88 PHYSIOLOGY 



trioxydodecoic acid already mentioned as occurring among the disintegration 

 products of proteins. 



THE BUILDING UP OF THE PROTEIN MOLECULE 



By simple hydrolysis the protein molecule may be broken down into a 

 large number of amino-acids. Analyses of various proteins show that these 

 amino-acids are present in different proportions in the individual proteins, 

 so that in many cases a large number of identical amino-acid groups must 

 be present in the protein molecule with smaller numbers of other groups. 

 In endeavouring to form an idea of the manner in which the amino-acids can 

 be linked together into one gigantic molecule, Hofmeister first put forward 

 the idea that the linkage follows the general formula : 



_CH 2 NH CO 

 or -NH CH 2 CO NH- 



This theory of the constitution of proteins was based on the fact that a 

 similar grouping was known to occur in leucinimide, obtained by the con- 

 densation of two molecules of leucine, 



C 4 H 9 



NH CO 

 CO' NH 



' \CH/ 



C 4 H 9 



and also by the fact that only a small proportion of the NH 2 groups present 

 in the separated amino-acids exist free in the protein molecule. By the 

 action of nitrous acid the terminal NH 2 groups are split off and replaced by 

 OH. When proteins are treated with nitrous acid only a small proportion of 

 the total nitrogen is split off in this way. The linking of the amino groups 

 must therefore take place by means of the nitrogen, i.e. by NH groups. 

 Synthetic experiments have fully confirmed this hypothesis. In 1883 

 Curtius obtained a substance giving the biuret reaction, the so-called 

 ' biuret base/ by the spontaneous polymerisation of glycocoll ester. This 

 base has been shown by recent researches to consist of four glycine molecules 

 arranged together in an open chain. The clue to the structure of this base 

 was given by Fischer, who has devised a number of ingenious methods for 

 combining together amino-acids of any character and in any number. Thus 

 from two molecules of glycine we may obtain the compound glycyl glycine, 

 as follows : 



NH 2 .CH 2 .COOH + HNH.CH 2 .COOH - H 2 - 

 NH 2 .CH 2 .CO.NH.CH 2 .COOH 



