Spectral differences between the alpha and beta heme groups within human deoxyhemoglobin.

The contributions of the alpha and beta heme groups to the absorbance change that accompanies the aggregation of hemoglobin subunits were measured by comparing the extinction changes produced by n-butyl isocyanide binding to deoxyhemoglobin A with those produced by binding of n-butyl isocyanide to the isolated chains. Almost the entire aggregation absorbance change is associated with the unliganded alpha subunits, which exhibit a 30% increase in molar extinction at 428 nm during tetramer formation. This marked spectral change indicates a substantial perturbation of the electronic properties of the alpha heme groups in going from the high-affinity, isolated chain state to the low-affinity, tetrameric state. The lack of a significant absorbance change as sociated with the beta heme groups implies that the change in reactivity of unliganded beta subunits produced by aggregation is not the result of a change in spin or coordination geometry of the iron atom, but rather involves primarily an alteration in the protein structure near the sixth coordination position.