THE PUOTEIXS AXD THEIR METABOLISM 93 



D 



III I V 



H II 



X< H X< 



II I H CH 2 X< / H 



(I!, X< -* j OC Cfl, 



II + HOOC CH. 



CO X CII 2 

 ,-D - X-CH 2 II / | 



/ i + >X CH-. II CO X~CH 2 



II COOH II / 



COOH H COOH 



riyeyl-glycin -f- 2 molecules of glyoocoll * Tetra-glycyl-glycin 



In these reactions we have illustrations of the various reactions that 

 olveocoll and its peptids may undergo. In E. we have a molecule of glycyl- 

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

 ii lycy 1-g lycy 1-glycin. 



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

 i: lycy 1-glycin, gi y i n g r i se to a tetra-peptid, while in D. one molecule of 

 iilyeyl-gl vein 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 XII 2 free 

 :\r one end, and one COOH at the other, making the possibility of the 

 i until of this chain indefinite. 



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

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

 chain that may thus he formed is theoretically endless. 



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

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

 v- ii'd he dealing with a straight chain of amino acids. The difference 

 ' \vf-cn one protein and another would depend only upon the number of 

 :n:;ino acid molecules that go to make the protein molecule. But in the 

 u.uirul proteins we have to deal with a union of about twenty-one amino 

 ; < i'ls. which introduces an entirely new factor, namely that of isomerism 

 am i .-rereo-isomerism. 



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

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

 alauin and alanyl-glycin. 



