aa8 Biological Chemistry. 



the necessary losses due to secondary decompositions and 

 the difficulties of separating the individual acids, and 

 partly, probably, to the fact that not all the hydrolysis 

 products are at present known. Nevertheless, it is obvious 

 that all the proteins can be characterized by the fact that 

 all yield amino-acids on hydrolysis, although there is no 

 apparent resemblance between them, and they differ so 

 markedly in physical appearances. This statement may be 

 illustrated by contrasting two such substances as egg-white 

 and horn. In spite of the deficiencies of the experimental 

 methods, there is no doubt that the various proteins differ 

 largely from one another as regards their hydrolysis 

 products. 



CONSTITUTION OF THE PKOTEINS. 



It is now generally assumed that the proteins belong 

 to a class of chemical substances called by Fischer poly- 

 peptides. These are formed by the condensation of 

 amino-acids by the elimination of the elements of water 

 between amino and carboxyl groups. As an example of 

 the simplest of such substances, that derived from the 

 condensation of two molecules of glycine may be cited. 

 Its formation may be represented by the following for- 

 mula (although it is actually obtained by a more complex 

 process) : 



NH 2 CH 2 -CO-JOH HjNH CH2-COOH 



'^oTNH^CHa-CO NH CH 2 -COOH 



Such a substance on hydrolysis will yield two molecules 

 of glycine 



NH 2 .CH 2 -CO NH-CH 2 COOH- +1^0 = 2NH 2 CHa-COOH 



This contains the so-called "peptide" linkage NH CO . 

 A polypeptide formed by the condensation of two amino- 

 acids is a dipeptide. Glycyl glycine is formed by the 



