The Proteins and Their Metabolism 



A. I. RIXGER 



NEW YORK 



Introduction 



The proteins are the most important constituents of the animal and 

 plant kingdoms. They are an ill-defined group, colloidal in character, 

 non-volatile and obtainable in a pure state with the greatest of difficulty. 



Just as the molecules of the simple chemical compounds are built up of 

 atoms and radicals, the protein molecule is composed of the union of a 

 great many amino acids. In all, about twenty-one different amino acids 

 have been found, and there is every reason to believe that more will be 

 found in the course of time. When one realizes that the amino acids them- 

 selves are of rather large size and that all of them may be present in most 

 of the proteins, one can readily appreciate the enormous size and complex- 

 ity of the protein molecule. The exact determination of the molecular 

 weight of the protein seems at present to be a hopeless task, in spite of 

 many ingenious attempts. By means of the freezing point method, egg al- 

 bumin is found approximately to possess a molecular weight of about 14- 

 000, and calculating the molecular weight of hemoglobin on the basis of 

 one atom of iron, one gets the figure of 16000. The protamins, which 

 are the simplest proteins, have a molecular weight of approximately 4000. 



Elementary Composition of Proteins 



The proteins are composed of the following elementary constituents: 

 Carbon, Hydrogen, Nitrogen, Oxygen and Sulphur. The quantitative 

 relationship of these elementary constituents is found to fluctuate in 

 the different proteins within narrow limits. Carbon, 50 to 55 per cent ; 

 hydrogen, 6.5 to 7.5 per cent; nitrogen, 15 to 17.5 per cent; sulphur, 0.3 

 to 2 per cent; phosphorus, 0.4 to 0.8 per cent; oxygen, 21 to 23 per cent. 



Classification of the Proteins 



Up to the present we have not yet arrived at, any definite knowledge 

 concerning the structural formula of the protein molecule, and until that 



81 



