254 ORIGIN OF STRUCTURES AND FUNCTIONS 



biologically important properties of the protein may change 

 while the composition of its peptide chains and their sequence 

 remain the same. 



The correctness of this general picture is confirmed by 

 an immense amount of factual material derived from the 

 fields of both enzymology and immunochemistry.^* Experi- 

 ments on the denaturation of biologically active proteins are 

 specially convincing in this respect. This phenomenon is 

 induced by the action of very diverse physical and chemical 

 factors such as heating, vibration, the action of urea or ultra- 

 violet radiations, etc. It is not accompanied by dissolution 

 or rearrangement of the covalent bonds of the peptide chains 

 of the protein.''^ The specific three-dimensional architecture 

 of the protein molecule is, however, severely disturbed.^® 

 The first more or less satisfactory theory of denaturation was 

 put forward by H. Wu."^ According to this theory, denatura- 

 tion occurs as a result of the disruption by the denaturing 

 agent of the weak bonds which subsist between the peptide 

 chains. When this takes place, they arrange themselves in a 

 random and disorderly way corresponding with the most 

 stable thermodynamic state. 



According to A. E. Mirsky and L. Pauling^^ the configura- 

 tion of the native protein molecule is maintained by hydrogen 

 and salt bonds, which unite the different parts of the peptide 

 chains. When denaturation occurs, these bonds are broken, 

 the chains fall apart and many radicals which were previously 

 hidden within the molecule become available for chemical 

 reactions. 



This explains the change in the reactivity of proteins on 

 denaturation.^" In particular, it was shown some time ago 

 that the number of sulphydryl and disulphide bonds avail- 

 able for reactions was greater in denatured proteins than in 

 the same proteins in their native state."" Denatured proteins 

 also give stronger reactions for tyrosine"^ and arginine"^ and 

 will combine with larger amounts of iodine."^ 



According to contemporary ideas the structure of native 

 proteins consists of closely packed, coiled or twisted peptide 

 chains which untwist on denaturation to give extended 

 chains without any significant rearrangement which would 

 involve disruption of their covalent bonds.""* 



