SPECIFICITY IN CHOLINESTERASE REACTIONS 



183 



3.25 3.30 3.35 3.40 3.45 3 50 3 55 3.60 

 T~' X 10^ 



Fig. 6 



Each substrate differs from the next by having one fewer methyl group, 

 thus acetylchohne, dimethylaminoethyl acetate, monomethylaminoethyl ace- 

 tate, and aminoethyl acetate. All are positively charged at neutral pH. 



The Arrhenius plots for these substrates are shown in Fig. 6 (Wilson and 

 Cabib, 1956). It will be noted that the fewer the methyl groups the lower 

 the hydrolytic activity. 



In the case of the poorer substrates we have straight lines, but in the case of 

 the better substrates the loci are very markedly curved. 



Now, if we had an infinite series of substrates, one better than the next, and 

 there was in fact a two step process in which the second step was always the 

 same, the first process must become faster and faster and eventually catch up 

 with — and possibly exceed — the second process. That is what I think has 

 happened here. With the two poorest substrates the first step is rate control- 

 ling, but with the better substrates both steps are rate controlling. 



If we accept this interpretation as correct we can evaluate the entropies and 

 enthalpies for step one by using the above equations provided, we can derive a 

 good value for the specific rate constant of step two by drawing an asymptote 



