POLYPEPTIDE STRUCTURE OF PROTEINS 151 



no means excluded that other linkages may coexist with these in the 

 protein molecule. One fact which would appear to afford indication 

 of the presence of non-peptide linkages in the native protein molecule 

 is that while native protein is attacked by the Pepsin of gastric juice 

 and hydrolyzed by it as far as the proteose stage of hydrolysis, the 

 various peptones and synthetic polypeptides are not hydrolyzable by 

 pepsin. This may mean that types of union exist in the protein 

 molecule which are susceptible to attack by pepsin and which are not 

 present in the peptones and polypeptides. On the other hand it may 

 simply indicate that the greater length of the amino-acid chain in the 

 native protein molecule confers upon it sufficient instability to lay it 

 open to attack and disintegration by the relatively weak hydrolyzing 

 enzyme, pepsin, while the relatively stable proteoses require a more 

 energetic hydrolyzing-agent such as the Trypsin of pancreatic juice. 

 This latter view receives substantial support from the fact that the 

 splitting of protein into proteoses already results in the uncovering 

 of free amino-groups, so that the proteoses are evidently united together 

 in the protein molecule through nitrogen atoms as the amino-acids 

 are linked together in peptides. Furthermore, it is definitely known 

 that greater length of an amino-acid chain confers upon it greater 

 instability toward hydrolyzing enzymes. Thus, Tetra-glycyl-glycine 

 is hydrolyzed by trypsin, while glycyl-glycin, diglycyl-glycine and 

 triglycyl-glycine are not attacked by this enzyme. 



CONSEQUENCES OF THE POLYPEPTIDE STRUCTURE OF 

 PROTEINS. 



The polypeptides are as essentially amino-acids as the amino-acids 

 out of which they are built up. Thus, glycyl-glycine is as typically 

 an amino-acid as glycine itself, since it possesses an NH 2 group as 

 well as a COOH group, and for this reason is presumably capable of 

 forming compounds, not only with acids and bases, but also, possibly, 

 even with neutral salts, by attaching the basic radical of the salt at 

 one point of the molecule (the carboxyl) and the acid radical at another 

 (the amino-group) . On undergoing electrolytic dissociation in aqueous 

 solution it may be supposed to yield either hydrogen (H + ) ions, or 

 hydroxyl (OH~) ions, owing to the occurrence of a reaction with water 

 of the type: 



/NH 2 /NH 3 OH 



R< + H.OH = R< 



X COOH X COOH 



just as ammonia, in aqueous solution, partially reacts with water to 

 form NH 4 OH. 



It was considered until quite recently, and is still thought in some 

 quarters, that these elements in the structure of the protein and 

 polypeptide molecules afford an explanation of the power which they 

 possess of uniting with both acids and bases, in other words the 



