90 PHYSIOLOGY 



Thus in the building up of the protein molecule there is an almost indefi- 

 nite coupling up of numerous amino-acid groups, the connecting element in 

 each case being the nitrogen. Of the two or more optical isomers possible of 

 each amino-acid containing more than two carbon atoms, only one is made 

 use of for this purpose. A still further flexibility in its reactions to its 

 environment is conferred on the protein molecule by changes occurring with 

 great readiness in the intra-molecular grouping of its constituent atoms. 

 Thus, if we take the simplest member of the class of polypeptides, glycyl 

 glycine, four structural formulae are possible, namely : 



(1) NH 2 CH 2 CO - NH.CH 2 .COOH 



(2) NH.CH 2 .CO \ 



I > 

 CO.CH 2 .NH/ 



(3) NH 2 .CH 2 .C(OH) = N.CH 2 .COOH 



(4) N.CH 2 .CO 



II > 

 C(OH)CH 2 .NH 3 / 



(2) and (4) being the intramolecular form of the formulae (1) and (3). (3) and 

 (4) are sometimes spoken of as the enolic form. If we consider that perhaps 

 some hundred of the amino-acid groups may go to making up a single protein 

 molecule, it is possible to form some conception of the enormous variability 

 in reaction possible to such a compound. 



THE CONSTITUTION OF DIFFERENT PROTEINS 



All the proximate constituents of proteins, so far as we know, are amino- 

 acids. Of these the following have been isolated, namely, glycine, alanine, 

 amino-valerianic acid, leucine, isoleucine, proline, oxyproline, serine, phenyl 

 alanine, glutamic acid, aspartic acid, tyrosine, tryptophane, cystine, lysine, 

 histidine, arginine, and ' di-amino-trioxydodecoic ' acid. 



The question now arises whether all the different varieties of protein owe 

 their peculiarities to the presence of different amino-acids or whether the 

 greater number of the amino-acids above mentioned are present in all pro- 

 teins, the differences between the latter being determined by differences in the 

 arrangement and relative amounts of their proximate constituents. A large 

 number of analyses of different proteins have been made by Abderhalden, 

 Osborne, and others, utilising the methods for the isolation of amino-acids 

 devised by Fischer. The constitution of some representative proteins as 

 determined in this way are given in the Table opposite. 



These results show that all the proteins contain a very considerable 

 proportion of the total number of amino-acids which have as yet been 

 isolated from acid digests of proteins. The differences in various proteins 

 cannot therefore be determined by qualitative differences in their constituent 

 molecules, but must depend on the relative amounts of the amino-acids 

 which are present and on their arrangement in the whole molecule. As regards 

 relative amounts of amino-acids we find very striking differences. Thus, 



