THE HEREDITARY ORDER: GENETIC INFORMATION 



are necessarily folded in the same way. Thus, differences in proteins 

 can be due only to differences in the sequence of amino acids which 

 constitute the polypeptide chain, and of course in the proportion 

 of each of the twenty molecular species. Many molecules of the 

 same amino acid may be present in a given protein. A given amino 

 acid, except the terminal one, is necessarily associated to its left and 

 to its right with another amino acid, thus forming a tripeptide 

 (Figure 7). A long polypeptidic chain can be arbitrarily subdivided 



OH 



NH3 + 



' A 



CH2 C^ CH 



CH2 ' ' COO- 



CHg f-H 1 c;h CH2 CONH2 



I 1^1 III 



CHg I HCH CH2 CHg CH2 



H I CH, CH2 H H H H I H I I 



\l K/ \ \ / \l \l I 



c c c c ")c c c 



+ H3N^ ^C — N C — N C — N C N C N C N 



II I II I II I II I II I III II 



OH OH OH OH OH OH 



leucyl isoleucyl tyrosyl glycyl cysteinyl glutamyl asparogine 



Figure 7. Fragment of a Polypeptide Chain. 



into peptides: di-, tri-, tetra peptides, etc. For the same groups of 

 amino acids, the same peptides may be repeated. When an enzyme 

 acts, the substrate is attached at a specific site, at a specific peptide 

 of the protein. But this specific peptide, if isolated from the rest of 

 the molecule, would not be enzymatically active. In a protein, no 

 individual function can be assigned to any isolated amino acid or 

 group of amino acids. Their functional value depends on the sequence 

 of all amino acids. The unit of function is the whole protein 

 molecule. Each enzyme, being a specific molecule, is necessarily 

 unique. This does not mean that in different species the same enzy- 



[23] 



