Functional Heterogeneity of the 
and ß Chains in the Oxidation-Reduction Reaction of Human Hemoglobin
Ronald A. MacQuarrie 1 and Quentin H. Gibson 1
From the
1 From the Section of Biochemistry and Molecular Biology, Cornell University, Ithaca, New York 14850
The 
and ß chains of human hemoglobin are functionally nonequivalent. In tetrameric human methemoglobin, the ß chains react with sodium dithionite about 10-fold faster, and with sodium azide about 6-fold faster than the chains. In addition, the isosbestic point for oxidized heme-reduced heme differs for the and ß chains within the intact tetramer. The differential reactivity of the chains toward azide and dithionite was used to measure the concentrations of the oxidized and ß chains in mixtures of methemoglobin and deoxyhemoglobin. The ratio of concentration of oxidized ß to oxidized chains decreased as the proportion of methemoglobin was progressively decreased, in the presence of catalytic amounts of the electron-mediating dye thionine. Therefore, the ß chains have a higher half-reduction potential than the chains. Furthermore, nearly the same difference in apparent oxidation potential was found at pH 6.1, where the Hill coefficient for the oxidation-reduction reaction is 1.4, and at pH 8.7, where the Hill coefficient is 2.5. We obtained an estimate of the heme-heme interaction energy contributed by the ß dimer from these results and from the known equilibrium curves for the oxidation-reduction reaction. The estimated ß dimer interaction energy is much smaller than the experimentally measured interaction energy of the tetramer. We conclude that since the ß dimer contributed only a small fraction of the total interaction energy at alkaline pH, the hemoglobin tetramer is the important functional unit in the oxidation-reduction reaction.
Submitted on September 10, 1970
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