ON THE NATURE OF HEMOPROTEIN REACTIONS 



accounting for the kinetic characteristics of inorganic and 

 organic reactions between ions and between an ion and a neutral 

 molecule, may also provide the explanation for the operation 

 of heme-linked groups. 



A similar detailed study has not yet been carried out on 

 the ferrihemoglobin-fluoride and -cyanide reactions, but it 

 seems probable that the data referred to earlier for the cyanide 

 reaction (2), which was regarded as evidence for the bonding 

 of HCN in the complex, could be accounted for by the same, 

 more complicated, mechanism developed for the ferrimyo- 

 globin reaction. More recently the pH variation of the equilib- 

 rium constants for the formation of ferrihemoglobin fluoride, 

 azide, cyanate, and thiocyanate has been investigated (41). 

 In interpreting the results it was assumed that the anions are 

 bonded, and no analysis was made taking into account the 

 ionization of heme-linked groups. However the pK values for 

 their ionization in ferrihemoglobin are known from data on the 

 cell reaction, and, since the pH variations are similar to that for 

 ferrimyoglobin fluoride given in Figure 1, it seems very likely 

 that the same analysis used in tlie case of myoglobin would 

 confirm anion bonding. 



The structure of the peroxidase and catalase complexes now 

 presents a very interesting problem, for, as mentioned above, 

 the simplest interpretation of kinetic and equilibrium data 

 again appears to suggest bonding of the undissociated acid 

 (2,17). Yet the physical properties of their fluoride, cyanide, 

 and other complexes, particularly absorption spectra and 

 magnetic susceptibilities, resemble so closely those of the ferri- 

 hemoglobin and ferrimyoglobin complexes, that on these 

 considerations alone it has long been accepted that combination 

 occurs with the anion (30,32). One explanation of this apparent 

 contradiction could be that anions are bonded in all the com- 

 plexes, but that in the case of catalase and peroxidase, one or 

 more linked groups ionize in such a way as to mask completely 

 the pH dependence of equilibrium and velocity constants, which 

 would otherwise be characteristic of anion bonding. 



351 



