92 PHYSIOLOGICAL CHEMISTRY 



The carbon forms the backbone of the protein as long chains 

 to which the other elements are attached. Oxygen is present 

 in a proportion smaller than in the carbohydrates, but greater 

 than in the fats. Nitrogen is present in various groupings, 

 chiefly as imid NH groups which form the links holding the 

 various parts of the protein together ; as amino groups NH 2 ; 

 and as CONH 2 . These last two forms represent only a small 

 part of the total nitrogen. Sulphur is present for the most 

 part in unoxidized form S in one or two of the constitu- 

 ents of the protein molecule, but may be present also in oxidized 

 form. Phosphorus is present perhaps in different forms, 

 chiefly in oxidized form as phosphate. 



Classification. A knowledge of the percentage composition 

 such as that given above affords us no clue to the structure of 

 the protein molecule, which is very complex. Our knowledge of 

 the actual structure of the proteins is so limited that a classi- 

 fication of the group has been worked out chiefly along other 

 lines, making use of solubilities, source, etc., to distinguish 

 groups, in connection with knowledge of chemical components 

 so far as such information was available. The classification 

 adopted by the American Society of Biological Chemists is as 

 follows : 



I. SIMPLE PROTEINS. II. CONJUGATED PRO- III. DERIVED PROTEINS. 



1. Albumins A. Primary Protein De- 



2. Globulins 1. Glycoproteins rivatives. 



3. Glutelins 2. Phosphoproteins j p ro teans 



4. Prolammes 3. Hemoglobins 2 .' Metaproteins 



5. Albuminoids 4. Nucleoprotems 3> Coagulated Pro- 



6. Histones 5. Lecithoproteins teins 



7. Protamines 



B. Secondary Protein 



Derivatives 



1. Proteoses 



2. Peptones 



3. Peptids 



This classification is by no means final or ideal. Compounds 

 are known which do not fall well into any group, whereas 

 others seem to be intermediate between two groups. This class- 

 ification serves very well, however, to bring comparative order 



