THE PROTEINS AND THEIR METABOLISM 93 



D 



III I V 



H H 



X< H X< 



H | H CH 2 X< / H 



CH 2 X< -^ | OC CH 2 



H -f HOOC CH 2 



CO - - X CH 2 

 CO - - X CH 2 H / 



/ + >X CH 2 H CO X CH 2 

 H COOH- H / 



COOH H COOH 



Glycyl-glycin -f- 2 molecules of glycocoll * Tetra-glycyl-glycin 



In these reactions we have illustrations of the various reactions that 

 glycocoll and its peptids may undergo. In B. we have a molecule of glycyl- 

 glycin unite with one molecule of glycocoll, giving rise to a tri-peptid 

 glycyl-glycyl-glycin. 



In C. one molecule of glycyl-glycin unites with another molecule of 

 glycyl-glycin, giving rise to a tetra-peptid, while in D. one molecule of 

 glycyl-glycin unites with two molecules of glycocoll, giving rise to the 

 same tetra-peptid. 



From these illustrations we also learn that no matter how many amino 

 acids are hooked on to one another, they will always have one NH 2 free 

 at one end, and one COOH at the other, making the possibility of the 

 length of this chain indefinite. 



We may therefore conceive of an amino acid as an individual with 

 an arm at either side, capable of clasping two other individuals. The 

 chain that may thus be formed is theoretically endless. 



If a protein were made up by the union of a large number of molecules 

 of a single amino acid the problem would be comparatively simple. We 

 would be dealing with a straight chain of amino acids. The difference 

 between one protein and another would depend only upon the number of 

 amino acid molecules that go to make the protein molecule. But in the 

 natural proteins we have to deal with a union of about twenty-one amino 

 acids, which introduces an entirely new factor, namely that of isomerism 

 and stereo-isomerism. 



Only one kind of union is possible between glycocoll and glycocoll. 

 Between glycocoll and alanin, however, two unions are possible, glycyl 

 alauin and alanyl-glycin. 



