162 PLANT PHYSIOLOGY 



Amino-Acid Constitution. — The best definition which can be 

 given of the simple proteins is that of Panzer (1903), who defined 

 proteins as "all substances which upon hydrolysis yield simple 

 amino acids." The amino acids are organic acids with an NH or 

 an NH 2 group substituted for one of the hydrogen atoms. Thus 

 if in acetic acid (CH 3 COOH) we substitute an NH 2 group for one 

 of the hydrogen atoms we get the amino acid, glycine (CH 2 NH 2 

 COOH). Similarly propionic acid (CH 3 CH 2 COOH) with hydro- 

 gens attached to two carbon atoms might give two amino acids, — 

 CH 3 CHNH 2 COOH and CH 2 NH 2 CH 2 COOH. The first, with the 

 substituted NH 2 near the carboxyl end, is called alpha-propionlc 

 amino acid or alanine, and the other beta. While these two are 

 possible, in natural proteins only the alpha amino acids are found, 

 although there may in addition be substituted NH 2 groups on 

 other carbon atoms as well, e. g., lysine (CH 2 NH 2 CH 2 CH 2 CH 2 

 CHNH 2 COOH). It is also possible to substitute NH 2 groups 

 for more than one hydrogen on a single carbon atom, but such 

 products are not found among the amino acids derived from natural 

 proteins. 



Owing to the presence of the amino acids, the proteins are given 

 peculiar properties. If an amino acid is examined closely it will 

 be seen to possess both basic and acid qualities. The carboxyl 

 end is acid and can unite with bases forming salts of the base. 

 The NH 2 , however, is basic and can combine with acids to form 

 salts of the acid. The amino acid can thus form salts with either an 

 acid or a base. The amino acid, alanine, can consequently combine 

 with sodium to form sodium-amino-propionate, CH 3 CH — COONa, 



NH 2 



and can also unite with hydrochloric acid to form alanine hy- 

 drochloride, CH 3 CH — COOH. In the one case the amino acid 



NH 2 HC1 



has behaved like an acid and in the other like a base. Such sub- 

 stances are called amphoteric, and the proteins which result from 

 the amino acids have this same property. 



Because of this dual nature of the amino acids, two of them can 

 be linked together with the elimination of water according to the 

 equation; 



