104 GENERAL BIOCHEMISTRY 



PEPTIDES 



The Peptide Bond 



Compounds of this class are made up primarily of amino acids. A 

 few natural substances placed in this group are not composed exclu- 

 sively of amino acids, but the additional components are limited in 

 both number and size. Hence the fundamental bonds connecting 

 the subunits of the peptides are those joining the amino acids. Such 

 bonds link the carboxylate ion of one amino acid with the ammonium 

 ion of another and are called the peptide bonds. The product is a 

 special type of amide known as a peptide and is represented in the gen- 

 eral reaction for the formation of a peptide bond: 



H3N+CHCOO- + H3N+CHCOO- -^ 



I I 



R R' 



H3N+CHCONHCHCOO- + H2O 



1 I 



R R' 



In this example the peptide consists of two amino acid subunits or 

 residues and thus is a dipeptide. Tripeptides contain three amino 

 acid residues, and so on to polypeptides with a large number of such 

 residues. Specific names are applied when the nature and relative posi- 

 tions of the amino acid residues are known. These names begin with 

 the amino acid bearing the free a-ammonium ion at one end of the 

 molecule and proceed stepwise to the amino acid carrying the terminal 

 carboxylate ion. In this way a name is based on that of the right-hand 

 amino acid when written as above and prefixed with the names of the 

 substituents in order. When the optical configurations of the in- 

 dividual amino acids are known, they are designated by the appro- 

 priate letters. Usually, in writing the structures of peptides, no effort 

 is made to represent the actual configurations of asymmetric carbon 

 atoms because of difficulties in projecting into two dimensions when 

 several asymmetric atoms are involved. The following examples illus- 

 trate these principles of nomenclature: 



H3N+CH2CONHCHGONHCH2COO- 



I 

 CH3 



glycylalanylglycinc 



(GH3)2GHGHGONHCH2CONHCHCOO- 



1 1 



NH3+ GH2CH2GOO- 



L-valylglycyl-L-glutamic acid 



