HYDROLYSIS OF POLYPEPTIDS 373 



is itself possessed of an optical structure which fits it to unite 

 with certain stereo-isomers and only with them. To pursue 

 Emil Fischer's classical illustration, the enzyme and these par- 

 ticular stereo-isomeric unions fit one another as a key does a lock. 

 The position is therefore this, that the proteolytic enzymes are 

 only adapted to "fit" certain modes of union between amino- 

 acids and that these are precisely the modes of union which occur 

 in the proteins which it is their function to digest. To a certain 

 type of mind, a teleological "explanation" of this fact would 

 doubtless prove very inviting. But the biochemist demands 

 something more satisfactory than teleology, or even than the 

 conveniently comprehensive neo-Darwinian generalization of 

 "adaptation through survival of the fittest." Our physico- 

 chemical view-point inclines us to suspect the existence of some 

 physico-chemical mechanism which, in the building up of these 

 bodies has necessarily brought about their mutual adaptation to 

 and dependence upon one another. The nature of this mechanism 

 is, I believe, not far to seek. In Chap. XVII we shall see that 

 not only the hydrolysis but also the synthesis of the proteins is 

 brought about through the agency of enzymes, and that the 

 enzymes which bring about the synthesis of the proteins are, 

 in all probabiHty, but slight modifications (probably anhydrides) 

 of the enzymes which bring about their hydrolysis. Synthesis, 

 just as hydrolysis, must involve the formation of intermediate 

 compounds between the enzyme and the substrate (in this in- 

 stance the constituent amino-acids out of which the protein is 

 to be built up). Since the enzyme is only adapted to unite with 

 certain stereo-isomers of the amino-acids only these stereo- 

 isomers can be transported into the structure which the enzyme 

 is engaged in building up. In other words, the stereo-isomeric 

 structure of the naturally occurring proteolytic enzymes neces- 

 sarily determines the stereo-isomeric structure of all of the natu- 

 rally occurring proteins, and for the same reason that these enzymes 

 can only split certain amino-acid unions, they can only cause 

 these unions to arise. Hence the occurrence of only those stereo- 

 isomeric groups within the protein molecule which are open to 

 attack by the proteolytic enzymes may be regarded as a conse- 

 quence of their power to react with the digestive enzymes (17). 



From the above-cited results of Fischer and Abderhalden it 

 is evident that not only the stereo-isomeric structure of the 



