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-derivati
ve spectra were consistent with a uniform association reaction. Stopped-flo
w 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 t
o 8. pH change is known to affect the spatial arrangement of AB-GH helical
entities. Molecular graphic analysis of modeled alpha degrees protein super
imposed over native ah protein revealed an apo Mb-like structure with well-
defined AB-GH segments. Repositioning of these core helical segments, resul
ting in increased conformational freedom of the alpha(1)beta(1) interface,
was apparently responsible for the enhanced association properties of the a
lpha degrees protein.