PHILIP GEORGE 



the particular reaction studied (32,47). There is no similar 

 evidence for a Bohr effect of different magnitude with myo- 

 globin from different species, nor has the method of extraction 

 or degree of purity been shown to affect it, although a more 

 systematic examination might well reveal these features. Myo- 

 globin, possessing only one heme per molecule, cannot, of 

 course, exhibit the other effect, heme-heme interaction. 



It is understandable that the magnitude of the Bohr effect 

 should depend on the particular reaction studied, for it is 

 governed by the difTerence between the acid strength of the 

 heme-linked ionizing group in reactant and product; this 

 quantity naturally takes on a different value, depending upon 

 the pair of hemoprotein derivatives involved, as a direct con- 

 sequence of structural differences. Earlier hemoglobin studies 

 suggested that the chief heme-linked group (with J&K2), respon- 

 sible for the marked oxygenation Bohr effect, had an identical 

 value in oxy- and carbonmonoxy-hemoglobin and in ferri- 

 hemoglobin of 6.68, compared to the value of 7.9 in ferro- 

 hemoglobin (47) . A structural interpretation of this was given 

 in terms of a model in which the heme was joined to the tertiary 

 N-atom of a histidine residue (6), and it was to be expected that 

 other covalently bonded complexes would have a pK. value 

 similar to that of the covalently bonded oxygen and carbon 

 monoxide complexes. Sufficient data are not yet available to 

 judge whether this is true for both ferri- and ferro-hemoglobin 

 derivatives, but in the case of myoglobin, ferrimyoglobin cyanide, 

 with covalent bonding, has a />K value of 7.0 compared with 6.0 

 in oxy- and carbonmonoxy-myoglobin and ferrimyoglobin. It 

 would thus appear that the acid strength is determined by more 

 factors than those taken account of in this generalization based 

 on bond type. 



The prospect of identifying a heme-linked ionizing group 

 by its thermodynamic properties is very attractive. The 

 magnitude of the pK. value and the heat and entropy of ioniza- 

 tion can be obtained from the variation of physical properties 

 with pH, from kinetic or equilibrium studies, or from differential 



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