34 



Life: Its Nature and Origin 



these are relatively simple, as shown in the diagrams of the com- 

 mon amino acids glycine and alanine (Fig. 13). Each amino acid 



Fig. 13. Structural formulae of three amino acids. The portion of each above 

 the dotted line is called the side chain or residue, R. 



molecule has both an acidic carboxyl group (— COOH) and a 

 basic or amino group (— NH2). Several amino acids (either all one 

 kind or a mixture) may join together, base to acid, base to acid, 

 and so on, lose H2O in the process, and form linear chains called 

 peptides. These chains become aligned so that the atoms — C— C— 

 N— C— C— N— form a central axis ( called a polypeptide chain ) from 

 which hydrogen and oxygen atoms and the distinctive residues of 

 the amino acids stick out as branches (Fig. 14). Proteins are es- 

 sentially very long polypeptide chains whose size may be judged 

 from their molecular weight which ranges from 2,000 to 10,000,000. 

 The amino acid residues may occur in a variety of combinations 

 along the chain, each combination with potentialities for entirely 

 different physical and chemical behavior. The best known protein 

 is insulin (Fig. 15), one of the smaller proteins with a molecular 

 weight of about 6,000 (Sanger, 1959). Entire chains may become 

 bonded together or combined with other types of compounds. 



What is the role of proteins? They form many of the structural 

 elements of cells, such as collagen in bone and myosin in muscle 

 fiber; they constitute those tremendously varied groups of com- 

 pounds, enzymes and hormones. Certain complex types, the prota- 

 mines, form what is believed to be an important part of the genetic 

 structure. Linderstrom-Lang (1953) and Pauling, Corey, and Hay- 

 ward (1954) gave instructive accounts of other features of proteins. 



