120 THE PROTEINS AND PROTEASES [CH. 



strong bases as acids. The amino-acid, alanine, for instance, forms salts, 

 sodium amino-propionate with a base, and alanine hydrochloride with 

 an acid : 



CH 3 CH COONa CH 3 CH COOH 



I I 



NH 2 NH 2 'HCI 



Substances behaving in this way have been termed "amphoteric" 

 electrolytes (see also p. 15). 



In the proteins, which are formed by condensation, as explained above, 

 there are always some NH 2 and COOH groups left uncombined. Hence 

 a protein must, in the same way, have the properties of both an acid 

 and a base. 



The amino-acids which are obtained by the hydrolysis of plant pro- 

 teins may be classified as follows : 



Aliphatic compounds. 



Mono-carboxylic morio-amino acids : 

 Glycine or a-amino-acetic acid 



CH 2 (NH 2 )*COOH 

 Alanine or a-amino-propionic acid 



CH 3 -CH(NH 2 )'COOH 

 Valine or a-amino-iso-valeric acid 



CH 3 



>CH -CH(NH 2 )- COOH 

 CH/ 



Leucine or a-amino-iso-caproic acid 



CH 3X 



>CH CH 2 ' CH(NH 9 ) COOH 

 CH/ 



Iso-leucine or a-amino-/3-niethyl--ethyl-propionic acid 



-CH(NH 2 ) -COOH 



Serine or a-amino-^-hydroxy-propionic acid 



CH 2 OH-CH(NH 2 )'COOH 



Dicarboxylic mono-amino acids : 



Aspartic acid or a-amino-succinic acid 



COOH CH 2 CH(NH 2 ) COOH 

 Glutaminic acid or a-amino-glutaric acid 



COOH CH 2 -CH 2 -CH(NH 2 )-COOH 



