NERVE ACTIVITY 



Extensive investigations of the molecular forces involved in 

 the interaction of acetylcholine with the esterase were carried out 

 during the last few years (Wilson (38)). The main results may 

 be only very briefly summarized : The active surface of the 

 enzyme protein contains a negatively charged site, the "anionic 

 site," which reacts by Coulombic forces with the positive electric 

 charge of acetylcholine and contributes hereby to the attraction, 

 fixation, and orientation of the substrate upon the enzyme sur- 

 face. The existence of such a negative site has been demon- 

 strated in various ways with suitable substrates as well as com- 

 petitive inhibitors (42). In addition to the Coulombic forces, 



ANIONIC SITE ESTERATIC SITE 



i I 



CHV PROTEIN ^ ^_Q(+) 

 I ^' ^ I 



CH3 ^N — CHj— CHg — 0-C— 0*^-^ 



! 



CH3 CH3 



Fig. 2. Schematic presentation of interaction between the 

 active groups of acetylcholinesterase and its substrate. 



the methyl or alkyl groups of the nitrogen contribute to the 

 binding by van der Waals forces. This has been demonstrated 

 by the use of methylated competitive inhibitors of the ammonium 

 and hydroxyethyl ammonium series (Wilson (36)). Besides the 

 anionic site attracting the cationic nitrogen there is also a group 

 in the enzyme surface which reacts with the ester group and has 

 been referred to as the "esteratic site." The carbonyl group has 

 a marked polar character. The importance of the electrophilic 

 character of the carbon has been demonstrated experimentally 

 (2). The observations suggest tliat a covalent bond is formed 

 between the carbon and some basic group in the enzyme. A 

 further clue was obtained from observations on the pYL depend- 

 ence of the catalysis. Since the constitution of acetylcholine 

 does not vary with /?H, changes in enzyme activity must be 

 attributed to changes in the protein. The changes were inter- 

 preted in terms of the dissociation of acidic and basic groups 



637 



