8o THE CHEMICAL COMPOSITION OF THE BODY 



THE PROTEINS. 



These nitrogenous substances constitute the most important and com- 

 plex compounds in the body. They are essentially the organic basis of all 

 living substance. At the same time they are the most important of our 

 organic food stuffs. The proteins are necessary as food material for the 

 continuance of life and cannot be replaced in the diet by any other organic 

 or inorganic substances. Without them all life, whether animal or vegetable, 

 is impossible. 



The proteins are substances containing carbon, hydrogen, and oxygen 

 (which are present in fats and carbohydrates). The proteins also contain 

 nitrogen and sulphur. Phosphorus and certain metallic elements are present 

 as constituents of some proteins. The elementary composition of most 

 protein substances falls within the following percentages: 



Carbon from 50 to 55.0 per cent. 



Hydrogen from 6 to 7.3 per cent. 



Oxygen from 19 to 24.0 per cent. 



Nitrogen from 15 to 19.0 per cent. 



Sulphur from 0.3 to 2.5 per cent. 



Phosphorus, when present from 0.4 to 0.8 per cent. 



The individual protein substances are chemical entities. As individuals 

 of a group they differ in elementary composition and in the derivatives 

 which they yield on cleavage of the protein molecule. 



Chemical Structure of Proteins. Proteins are combinations of 

 a-amino acids, the simplest example of which is glycocoll or a-amino acetic 

 acid. Acetic acid has the formula CH 3 COOH; if the NH 2 group is sub- 

 stituted for one of the H's in the CH 3 radical it is an amino acid. The 

 introduction of the amino group in this way yields bodies which combine 

 both with acids and with bases. It is also possible for the amino acids to 

 combine with one another, with the elimination of water. The reaction, 

 however, can only be brought about under certain conditions. For instance, 

 glycocoll can be combined with itself as a dipeptid or combined with any 

 other amino acid. The combination may be indicated by the following: 



CH 2 - NH 2 - CO I OH +H j HN - CH 2 - COOH= 



glycocoll plus glycocoll 



CH 2 - NH 2 - CO - NH - CH 2 - COOH + H 2 O 



glycyl-glycine plus water 



A glance at the chemical formulae of the resulting dipeptid indicates 

 that in this new substance there is still an amino group (NH 2 ) and a carboxyl 

 group (COOH) which are not combined. Another amino acid may be 

 joined on to the carboxyl root, and yet a fourth. on to the remaining amino 



