Soret spectroscopic and molecular graphic analysis of human semi-β-hemoglobin formation

The interaction of heme-free alpha (alpha degrees) and heme-containing beta (beta(h)) chains of human hemoglobin has been monitored in 0.1 M potassium phosphate buffer, pH 7 or 8, at 5 degrees C. Soret zero and first-derivative spectra were consistent with a uniform association reaction. Stopped-flow investigations demonstrated association rates on the order of 10(7) M-1 s(-1). This was 100-fold more rapid than the reported rate of combination of alpha(h) and ph proteins. This encounter-like rate of semi-beta-hemoglobin (alpha degrees beta(h)) formation was increased by raising the pH from 7 to 8. pH change is known to affect the spatial arrangement of AB-GH helical entities. Molecular graphic analysis of modeled alpha degrees protein superimposed over native ah protein revealed an apo Mb-like structure with well-defined AB-GH segments. Repositioning of these core helical segments, resulting in increased conformational freedom of the alpha(1)beta(1) interface, was apparently responsible for the enhanced association properties of the alpha degrees protein.