Modification of the Secondary Structure of Haenwprotein Molecules 97 



the present findings require that its acid strength should be increased by about 7 pH 

 units as compared to the ionization in the free ligand, and by about 4 pH units 

 compared to the ionization of the group in the imidazole complex. Although they 

 do not render it inconceivable, these data cast doubt on the imidazole hypothesis as 

 far as it is invoked to explain the linked ionization effects. It could well be that the 

 haem is co-ordinated to a histidine residue, but the linked effects arise through the 

 ionization of another group entirely. 



Chance: In Kaziro's Figs. 4 and 6 (this volume, p. 80) it would appear the reaction 

 with oxygen occurs with a considerably smaller extent of structural degradation than 

 is needed for reaction with carbon monoxide. This agrees with the observation that 

 autoxidation occurs at higher values of pH than does reaction with CO (Chance and 

 Paul, unpublished observations). It seems to me that the differences are sufficiently 

 great to raise the question of whether oxygen reacts to cause autoxidation at the same 

 site as does CO. In view of spectroscopic evidence, we can accept that CO combines 

 with the iron atom. Thus the oxygen reaction may occur elsewhere, perhaps at a 

 histidine group. 



Lemberg: At my suggestion, Kaziro has used the term 'modification' for the type of 

 change discussed in his paper. I feel that this is preferable to the term 'perturbation', 

 suggested by Holden {Aust. J. exp. Biol. med. Sci. 25, 47, 1947). 



Kaziro: I agree. I have frequently used 'perturbation' in publications, but the term has 

 not yet found general acceptance. 



