292 VI. HEMOGLOBIN 



8.2. Pathway of the Interaction 



Although the concept of interaction between hemes has been of 

 great assistance in the quantitative expression of many of the reac- 

 tions of hemoglobin, there has been little discussion of the interaction 

 pathway. We put forward the following speculations on the problem 

 in order to stimulate further work. As a basis, the imidazole hypoth- 

 esis of the heme-protein linkage is taken, but attempts are made 

 to extend this to include those facts which this hypothesis cannot 

 yet explain (c/. Section 3.2.2.4.). 



In hemoglobin the hemes are assumed to be held between two 

 imidazole groups, with the possibility that electrostatic bonds might 

 also be present between the ionized carboxyl side chains of the 

 porphyrin and basic — NH^ or — OH groups in the protein. The 

 latter linkages would not be expected to transmit changes in the 

 energy levels of the resonance system of the porphyrin to the protein 

 on account of the aliphatic carbon chain between porphyrin ring and 

 carboxyl group in the propionic acid side chains. The interaction 

 therefore takes place via the iron-imidazole linkage. The imidazole 

 rings, however, are separated from the mesh of peptide linkages in 

 the protein by aliphatic chains, which must insulate the resonance 

 system of the iron porphyrin with its two satellite imidazoles from 

 the protein. One heme must therefore be able to influence another 

 along some pathway which lies between these "insulating" linkages. 

 The only positions free to take part in interaction between the hemes 

 are the imidazole groups with their two dissociable protons. It has 

 been shown above how the proton-escaping tendency from these 

 groups is affected by changes in iron bonding when groups combine 

 with the heme, and it has generally been assumed that interaction is 

 absent between the heme-linked groups of adjacent hemes. It seems 

 to us, on the contrary, that interaction must occur between adjacent 

 heme-linked groups, if any reasonable explanation is to be given of 

 heme-heme interaction as the sharing between adjacent hemes of 

 the energy required to bring about the alterations occurring in the 

 dissociation of protons which accompany changes in bond type. 

 RT log a must therefore always be less than the change in free 

 energy which occurs when the first heme in the molecule reacts with 

 oxygen. In the data from which Pauling developed his equation 

 RT log a was in fact less than the value oi RT log /3, which he defined 

 as the energy of interaction between a single heme and its heme-linked 

 groups. 



