AMINO ACID COMPOSITION AND ORDER 239 



natural proteins, especially in those of higher animals and 

 plants. It is interesting to note in this connection that, as the 

 number of substances studied grows greater and the accuracy 

 of the results improves, the number of so-called ' common ' 

 amino acids found in proteins does not increase ; in recent 

 years it has even shown a certain tendency to decrease. Thus 

 H. B. Vickery and C. L. A. Schmidt*" considered, in 1931, 

 that there were twenty-two common amino acids, this number 

 later fell to twenty-one, and now, as P. Desnuelle writes, " we 

 shall therefore assume twenty common amino acids only, and 

 this number will probably never be much modified ". We 

 give here a table of these amino acids and the chemical char- 

 acteristics of their radicals, borrowed from Desnuelle's paper 

 (Table 2). 



The question inevitably arises as to why the endless variety 

 of proteins which we can isolate from contemporary animals 

 and plants should be made up of such a limited number of 

 structural elements. It is clear that as a result of the physical 

 and chemical laws discussed in the previous chapter there 

 could and must have been formed many, many other amino 

 acids as well as those given in the list. Why, then, do we not find 

 them in contemporary proteins? Obviously, in the formation 

 of these latter, there must have occurred a strict selection of 

 those amino acids indispensable for life. It would seem that 

 the chemical fimctions which we have just discussed are 

 quite sufficient for the catalysis and co-ordination of all the 

 various metabolic reactions. Combined with one another into 

 protein molecules the twenty amino acids listed form all the 

 enzymes necessary for metabolism and the other important 

 internal chemical mechanisms of living protoplasm. There 

 can be no doubt that in the process of evolution of living 

 matter there took place a rationalisation of these mechanisms 

 and consequently a standardisation of them, analogous to 

 that occurring in technical processes. All those amino acids 

 which were not absolutely necessary to life were eliminated 

 in later generations by natural selection. 



R. L. M. Synge^^ has written very pointedly: 



If we assume, on the basis of evolutionary theory, that the 

 proteins of highly organised beings became progressively more 



