THE PROTEINS 31 



and recent research has since shown that the variations are even 

 greater than those just stated. 



The same fact is brought home more vividly when the cleavage 

 products are separated and estimated. These differ both in kind 

 and in amount, but nearly all of them are substances which are 

 termed amino-acids. Emil Fischer, to whom we owe so much of our 

 knowledge in this direction, considers that the proteins are linkages 

 of a greater or lesser number of these amino-acids, and there is great 

 hope that in the future his 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 the laboratory 

 by processes similar to those brought about by the digestive 

 ferments which occur in the alimentary 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 

 first 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 for- 

 mation. The peptones in their turn are decomposed into short link- 

 ages 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, we see that 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- 

 grotip, 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. Propionic 

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

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

 propionic acid or alanine. Going a little higher in the scale, and taking 

 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. 



All the three amino-acids mentioned glycine, alanine, and leucine 



