STRUCTURAL BASIS OF HEME-HEME INTERACTION 



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8. STRUCTURAL BASIS OF HEME-HEME INTERACTION 



8.1. Classification of Hemoglobin Equilibria 



Coryell's demonstration of the relation between n and a {If98) provides 

 a useful criterion for the classification of hemoglobin reactions according 

 to the degree of heme-heme interaction. He summarized data for a number 

 of systems in order to see if the presence or absence of interaction could be 

 correlated with the presence or absence of a change in the bond type. In 

 Table V data are given for a number of these systems. 



TABLE V 

 Classification of Hemoglobin Equilibria 



Reaction 



Bond changes" 



Change in H "i" ion 

 activity on 



CO -I- HbOs ;=± HbCO + O. 1 



Hi+ -l-OH-^HiOH 1 



Hi+ + F-^HiF 1 



Hb + 02(C0) ^ Hb02(HbC0) 2 ^ « ^ 3 



Hb-e^Hi+ 1^"^2 

 HbCO- e^Hi+ +C0 2 



Hi++SH-^HiSH 1.8 



Hi + N3-^HiN3 1.7 



" « is the sigmoid coefficient. 



* cov. = covalent, ion. = ionic. 



" Any reaction of heniiglobin or any reaction in which the valency of iron changes 



must be accompanied by such changes. = no change, ? = undetermined, + 



= change present. 



As Coryell pointed out, a change in the character of the iron bonds is not 

 sufficient criterion for the presence or absence of interaction. This may be 

 the basis for interaction in the combination of oxygen or carbon monoxide 

 with hemoglobin, and the explanation for the absence of interaction in the 

 replacement of oxygen by carbon monoxide, but fails to explain the behavior 

 of the systems in which hemoglobin is present. It may be necessary to take 

 into account the effect introduced by the charge on the iron atom. Even 

 when there is no change in bond type, such as in the reaction Hb — e ;=^ Hi+, 

 the appearance of the positive charge on the iron atom of the first heme 

 oxidized may be transmitted to the second heme {cf. Section 8.2.) and may 

 there facilitate the removal of the second electron and so on. Further 

 speculation on this aspect of the interaction problem is, however, unprofitable 

 in view of the relatively small number of systems so far analyzed. There 

 would seem to be no great difficulty in accumulating more data on other 

 systems. 



