Studies on Relationships between Structure and Function of Hemoglobin M_iwate

From the ¹ From the Department of Biochemistry, Tohoku University School of Medicine, Sendai, Japan

Functional properties of hemoglobin M_iwate (₂^{87 tyr}ß₂^a) were studied with particular reference to their relations with the structure of this abnormal hemoglobin.

1. The oxygen affinity of hemoglobin M_iwate (in reality the ß subunits) was far lower than that of hemoglobin A, and the interaction constant n in oxygen equilibrium was 1.0 at any pH. Apparently no Bohr effect was observed in the pH range 6.3 to 7.5, while a very small Bohr effect (log p/pH = 0.19) could be observed at higher pH values. A lowered Bohr effect was confirmed by the differential titration experiments.

2. Hemoglobin M_iwate, like hemoglobin A, dissociates into ß dimers at higher concentrations of sodium chloride, but the features of the oxygen equilibrium of the half-molecules of hemoglobin M_iwate were not appreciably different from those of the original molecule.

3. Hemoglobin M_iwate has two reactive —SH groups per molecule. However, "—SH blocking" by p-chloromercuribenzoate resulted in no particular change in the value of n and in oxygen affinity, except that the Bohr effect at higher pH values appeared to be lost.

4. Characteristics of the carbon monoxide equilibrium of hemoglobin M_iwate were similar to those observed in oxygen equilibrium. However, when the subunits were reduced by sodium dithionite, and hence enabled to bind carbon monoxide, the carbon monoxide affinity and the Bohr effect were increased to nearly the same levels as observed with hemoglobin A. Nevertheless, the value of n remained nearly at 1.0.

5. Rates of autoxidation of hemoglobin M_iwate were about 6 times higher than those of hemoglobin A under comparable conditions.

6. The subunits of hemoglobin M_iwate were more resistant to denaturation by sodium benzoate than the normal hemoglobin.

These data strongly suggest that the substitution of only a single amino acid in one type of subunits profoundly affects the functional properties not only of the abnormal subunits, but also of structurally normal subunits in the same molecule, and possibly of the molecule as a whole. Also it was suggested, because of these and some other findings, that the conformational changes of the hemoglobin molecule responsible for the Bohr effect, the affinity for ligands, and the shape of the oxygen or carbon monoxide dissociation curve, respectively, may be different, even if closely related.

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