ON THE NATURE OF HEMOPROTEIN REACTIONS 



(5-12) is in progress. Their relative magnitudes are, however, 

 unlikely to change significantly. The effect of the ionization 

 of the heme-linked group on the velocity constants and equilib- 

 rium constants for the corresponding reactions can thus be 

 examined, bearing in mind that either ki and ^_i, or k^ and 

 A- 2, but not both, may have much smaller values and so in- 

 validate one of the comparisons. 



When the attacking species is an ion it can be seen that the 

 velocity constant is considerably greater for the path most 

 favored by coulombic attraction (or cancellation of charge), 

 e.g., with CN~ as reactant, k^ ~ 44 ki, and with H3O+ as 

 reactant, k_'i « 21 k_2. But when the attacking ligand is a 

 neutral molecule, the difference between velocity constants is 

 very much smaller, as may be seen by comparing k[ and ^2? 

 k-i and k-2. Since the velocity constants change in this way 

 when the group ionizes, ferrimyoglobin has the greater afhnity 

 for cyanide when the linked group is in its conjugate acid form, 

 i.e., K2 is about 10 times greater than ^1, and K2 about 10 times 

 greater than K[. Now the influence of charge on the magnitude 

 of the velocity constants is important in the formation reaction 

 for the path involving CN~, and in the dissociation reaction 

 for the path involving HCN. As a consequence the greater 

 affinity for cyanide shown by the conjugate acid form of ferri- 

 myoglobin (H+Y) arises in a different way for each path. The 

 reaction of the conjugate acid with HCN is favored with respect 

 to its conjugate base, because the dissociation velocity constant 

 increases more than the formation velocity constant increases 

 upon ionization {k_[ » k_2, k[ > ^2)- The reaction with 

 CN~ is favored because the formation velocity constant de- 

 creases more than the dissociation velocity constant decreases 

 upon ionization {ki <c k2, k-i < k-2)- 



Corresponding kinetic data, summarized in Table III, are 

 available at present only for the fluoride reaction of ferrimyo- 

 globin with the heme-linked group in the conjugate base form 

 (16,18). The most striking difference between this reaction 

 and the cyanide reaction is seen in a comparison of the relative 



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