230 Biological Chemistry. 



is essentially a polypeptide, which may be represented by 

 the formula 



B' R" R"' 



I I I 



NH 2 CH CO NH-CH CO NH CH CO NH 



R" 



CO NH CH COOH 



A substance of this character would yield on hydrolysis 

 a mixture of amino-acids. Whether groups other than 

 amino-acids are essential to the constitution of proteins 

 cannot yet be definitely decided, as in most cases the 

 total amount of these acids obtainable by hydrolysis will 

 only account for a fraction (though generally a large 

 fraction) of the whole molecule. 



CLASSIFICATION OF THE PEOTEINS. 



No exact classification of proteins based on essential 

 chemical differences is yet possible. The provisional 

 system adopted is based partly on physical differences, 

 partly on the biological origins, and, to a small extent 

 only, on the chemical characters. The proteins are divided 

 into the following classes : 



I. The Simple Proteins. 



(a) The Protamines. This group of proteins has a 

 very limited distribution, and is found only in the sperma- 

 tozoa of certain fish. The members of the class are 

 interesting from a chemical standpoint, in that they yield 

 on hydrolysis chiefly diamino-acids. They are therefore 

 strongly basic in character, containing a large number of 

 amino groups, and as much as 32 per cent, of nitrogen. 



(b) The Histories. These are somewhat similar to the 

 protamines, but they yield a smaller percentage of diamino- 



