PROTEINS 471 



proteins it is combined with a purine base or carbohydrate, 

 forming nucleic acid. 



Amino Acids. — When proteins are broken down into their 

 simple component parts by the process of hydrolysis through the 

 intermediary of heat, enzymes, or other reagents, there result, 

 first, the polypeptides (and other protein derivatives mentioned 

 in group III) and, finally, substances known as the amino acids. 

 These latter are the "building stones," or structural units, of the 

 proteins. So far, 31 amino acids are known. The first and 

 simplest of them is glycine, or glycocoU (alpha amino acetic acid). 

 It was the second amino acid to be discovered (by Braconnot 

 in 1820), having been preceded, historically, by leucine (dis- 

 covered by Proust two years earlier). 



The basis for recognizing an amino acid is now generally 

 conceded to be its isolation by some worker other than its dis- 

 coverer, and the determination of its constitution by synthesis. 

 On this basis, twenty-one amino acids are recognized. They 

 are (in order of their discovery) : 



Leucine 3.5 di-iodo-tyrosine 



Glycine Cystine 



Tyrosine Tryptophane 



Serine Proline 



Aspartic acid Cysteine 



Glutamic acid Hydroxyproline 



Alanine Isoleucine 



Phenylalanine Valine 



Lysine /3-alanine 



Arginine Thyroxine 

 Histidine 



The total number reported is now nearly double that given 

 above, some of which may be authentic but not yet have met the 

 test of recognition. Their technical names are of interest only 

 to the specialist; for example, leucine is /3-isopropyl-a-amino- 

 propionic acid. The amino acids are listed above in the order 

 of their discovery, which is also almost the order of complexity. 

 Glycine is the simplest. The list is not yet complete, as in no 

 instance does the total number of amino acids isolated come to 

 much more than 70 per cent of the weight of the original protein. 

 The majority of the amino acids are commonly found among 

 the hydrolytic products of the proteins, although their relative 



