PHILIP GEORGE 



the same extent (6,47). Analysis of the equilibrium constant 

 data has shown that the effect can be attributed to two heme- 

 linked groups with pK values, denoted by pKi and PK2, which 

 are about 5.3 and 7.9 in ferrohemoglobin, changing identically in 

 oxyhemoglobin and ferrihemoglobin to 5.8 and 6.7, respectively. 

 In the case of myoglobin, although it has often been stated 

 that the same two reactions are little affected by pH, a similar 

 analysis has been made only recently (16). The data required 

 were taken from the papers of Taylor and Morgan (42) and of 

 Theorell (43) and are plotted in Figure 1 , curves A and B, along 

 with some equilibrium data for the fluoride reaction of ferri- 

 myoglobin, curve C (18). The similar trend of all three curves 

 suggests that, despite differences in temperature and ionic 

 strength, the pK. values for the heme-linked ionizations are 

 quite similar in oxymyoglobin and ferrimyoglobin, and in 

 ferromyoglobin and ferrimyoglobin fluoride. Mathematical 

 analysis of curves A and B shows that one linked group, with 

 pK. values of about 6.0 for the first two compounds and about 6,4 

 for the second two compounds, is sufficient to account for the 

 observed variation of log K with pH (16). 



Having obtained an approximate pK value for this single 

 heme-linked ionization in ferrimyoglobin, it was found possible 

 to make a consistent analysis of equilibrium constant data for 

 both the fluoride and cyanide reactions on the assumption that 

 the anion is bonded to the iron in both complexes. The data 

 cover a pH range which is considerably greater than that for the 

 oxygenation and cell reactions {cf. Fig. 1), and lead to the 

 following provisional pK. values for the heme-linked group at 

 20 °C. and 7=0.10: 6.0 in ferrimyoglobin and 6.4 and 7.0 in 

 its fluoride and cyanide complexes, respectively (16). These 

 results were confirmed by an independent and direct measure- 

 ment of the />K value in the fluoride complex, where the intensi- 

 ties of the absorption bands increase slightly owing to the 

 ionization of the heme-linked group as the pH rises from 5.5 to 

 7.0 (17,45) — a similar change to that found with ferrihemoglobin- 

 fluoride in the pH range 5.4 to 6.2 (17). Additional confirma- 



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