DENATURATION 253 



former case the heme-imidazole Hnkages on alternating sides of the 

 hemes may be ruptured. So far, it is not known if the half molecules 

 of hemoglobin can be made to dissociate still further to particles of 

 the same weight as the myohemoglobin molecule. At the present 

 time, no x-ray data are available for myohemoglobin crystals; this 

 w'ould be of great interest in view of the above problem. 



4.3. Denaturation 



4.3.1. Definition. Theoretically denaturation can be defined as a 

 change in the structure of a native protein which involves the spatial 

 arrangement of the peptide chain without breaking the chain itself. 

 Various degrees of such changes are feasible, from the disarrangement 

 of a few amino acids to a complete unfolding of the chain. This 

 (denaturation in the proper sense) is apparently preceded by the 

 breaking of hydrogen bonds or salt linkages (and possibly of disulfide 

 bridges) between the side chains of the peptide backbone. These 

 primary alterations cause, or at least facilitate, the disarrangement 

 of the peptide chain, and it is therefore impossible to exclude them 

 from the concept of denaturation. In this way a certain lack of 

 preciseness is introduced into terminology but attempts to separate 

 the preliminary reactions from denaturation under a separate name, 

 such as "perturbation" (Holden, 1308), would appear to be premature 

 at present. 



Denaturation of a native protein usually involves the loss of its 

 biological activity and of its crystallizability, a decrease of its solu- 

 bility at a pH close to its isoelectric point, changes of molecular size 

 and shape, and alterations in the accessibility of certain groups in 

 the protein {e.g., sulfhydryl groups) to chemical reagents. Some of 

 these changes may not be found in a particular denaturation, while 

 at least some of them may occur without denaturation (c/. Chapter 

 VII, Section 4.4.). Oxyhemoglobin, for instance, forms an insoluble 

 zinc salt, in which the oxygen is still held in reversible combination; 

 or the biological activity of a protein may disappear by reactions 

 involving the active center or prosthetic group, not the protein, as 

 for instance in a conversion of hemoglobin to hemiglobin, by which 

 the ability of reversible combination with oxygen is destroyed. While 

 these should not be considered denaturations, in other instances the 

 distinction may be more difficult. One and the same reagent may 

 react with the prosthetic group as well as with the protein, as probably 

 salicylate does in its reaction with hem?globin (Roberts, 2280). It is 



