EFRAIM RACKER 



the presence of substrate excess may lead to the formation of 

 inactive enzyme. A modified MichaeUs-Menten equation was 

 derived for this case by Foster, McRae, and Bonner (7) : 



Ve. (S) 

 V = 



k: + (S) + (S)7C 



in which K^ and V^^ are analogous to K^ and F^^ of the 

 Michaelis-Menten equation. The term {S)^/C is an index of 

 the probability of two substrate molecules combining with the 



ACTIVE COMPLEX INACTIVE COMPLEX 



Figure 1. 



enzyme, resulting in the formation of inactive enzyme substrate 

 complex. At low concentrations of substrate the term (S)VC' 

 is negligible ; with increasing substrate concentrations it becomes 

 appreciable, the velocity v decreases. When the concentration 

 of substrate reaches the value of C, the velocity becomes half 

 of maximal ; thus one half of the enzyme molecules is in the form 

 of the inactive, oversaturated enzyme complex. Experimental 

 data on the effect of indoleacetic acid concentration on the 

 growth of oat coleoptile sections fit in a remarkable manner the 

 theoretical curve for a two-point attachment. Indeed, studies on 

 the effectiveness of various indoleacetic acid analogues revealed 

 that two specific groupings on the molecule are required for 

 biological activity. 



A similar interpretation has been given to the inhibition of 

 acetylcholinesterase at high substrate concentration (cf. 55). 

 An extensive analysis of the mechanism of action of this enzyme 

 has been carried out by Nachmansohn, Wilson, and their 

 collaborators (31,54). They investigated the formation of the 

 enzyme-substrate complex with the aid of ionizable and non- 

 ionizable substrates and inhibitors at various pH values. These 

 kinetic studies revealed some interesting properties of the two 

 sites of the enzyme, which are referred to as the anionic and 



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