438 NITROGEN METABOLISM 



Most of our knowledge of the structure of protein molecules has been 

 gained by studying their hydrolytic products. Proteins can be hydrolyzed 

 by treating them with acids, alkalies, or suitable enzymes. The end product 

 of the complete hydrolysis of any protein is always a mixture of amino acids. 

 During the course of protein hydrolysis a number of types of compounds are 

 produced which are intermediate in complexity between the proteins and the 

 amino acids : 



Proteins — > Proteoses — » Peptones -^ Polypeptides 



— > Dipeptides — > Amino x^cids 



It is evident, therefore, that amino acids are the structural units from 

 which the proteins and intermediate products of protein hydrolysis are syn- 

 thesized in living cells. A consideration of the chemical nature and synthesis 

 of the amino acids is therefore necessary before discussing the proteins further. 



The Amino Acids.^ — Amino acids are, as the name suggests, compounds 

 with the properties of both acids and amines. Eveiy amino acid contains at 

 least one carboxyl ( — COOH) group and one or more amino ( — NH2) 

 groups. The simplest amino acid is glycine. Gl^'cine may be considered as 

 acetic acid in which one of the hydrogen atoms of the methyl group has been 

 replaced by an amino group : 



CH3COOH CH2COOH 



Acetic acid 



NH2 



Glycine 



In naturally occurring amino acids the amino group, or one amino group 

 if several occur in the molecule, is always attached to the a carbon atom, 

 which is the one next to the — COOH group. Thirty or more different 

 amino acids have been reported by various investigators, but since all of these 

 have not been confirmed, the number of definitely known amino acids is some- 

 what less. It is also probable that other amino acids remain to be discovered. 

 The names and chemical formulas of the better known amino acids are listed 

 in Table 42. 



It should be noted that although proline is a product of protein hydrolysis 

 and is usually classed as an amino acid it contains only an NH group and no 

 NH2 group. 



Absorption of Nitrogen Compounds from the Soil. — No reliable evi- 

 dence has been obtained that green plants can utilize directly the gaseous 

 nitrogen of the atmosphere in the synthesis of nitrogen-containing organic 

 compounds. Nitrogenous compounds absorbed from the soil serve as the sole 

 source of nitrogen for all terrestrial green plants. Such plants can utilize four 



