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JBC, Vol. 250, Issue 3, 961-965, Feb, 1975

The effect of functional differences in the alpha and beta chains on the cooperativity of the osidation reduction reaction of hemoglobin

S. J. Edelstein and Q. H. Gibson

Partially oxidized solutions of hemoglobin have been reacted with azide to determine the extent of oxidation, of the alpha and beta chains according to the method of McQuarrie and Gibson (J. Biol. Chem. (1971) 246, 517-522) In 2, 2'2'' nitriloethanol buffer the fraction of oxidized material represented by the beta chains decreases with decreasing extent of total oxidation, of the alpha chains. Upon addition of insitol hexaphosphate, the degree of perferntial oxidation in terms of a two-state model similar to the description of oxygenation by Edelstein (nature(1971) 230, 224-227) but with the incorporation of chain heterogeneity. The results indicate that the pH-dependent cooperativity of the oxidation-reduction reaction can be described in terms of a bell curbe of n versus log l, the allosteric somewhat lower and shifted slightly to the left, due in part to an affnity of beta chains for electrons approximately twince that of alpha chains. Because the curve is shifted to the left, oxidation-reduction equilibria at l values corresponding to pH 6 to lie on the right side of the bell curve where cooperativity the preferntial affity of beta chains for electrons rises to about 4 times that of alpha chains. As a consequence, the coreesponding bell curve is lowered with the Hill coeficient falling to unity or below in the range of l encountered. Thus the principal cause of decreased cooperativity is chain heterogeneity and not stabilization in the t state as suggested by Perutz; under these conditions the molecules of methemoglobin in the t state are only a fractional part of the population.
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