130 PROTEINS 



taken once each, there would be 10^^ isomers." There is an adequate 

 mathematical basis for the existence of literally billions of proteins — 

 many more than probably occur in nature. The surprising thing is that 

 with so many possibilities, cells through countless generations produce 

 proteins of identical chemical and physical characteristics. 



Attention is called to the occurrence of a free amino and a free car- 

 boxyl group at the left and right ends, respectively, in the formulas of 

 the above peptides. Because of these groups, peptides possess both basic 

 and acidic properties. The side chains may also contribute to the basicity 

 and acidity of the peptide because of free amino and carboxyl groups 

 contained therein. Lysine and aspartic acid are examples of amino acids 

 containing basic and acidic groups in the side chains. There are several 

 more such dicarboxylic and basic amino acids. (See formulas of the 

 amino acids.) In long peptides such as proteins it is the side chains, 

 and not the end groups, that contribute most to the basicity or acidity 

 of the molecule. Note also that the phenol group of the tyrosine is free 

 in the above peptide, and, hence, it will have properties characteristic 

 of this group, e.g., positive Millon and xanthoproteic tests. In proteins 

 there will be many such distinctive groups free, e.g., phenol, indole, 

 imidazole, to impart their characteristic properties to the molecule. 



Hundreds of peptides have been made in the laboratory. Fischer, 

 one of the most famous investigators of the "composition of the proteins, 

 made a large number of polypeptides. One of these contained 3 leucine 

 and 15 glycine radicals, which gives a molecular weight of 1213 — one 

 of the largest molecules that has ever been produced synthetically. These 

 synthetic polypeptides possessed many of the properties of native pro- 

 teins such as solubility, color tests, and hydrolysis by enzymes. Although 

 they are far from being as complex as the native proteins, their synthesis 

 is a considerable step toward an understanding of the way in which a 

 protein molecule is put together. 



In the preceding discussion of peptides no consideration has been given 

 to the possibility of the second carboxyl group of aspartic acid and 

 glutamic acid being involved in the linkages. Glutathione, an important 

 constituent of all cells, is a tripeptide in which glutamic acid is linked 

 to the next amino acid through the y-carboxyl instead of the a-carboxyl. 

 Thus, 



CO— NH-CHCONHCH-COOH 

 I I I 



CH2 CH2 H 



I I 



CH2 SH 



I 



HCNH 



2 



COOH 

 Glutathione (7,1- glutamyl- L-cysteyl-glycine) 



