Which one of the following statements about Hemoglobin S (HbS) is not true -
Sickle cell anemia is caused by replacement of normal Hemoglobin by sickled hemoglobin (HbS).
HbS is formed by replacement of Glutamine by Valine at position 6
This substitution replaces the polar Glutamine residue with a nonpolar Valine.
The replacement of Glutamine by Valine generates a sticky patch on the surface of HBS.
The sticky patch is present on both oxygenated and deoxygenated HbS
The deoxygenated HbS also contains a complementary site for the sticky patch.
In oxygenated hemoglobin this complementary site is masked.
When HbS is deoxygenated the sticky patch present on its surface binds to the complementary patch on another deoxygenated HbS molecules.
This binding leads to polymerization of deoxyhemoglobin S forming long fibrous precipitates.
These extend throughout the erythrocyte and mechanically distort it, causing lysis and multiple secondary clinical effect.

So, if HbS can be maintained in an oxygenated state or if the concentration of deoxygenated HbS can be minimized, formation of these polymers will not occur and sickling can be prevented (Remember the complementary site for the sticky patch remains masked in case of Oxygenated HbS).
Role of HbA in polymerization —

Unlike HbS, A does not contain any sticky patch, but it does have a binding site for the sticky patch of 1,,S.
Thus it can bind to HbS through its receptor site but this binding cannot extend the polymer because HbA does not contain any sticky patch to promote binding to still another hemoglobin molecule.
So HbA interferes with the polymerization and aggregation of  HbS and reduces the intensity of sickle cell anemia.
Hemoglobin HbS differs from hemoglobin HbA by the substitution of Val for Glu in position 6 of the beta chain
One altered peptide of HbS migrates faster towards the cathode (-) than the corresponding peptide of HbA
Binding of HbS to the deoxygenated HbA can extend the polymer and cause sickling of the red blood cells
Lowering the concentration of deoxygenated HbS can prevent sickling