Basic defect in HbS is?: (A) Altered function, (B) Altered solubility, (C) Altered stability, (D) Altered O2 binding capacity
Answer is B. Basic defect in HbS is altered solubility. A molecule of hemoglobin S (HbS) contains two normal a globin chains and two mutant b globin chains- glutamate at position 6 is substituted with valine. This replaces the polar glutamine residue with a nonpolar valine. The replacement of glutamate by valine generates a sticky patch on the surface of HbS. The sticky patch is present on both oxygenated and deoxygenated HbS but deoxygenated HbS also contains a complementary site for the sticky patch. When HbS is deoxygenated the sticky patch present on its surface binds to the complementary patch on another deoxygenated HbS molecule. Binding of a number of deoxygenated HbS leads to formation of long fibrous polymers of HbS. This stiffens and disto the red cells producing rigid misshaped erythrocytes.