PHILIP GEORGE 



to form its cyanide complex throughout the entire /?H range, so 

 the phenomenon is one of a difference in degree rather than 

 kind. Kinetic and equihbrium studies (26) suggest that both 

 HCN and CN~ react to form the complex as in the case of 

 ferrimyoglobin, but there is no evidence for the presence of a 

 similar linked ionizing group between pH. 6 and 8. The data, 

 listed in Table VI, show that there is relatively little difference 

 between the affinity of ferrimyoglobin and ferricytochrome c 

 for cyanide, the equilibrium constants being, respectively, 

 2.3 X 10^ and 1.2 X 10^ M~^, corresponding to a free-energy 

 difference of only 3.1 kcal. /mole. The difference in reactivity 



TABLE VI 

 Kinetic and Equilibrium Data for Cyanide Complex Formation (16,18,26) 



" 25 °C., I = 0.10, heme-linked group in conjugate acid form. 

 6 24.6°C., I « 0.15. 



is a kinetic feature. Ferrimyoglobin cyanide is formed by the 

 CN~ path about 10^ times more rapidly, and by the HCN path 

 about 10^ times more rapidly: dissociation occurs about 10 

 times and about 4 times more rapidly, respectively (the com- 

 parison involving the CN~ path for ferrimyoglobin being subject 

 to the proviso mentioned above). In contrast to other hemo- 

 protein complexes of similar affinity, e.g., O2 and CO complexes, 

 ferricytochrome c cyanide owes its existence to extremely slow 

 dissociation rather than rapid formation. 



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