PROTEINS 111 



reduction potential of the cell, helping to keep a variety of critical 

 — SH groups in the reduced form. This peptide may be a key inter- 

 mediate in electron-transfer systems. It also is a recjiiired cofactor in 

 certain enzyme reactions, and it can serve in peptide syntheses as a 

 source of glutamic acid, which occurs in the peptide with both 

 a-carboxyl and amino groups free. 



Effective methods for the isolation and study of peptides have been 

 available for only about 10 years. Hence there must be a greal deal 

 more to be learned of the biochemistry of these compounds, and new 

 roles will undoubtedly be discovered. 



PROTEINS 



Proteins are combinations of amino acids and are still larger than 

 natural peptides. They often contain some other structure in addition 

 to the amino acids, these latter being united principally by means of 

 peptide bonds. Proteins always appear to be giant or macromolecules 

 with molecular weights in the range of 10* to 10". Although this class 

 is difficult to define, one may say that a protein is a macromolecule of 

 biological origin consisting largely of amino acids condensed via pep- 

 tide bonds. 



Composition and Structure 



Although there are many natiually occiUTing amino acids, only the 

 22 listed in Table 5-1 have been isolated from proteins. Moreover, 

 only the l isomers have been definitely found. Reports of d isomers 

 seem to refer to artifacts of isolation arising from racemization of one 

 of the natural forms. In addition, it is now quite clear that not all 

 proteins contain all of even these 22 amino acids. As mentioned 

 earlier, hydroxyproline is found only in gelatins and collagens. There 

 are many proteins, especially smaller ones, which do not contain all 

 the amino acids listed. For example, the protein hormone insulin is 

 without hydroxyproline, tryptophan, cysteine, and methionine. How- 

 ever, these particular amino acids do occur in other proteins. 



Even though the list of amino acids in proteins is limited, the pos- 

 sible variety of protein structures is not. The number of each kind of 

 amino acid per protein molecule is widely variable. The sequence of 

 the amino acids in the peptide chain is specific for each protein and 

 potentially capable of enormous diversity. Finally, proteins may dif- 

 fer in the number of peptide chains per protein molecule. A peptide 

 chain is a sequence of Rmino acids condensed by way of peptide bonds. 



