172 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



The enzyme molecule bears — NH3+ and — COO~ groups. The former 

 may lose a proton and the latter can accept a proton. It can be postulated 

 that when an ester is hydrolysed by an esterase, the — NH3+ grouping 

 approaches the alcohol oxygen and donates a proton to it. Simultaneously, 

 the — C00~ approaches the carbon of the carbonyl group. Also, it must be 

 postulated that a water molecule remains sandwiched between the — C00~ 

 and the C atom, and that the — C00~ tends to draw to itself a proton from 

 the water molecule at the same time assisting the 0H~ ion to approach 

 the carbon of the carbonyl group. The picture is as follows : 



O 



II 

 R— C— O— R' 



H— O 



I 

 H H 



o- 



0=C N+ 



o- 



R— C 0+— R' 



I 

 H- H 



I 

 O 



I 

 0=C N+ 



Enzyme 



Enzyme 



The enzyme-substrate complex thus formed can then split in the follow- 

 ing manner : 



R— C 



OH 4- H— O— R' 



H 



O 



I 

 0=C N 



Enzyme 



This proposed mechanism is based upon the idea that a protein is a semi- 

 conductor, in which, as in a metal, certain electrons are not strictly local- 

 ized to definite points in the macromolecule, but are, to a certain degree, 



