CHAPTER IV 

 PROTEINS: 1 THEIR DECOMPOSITION AND SYNTHESIS 



THE proteins are a class of substances, which in the light of our pres- 

 ent knowledge, consist, in the main, of combinations of a-amino acids or 

 their derivatives. These protein substances form the chief constituents 

 of many of the fluids of the body, constitute the organic basis of animal 

 tissue, and at the same time occupy a decidedly preeminent position 

 among our organic food-stuffs. They are absolutely necessary to the 

 uses of the animal organism for the continuance of life and they cannot 

 be satisfactorily replaced in the diet cf such an organism by any other 

 dietary constituent either organic or inorganic. Such an organism may 

 exist without protein food for a period of time, the length of the period 

 varying according to the specific organism and the nature of the substi- 

 tution offered for the protein portion of the diet. Such a period is, how- 

 ever, distinctly one of existence rather than one of normal life and one 

 which is consequently not accompanied by such a full and free exercise 

 of the various functions of the organism as would be possible upon an 

 evenly balanced ration, i.e., one containing the requisite amount of 

 protein food. These protein substances are, furthermore, essential 

 constituents of all living cells and therefore without them vegetable life 

 as well as animal life is impossible. 



The proteins, which constitute such an important group of sub- 

 stances, differ from carbohydrates and fats very decidedly in elementary 

 composition. In addition to containing carbon, hydrogen, and oxygen, 

 which are present in fats and carbohydrates, the proteins invariably 

 contain nitrogen in their molecule and generally sulphur also. Pro- 

 teins have also been described which contain phosphorus, iron, copper, 

 iodine, manganese, and zinc. The percentage composition of the more im- 

 portant members of the group of protein substances would fall within 

 the following limits: = 50-55 per cent, H = 6-7-3 per cent, O = 

 19-24 per cent, N=i5~i9 per cent, 8 = 0.3-2.5 per cent, P = o.4- 

 0.8 per cent when present. When iron, copper, iodine, manganese, or 

 zinc are present in the protein molecule they are practically without 



1 The term proteid has been very widely used by English-speaking scientists to signify 

 the class of substances we have called proteins. 



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