PROTEINS 107 



structure study have not been available long enough to allow even a 

 rough estimate of the nimibcr. The presence oE free peptides has 

 been demonstrated in biological fluids like blood, urine, and plant 

 juices, but work has not progressed far enough to permit enumeration 

 and identification of the compounds involved. 



As intermediates in the hydrolysis of proteins, peptides are formed 

 in great abundance and variety and have been studied extensively. 

 The digestion of proteins by animals and microorganisms alike de- 

 pends upon catalytic hydrolyses with proteolytic enzymes. These 

 catalysts specifically break certain peptide bonds, forming peptides 

 whose size and structure depend upon the nature of the protein split 

 and the enzyme participating. Hydrolysis of proteins in xiitro, mean- 

 ing outside a living system, has been studied widely in work on protein 

 structure. Many peptides have been isolated and their structures estab- 

 lished, revealing the possibility of occurrence of an almost infinite 

 variety. 



Peptides are at least potential intermediates in biological protein 

 syntheses. If so, then they might occur in still other structural types 

 than those demonstrated during protein breakdown. In addition, a 

 number of peptides have been isolated from special sources and pos- 

 sessing special physiological actions. The sources of some members of 

 this group are listed below in connection with their structures and 

 actions. 



Structures and Properties 



Peptides consist primarily of amino acids linked together through 

 peptide bonds. Other structures of various kinds may also be involved 

 in certain special peptides and are usually present as amides. These 

 groups may be represented by ethanolamine, one of several different 

 simple organic acids, glucuronic acid, or a ribose derivative. The con- 

 tribution of these additional components to structure and properties 

 can be inferred from their nature and is quite specific; therefore it will 

 not be considered. 



The specialized chemistry of peptides is the chemistry of the pejj- 

 tide bond. For each such bond formed at physiological values of pH, 



H3N+CHCOO- + H3N+CHCOO- ^ 



I I 



R R' 



H3N+CHCONHCHCOO- + H2O 



I I 



R R' 



two charged groups disappear. Conversely, two charges appear when 



