CH. XXVIII.] THE PROTEINS 417 



direction, considers that the proteins are linkages of a greater or 

 lesser number of these amino-acids, and there is little doubt that in 

 the future this work will result in an actual synthesis of the protein 

 molecule, and with that will come an accurate knowledge of its 

 constitution. 



When the protein molecule is broken down in laboratory 

 processes, or by the digestive enzymes which occur in the alimen- 

 tary canal, the essential change is due to what is called hydrolysis ; 

 that is, the molecule unites with water and then breaks up into 

 smaller molecules. The early cleavage products, which are called 

 proteases, retain many of the characters of the original protein, and 

 the same is true, though to a less degree, of the peptones, which come 

 next in order of formation. The peptones, in their turn, are 

 decomposed into short linkages of amino-acids, which are called 

 polypeptides, and finally the individual amino-acids are obtained 

 separated from each other. 



What we have already learnt about the fatty acids will help us 

 in understanding what is meant by an amino-acid. 



If we take acetic acid, which is one of the simplest of the fatty 

 acids, its formula is 



CH 3 . COOH. 



If one of the three hydrogen atoms in the CH 3 group is replaced 

 by NH 2 , we get a substance which has the formula 



CH 2 . NH 2 . COOH. 



The combination NH 2 , which has stepped in, is called the amino- 

 group, and the new substance now formed is called amino-acetic 

 acid ; it is also termed glycine or glycocoll. 



We may take another example from another fatty acid. Pro- 

 pionic acid is C 2 H 5 . COOH ; if we replace an atom of hydrogen by 

 the amino-group as before, we obtain C 2 H 4 . NH 2 . COOH, which is 

 amino-propionic acid or alanine. 



If instead of propionic we take hydroxy-propionic acid, its amino- 

 derivative (amino-hydroxy-propionic acid) is termed serine. 



A fourth amino-acid is similarly obtained by the introduction of 

 the NH 2 into valeric acid C 4 H 9 . COOH. Amino-valeric acid 

 C 4 H 8 . ISrH 2 . COOH is called Valine. 



Going to the next fatty acid in the series, caproic acid 

 C 5 H n . COOH, we obtain from it in an exactly similar way, 

 C 5 H 10 . NH 2 . COOH, which is amino-caproic acid or leucine. 



According to the way in which the amino-group is linked, a large number of 

 isomeric amino-caproic acids, all with the same empirical formula, are theoretically 

 possible. Many of these have been prepared synthetically, and it has been shown 

 that the amino-caproic acid called leucine, formed by hydrolysis from proteins, is 

 the laevo-rotatory variety, and should be more accurately named a-amino-isobutyl- 



2 D 



