THE PROTEINS 83 



portant. The animal organism cannot build up protein material from nitro- 

 gen of the atmosphere or from combinations, such as ammonia, nitrates, and 

 nitrites. Plants, however, have the property of synthesizing proteins from 

 inorganic nitrogen. The nitrogen in the protein is directly utilizable by the 

 body, so that the animals are ultimately dependent upon plants for their 

 protein supply. 



Nitrogen in the protein molecule occurs in four different forms: 



1. The monamino acid nitrogen, or the nitrogen that is in the NH 2 (amino) 

 group of the a position. 



2. The diamino acid nitrogen, or the basic nitrogen, as shown in the 

 amino group in lysine. 



3. Amide nitrogen; the OH of the second COOH group in the dibasic 

 glutaminic and aspartic acids in protein may be replaced by the amino group. 

 On cleavage, the NH 2 is split off from the acid amide as ammonia. 



4. The guanidine residue as in arginine. 



The distribution of nitrogen in the protein accordingly depends on the 

 amino acids entering into its composition. 



Sulphur of the protein is present in the amino acid cystine. 



It has been stated that the amino acids are combined together in a pro- 

 tein molecule, carboxyl with amino radical. On boiling the proteins with 

 mineral acids, the reaction is reversed and the protein substances are split, 

 with the combining of water, into the individual amino acid components. 

 This change is termed a "hydrolytic cleavage." The qualitative and quan- 

 titative determination of the products thus obtained have shown us that the 

 proteins differ chemically both as to the individual amino acids which enter 

 into the complex protein molecule and the amount of each acid present. 

 Proteins, then, which may give exactly the same percentage composition and 

 elementary analysis and which show practically the same physical prop- 

 erties are found to be actually different individuals of the protein group 

 when the products of their hydrolytic cleavage are investigated. For exam- 

 ple, the following tables give the elementary composition and the amino 

 acids obtained from three proteins which are present in wheat flour. 



ELEMENTARY COMPOSITION OF WHEAT PROTEINS. 



