DECOMPOSITION OF PROTEINS 471 



No molecular formula can be ascribed to the proteins and their 

 molecular weight is still a subject of study. 



Structurally, the proteins are characterized as condensation products 

 of amino acids which are united chiefly by the peptide linkage (R — CO- 

 NH— R), similar to the polypeptides. On hydrolysis by acids, alkalies, 

 or specific enzymes, the proteins break up into the constituent amino 

 acids. From 10 to 25 per cent of the nitrogen is present in the proteins 

 in the unstable form of the amide linkage (R — CONH 2 ). About 60 

 per cent of the nitrogen is assumed to be in the peptide linkage. 



The physical properties of the proteins vary very widely. When 

 dried in the presence of moisture, when boiled with acids and alkalies, 

 and when acted upon by some microorganisms, the proteins show a tend- 

 ency to coloration. This is due to the formation of insoluble pigmented 

 substances, called melanins, probably related to the so-called "humins." 

 Most proteins are soluble in water or in dilute acids or alkalies; a few, 

 like keratin from horn, are insoluble in water and require strong acids 

 and alkalies to bring them into solution. The proteins are ampho- 

 teric substances, being capable of combining with both acids and 

 alkalies, neutralizing them, and causing a decrease in the hydrogen- 

 or hydroxyl-ion concentration. Coagulation, precipitation and color 

 reactions vary with the different proteins, depending on their consti- 

 tution and state of purity. 



On hydrolysis by acids or enzymes, the proteins are broken down 

 to proteoses, then to peptones and finally to amino acids, which are 

 simple crystallizable substances. Some amino acids, like tyrosine, 

 may appear in the early stages of hydrolysis. The proteoses and 

 peptones consist of several groups of amino acids, these groups being 

 smaller than the original protein. The majority of amino acids in the 

 protein molecule are characterized by the fact that one hydrogen in the 

 a position is replaced by NH 2 . In the case of two of the basic amino 

 acids, viz., arginine and lysine, a second amino group is present. A third 

 amino acid, histidine, contains an imidazol nucleus and is basic. Ace- 

 tic acid, CH3COOH, gives amino-acetic acid, glycocoll or glycine, 

 CH 2 NH 2 COOH. The general formula of a mono-ami no-monocar- 

 boxylic acid is R — CH(NH 2 )COOH. When two hydrogens are re- 

 placed by amino groups, we have di-amino acids. 



Due to the presence of amino groups, the amino acids possess both 

 acid and basic properties, so that glycocoll can form salts both with 

 bases (CH 2 NH 2 COOK) and acids (CH 2 NH 2 COOH • HC1) under proper 

 conditions of hydrogen-ion concentration. The dicarboxylic acids, 



