Structural insights into the inhibition of sickle hemoglobin polymerization by asymmetric hemoglobin tetramer HbFS (α2γβS)

Sickle cell disease (SCD) is caused by a single amino acid substitution in the {beta}S globin chain at 6th position (6E[->]V). This results in polymerization of deoxy state of sickle hemoglobin (HbS), followed by its precipitation and subsequent sickling of erythrocytes. These deformed cells can block small capillaries (vaso-occlusion), causing cardiovascular complications, ultimately leading to ischemia-reperfusion injury, severe oxygen deficiency, and progressive systemic damage. Occasionally, patients with SCD have been observed to produce exorbitantly high levels of fetal hemoglobin (HbF), which has been linked with the inhibition of HbS polymerization. One of the effects of hydroxyurea, the most commonly used therapeutic for SCD, is to elevate HbF levels. However, the mechanism of inhibitory role of HbF on HbS polymerization is largely unknown. This study attempts to gain insights into the mechanisms involved in this process by means of native mass spectrometry, ion mobility mass spectrometry, and hydrogen deuterium exchange-based mass spectrometry (H/DX-MS). The conformational flexibility of asymmetric hemoglobin, HbFS (2{gamma}{beta}S), for the observed regions in the tetrameric molecule appears to be more in the deoxy state as compared to the oxy state, eventually leading to reduced polymerization of sickle hemoglobin in patients with SCD that express elevated HbF levels.