 1958) but may occur if the 

 pyruvate concentration is low, indicating a competitive situation. 



Substrate inhibition of cholinesterase has been studied extensively and 

 illustrates several interesting principles. Zeller and Bissegger (1943) postu- 

 lated two binding sites in the active center, an anionic site with a negative 

 charge to bind the —N( 0113)3+ end of the acetylcholine molecule and an 



