PROTEINS 3 



Glutamic acid and aspartic acid are the only dibasic acids which 

 have thus far been obtained as decomposition products of proteins. As 

 there is an apparent relation between the proportion of these acids 

 and that of ammonia which the different proteins yield it is possible 

 that one of the carboxyl groups of these acids is united with NH 2 as 

 an amide, the other carboxyl group being united in polypeptide union 

 (see page 32) with some other amino acid. This might be represented 

 by the following formula: . 



. R CHNH COOH 



CO CHNH 2 CH 2 CH 2 CONH 2 . 



It has been shown by Thierfelder and Sherwin 1 that the amide, 

 glutamine, is a product of normal metabolism and hence this substance 

 rather than glutamic acid is present in the protein molecule. 



FIG. 31. GLUTAMIC ACID. 



Reproduced from a micro-photograph made by Prof. E. T. Reichert, of the University 



of Pennsylvania. 



The glutamic acid, yielded by proteins upon hydrolysis, is dextro- 

 rotatory. Crystals of glutamic acid are reproduced in Fig. 31. 



Proline, C 5 H 9 N0 2 . Proline is a-pyrrolidine-carboxylic acid and 

 possesses the following graphic structure: 



H 2 C CH 2 



H 2 Cl /CH-COOH. 

 NH 



Proline was first obtained as a decomposition product of casein. Pro- 



1 Thierfelder and Sherwin: Zeit. Physiol. Chemie., 94, 1, 1915. 



