236 ORIGIN OF STRUCTURES AND FUNCTIONS 



Having made a thorough review of the facts which we 

 have referred to, Tristram draws from them the following 

 conclusion: "That proteins do appear to remain more or 

 less constant in composition may well be a reflection of the 

 constancy of an environment, rather than evidence that 

 proteins are compounds of unvarying composition." 



The amino acid composition and sequence in the 



structure of the macromolecules of 



proteins. 



Having made these indispensable remarks about proteins 

 as individual chemical substances we can now proceed to a 

 proper description of the fundamentals of protein chemistry. 



It may now be held to be firmly established, in the first 

 place, that protein molecules are made up of residues of 

 various amino acids and, in the second place, that these 

 residues are linked together in the protein molecule mainly 

 by peptide bonds between the a-amino groups and a-carboxyl 

 groups of amino acids, as was first suggested by A. Ya. 

 Danilevskii" and afterwards proved experimentally by E. 

 Fischer^* and F. Hofmeister^*' and a number of later workers. 



Thus, as a first rough approximation, a protein molecule 

 may be described schematically as a polypeptide chain: 



— CO.CH.NH.CO.CH.NH.CO.CH.NH.CO.CH.NH — 



I I I I 



1 2 3 4 



where Rj, Ro, R3, R.,. etc., the side chains, represent the free 

 atomic groupings of the amino acid residues, which have very 

 diverse chemical properties (those of hydrocarbons, alcohols, 

 thiols, phenols, acids, bases, etc.). 



This sort of structure fundamentally distinguishes proteins 

 from other organic polymers such as cellulose or rubber, in 

 the molecides of which the same atomic grouping (residues 

 of glucose, isoprene, etc.) is repeated over and over again. 



Thanks to the variety of amino acid residues entering into 

 their composition, and also to the great chemical variety of 

 their functional groups, proteins have enormous chemical 

 potentialities. They can react with the countless multitude 

 of substances of living protoplasm to form either true com- 



