I I 



108 GENERAL BIOCHEMISTRY 



a peptide bond is hydrolyzed. The greater the number of ionic 

 charges present in solution, the higher the dielectric constant of the 

 solution. Both solubilities of materials and the rates of many reac- 

 tions are influenced by dielectric constant. Hence, the number of 

 charges and therefore the size of peptides indirectly influence many 

 metabolic processes. 



Most ordinary amides are not very polar and not very soluble in 

 water. The charged groups of peptides contribute polarity, and water 

 solubility becomes commonplace. However, in peptides the ionic 

 charges are further apart than in the amino acids themselves. Thus 

 one would expect somewhat less effect of the charges on each other 

 than when close together. One result of the separation of charges 

 is the alteration of the ionization constants of the carboxyl and amino 

 groups. For example, the constants for glycine are Ki = 10"^^ and 

 Ko = 10-^^, whereas those for glycylglycine are Ki = lO-^.i and 

 K2 = 10-^1. These differences lead to differences in the buffering 

 action of the two kinds of compounds, one kind being more effective 

 in the pH ranges of about 1.5 to 3.1 and 9.0 to 10.6, while the other 

 covers the pH ranges of 2.3 to 3.9 and 7.3 to 8.9. 



As molecules become larger, they diffuse more slowly. This effect is 

 increased, of course, when a semipermeable membrane serves as a bar- 

 rier to diffusion. Hence large peptides diffuse out of cells more slowly 

 than either smaller peptides or amino acids. Perhaps these differences 

 determine in part the concentrations inside cells and influence the 

 forms in which waste nitrogenous materials are excreted by marine 

 forms and higher animals. 



Biological Action 



Peptides play a number of biological roles. They may possibly serve 

 as intermediates in the formation of proteins and in this way would 

 play fundamental roles in almost all biological activities. However, 

 some current investigators believe that proteins are formed directly 

 from amino acids without intervening peptides. Others do not agree 

 with this concept; clarification of the problem must await further ex- 

 perimentation. 



Certain plant peptides are placed in the diverse group of com- 

 pounds called alkaloids. The peptide members of this class produce 

 striking effects when administered in small amounts to animals. Most 

 of these peptides with pronounced pharmacological properties have 

 been isolated from fungi, although they may turn up in numbers in 

 higher plants, since one example has been obtained from mistletoe. 

 Ergotamine, shown in the accompanying structural formula, is a pep- 



