STRUCTURAL BASIS OF HEME-HEME INTERACTION 293 



Since the distance separating one heme from another is of the 

 order of 40 A, the interaction pathway must be at least this length, 

 and we suggest that it consists of a pattern of electrostatic bonds 

 between the charged side chains of the protein and other adsorbed 

 molecules. If, for example, the amino groups which bind carbon 

 dioxide as carbamino compounds are situated on this pathway, it 

 becomes possible to reconcile the influence of oxygenation on the 

 bonding of carbon dioxide and vice versa without having carbon 

 dioxide actually bound to the same group which dissociates a proton 

 on oxygenation. 



Further, it may be of significance for this hypothesis that the 

 imidazoles binding the heme suffer opposite shifts in their pK values. 

 Our view that facilitation of the proton dissociation on the oxygena- 

 tion of one heme may facilitate the proton dissociation of a second 

 heme would be explained if interaction took place between those 

 imidazoles which dissociate in the opposite sense. 



The following may serve as an example of the suggested form of interaction : 

 One of the hemes combines with an oxygen molecule and the proton-escaping 

 tendency of the imidazole groups is altered. The pK value of the dissociation 

 from the distal group changes from 5.3 to 5.7. This is associated with a shift 

 in the equilibrium position of the proton, which, we assume, may be trans- 

 mitted along the chain of hydrogen bonds and electrostatic linkages to the 

 proton of the proximal imidazole of heme number 2. The interaction will 

 therefore increase the proton-escaping tendency of this group, since the pK 

 value of the proximal imidazole shifts from 7.8 to 6.8 when covalent bonds 

 are formed on combination with oxygen. The free energy required to bring 

 this change about, when combination with oxygen actually occurs with the 

 second heme, will thus be diminished. Similarly, the behavior of the proximal 

 imidazole group when the first heme combines with oxygen, involving a 

 repulsion of the jaroton, is transmitted to the distal imidazole of heme number 

 4, diminishing the energy required for the proton attraction which takes 

 place with the shift in the pK value from 5.3 to 5.7 when the heme combines 

 with oxygen. 



This hypothesis is illustrated in Figure 13. Im H+ represents the imidazole 



proton, Im H+ and Im H+ representing the decrease and increase respectively 

 of the escaping tendency of the proton. The hemes are numbered and are 

 represented by the symbol Fe02, while the distal and proximal imidazoles 

 are distinguished by the distance they lie from the heme and the position of 

 the oxygen molecule. The interaction pathways are shown by dotted lines 

 and distinguished by the symbols F,2, Fu . . . F41 etc., where the direction 

 of the interaction is given by the sequence of the subscript. When oxygen 

 combines with heme 1, interaction occurs along the pathways Fu, Fu to 

 hemes 2 and 4. A second oxygen molecule, combining at heme 2, now 



