AMINO ACID COMPOSITION AND ORDER 24I 



above, of many chemists of the past and present centuries 

 beginning with Ritthausen and ending with Vickery. It has 

 been shown that even the most complete amino acid analysis 

 of a particular protein taken by itself is still far from char- 

 acterising the physical and chemical properties of that pro- 

 tein, let alone its biological functions. As K. Bailey^" wrote 

 recently : 



One of the most disheartening features of the amino-acid 

 analysis of proteins is that the results have little meaning. To 

 a limited extent they are useful for assessing the nutritional 

 value of a protein, but they do not explain at all the true bio- 

 logical function ; why one protein is an enzyme, another a 

 hormone, another a toxin. 



This is quite understandable even on purely theoretical 

 grounds. Never in the history of science could it be main- 

 tained that the whole is nothing but the sum of its com- 

 ponent parts. (" The whole is always somewhat different 

 from the sum of the separate parts," M. Planck, 1935.) In 

 proteins it is the structure which determines this difference. 

 Even a study of the chemical properties of artificially syn- 

 thesised polypeptides shows that the chemical activities of 

 free amino acid groups (the so-called radicals, R) are markedly 

 changed when they are included in peptide chains, and also 

 depend on the order in which they are arranged in the 

 chains.^* 



Even by simply comparing the effects of various substances 

 and physical factors on a mixture of amino acids and a poly- 

 peptide composed of those same amino acids, it will be found 

 that the amino acid residues of peptides are considerably 

 more labile. For example, they are far easier to racemise 

 than the corresponding free amino acids. In just the same 

 way, the chemical reactivity of particular functional groups 

 such as the hydroxyl group of serine, the phenolic hydroxyl 

 gi'oup of tyrosine, the co-amino group of lysine, etc., is very 

 substantially altered according to which chemical groups are 

 immediately adjacent to them in the polypeptide chain. In 

 a number of cases amino acid radicals, when forming part 

 of a polypeptide chain, can react with compounds to which 



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