THE PROTEINS 85 



CH NH 

 I! /CH 



CH W 



and is known as HISTIDINE. Its structural formula is as follows: 



CH NH x 



CH 



CH 2 .CH.NH 2 .COOH 



i.e. it is iminazol a-amino-propionic acid or iminazol alanine. Since it occurs 

 in the phosphotungstic precipitate from the products of acid disintegration 

 of proteins and contains six carbon atoms, it was formerly classified with 

 lysine and arginine as a hexone base. 



D. SULPHUR-CONTAINING AMINO-ACIDS 



Sulphur forms an integral part of the molecule of all classes of proteins 

 except protamines. In some substances allied to proteins, such as keratin, 

 it may occur to the extent of 3 per cent. On boiling proteins with caustic 

 potash or soda, a portion of the sulphur is split off to form a sulphide, which 

 gives a black precipitate on addition of copper salts. On this account it was 

 formerly thought that the sulphur must be present in two forms, the oxidised 

 and the unoxidised, in the protein molecule. Recent investigation has 

 shown however that practically the whole of the sulphur is present in the 

 form of CYSTINE, and that this body on boiling with alkaline solutions gives 

 up only a little more than half its content in sulphur. 



This substance, which has been known for many years as the chief con- 

 stituent of a rare form of urinary calculus and as occurring in the urine in 

 certain cases of disordered metabolism, is again a derivative of the three- 

 carbon propionic acid. On reduction it gives a body known as cysteine, 

 which is a-amino-thiopropionic acid. 



CH 2 SH 



I 

 CH.NH 2 



I 

 COOH 



Cystine itself is compounded of two cysteine molecules joined together by 

 their sulphur atoms and has the formula 



I I 



CH.NH 2 CH.NH 2 



I I 



COOH COOH 



E. OTHER CONSTITUENTS OF THE PROTEIN MOLECULE 



When we add together the total amino-acids obtainable by the acid 

 disintegration of any given protein, a considerable proportion of the original 

 protein remains unaccounted for. This remainder must have a greater 

 content in hydrogen and oxygen than the amino-acids enumerated above, 



