PHILIP GEORGE 



for cyanide complex formation by ferrimyoglobin and ferri- 

 cytochrome c illustrate, in what is probably an extreme case, 

 how configurational changes can modify the thermodynamic 

 data for a simple reaction. 



There can be no question that some kind of configurational 

 change is involved in heme-heme interaction, for otherwise 

 there would be no reason for the free-energy change for the 

 successive addition of oxygen to the four hemes to be different. 

 The interaction is greatest with lysed cell preparations, becomes 

 much less with recrystallized samples (32), and is completely ab- 

 sent in reconstituted hemoglobin (39). But no evidence for 

 interaction has been found in recent potentiometric titrations of 

 recrystallized ferrihemoglobin, or in studies of the ionization 

 of the iron-bound water molecule and complex formation with 

 various ligands (15,18,41). The fugitive nature of the effect 

 can be regarded as experimental evidence for its origin in 

 configurational changes within the molecule; and one of the 

 problems now is to specify precisely what this change is. 



The thermodynamic data for the cell reaction, the ioniza- 

 tion of the iron-bound water molecule, and fluoride complex 

 formation are almost identical for recrystallized samples of 

 ferrihemoglobin and ferrimyoglobin (15,18,40). This suggests 

 very strongly that the heme is similarly situated in both hemo- 

 proteins in these particular preparations. Data given in Table 

 VII for the oxygenation of a recrystallized myoglobin prepara- 

 tion (43) and extremely fresh hemoglobin solutions, in which 

 interaction was very marked (38), show the reaction of the first 

 heme in hemoglobin to resemble most the oxygenation of myo- 

 globin. The data for the oxygenation of the fourth heme are 

 very different. The first oxygen is 7.0 kcal./mole (i.e., 12.1 — 

 5.1) more strongly bound than the last, whereas the entropy 

 change is unfavorable for the entry of the first ( — 22.2 e.u.), but 

 favorable to about 13.0 e.u. for the last. These data do not 

 support the hypothesis that the interaction has its origin in 

 steric effects, whereby, the hemes being "buried" within the 

 protein, the entry of the first oxygen loosens the structure, 



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