Comparative spin-label study of the dynamic structure of human hemoglobins A and F and the influence of the antibiotic, chloramphenicol

Hemoglobin A (HbA) and hemoglobin F (HbF) dynamic structures have been studied using spin-label ESR spectra analysis technique, which permits quantitative separation of slow macromolecular rotation (described by rotational correlation time, tau c) and fast anisotropic nitroxide radical motion (described by the 'order parameter', S). The hardly restricted motion of the maleimide spin-label reflects the overall macromolecular rotation and small dynamic structure differences between HbA and HbF were observed (tau c is equal to 26 and 27 ns, respectively). On the other hand, the dynamic equilibrium of the iodoacetamide spin-label demonstrates significant differences between beta- and gamma-chain C-terminus flexibility. Thus, there are different states of alpha,beta and alpha,gamma intersubunit contacts which may be expected to determine the different O2 affinity of HbA and HbF. The antibiotic, chloramphenicol, strongly affects the O2 affinity and the Hill constant of HbF, and also provides detectable changes of gamma-subunit C-terminus flexibility (tau c changes from 20 ns to 27 ns after chloramphenicol treatment of HbF), while the HbA tetramer structure remains almost unaffected. The HbF domain structure rearrangements are accompanied by a decrease of the steric restriction of the spin-label motion (S changes from 0.75 to 0.72).