GENERAL STRUCTURE AND COMPOSITION 9 



teins, and derived proteins nitrogenous extractives, and pigments. 

 The interesting substances known as enzyms probably form like- 

 wise a group under this head. 



Simple Proteins. Under this head are grouped those proteins 

 whose molecules contain only protein material; in contradistinc- 

 tion to the conjugated proteins whose molecules contain protein 

 in combination with a non-protein substance. 



Each of them contains carbon, hydrogen, oxygen, and nitrogen; 

 most of them contain sulphur also, and a few phosphorus in ad- 

 dition. These elements are united into very complex molecules, 

 and although different members of the group of simple proteins 

 differ from one another in minor points they all agree in their 

 broad features. The common body proteins have a similar per- 

 centage composition, falling within the limits given in the follow- 

 ing table: 



Carbon 50 to 55 per cent. 



Hydrogen 6.5 to 7.3 " " 



Oxygen 19 to 24 " 



Nitrogen 15 to 17.6 " " 



Sulphur 0.3 to 2.4 " 



In addition a small quantity of ash is usually left when a protein 

 is burned, showing that some inorganic salts are held in com- 

 bination with it. 



Recent chemical investigation has shown that the protein 

 molecule is a complex, made up of a number of simpler molecules 

 joined together. When a protein is boiled with a dilute acid its 

 molecules are decomposed, and the resulting solution is found 

 when examined to contain a mixture of the substances whose in- 

 dividual molecules were formerly parts of the complex protein 

 molecules. Sixteen such substances have been obtained from de- 

 composed proteins; they all contain nitrogen, and they all belong 

 chemically to the group of amino acids. Some proteins contain 

 all of them; others only a few. The characteristics of different 

 proteins are supposed to depend on which of these amino acids 

 are present in the molecules and also on their arrangement or 

 grouping therein. 



There are a number of chemical tests which may be used in de- 

 tecting the presence of proteins; but only a few of them apply to 



