Biochemical basis of difference in the electrophoretic mobilities of Hb A and HbS

Normal hemoglobin (Hb A) and sickle-cell hemoglobin (Hb S) have different electrophoretic mobilities because these two proteins have different what?

A. Amount of bound oxygen

B. Amount of bound 2, 3 bisphosphoglycerate

C. Charges

D. Size

E. Hydrophobicity

Answer- The correct answer is-C- charges.

In Sickle cell disease, also called as sickle-cell anemia, a single nucleotide alteration (point mutation) in the β globin gene of hemoglobin causes a change of thymine for adenine (GAG to GTG.), at the sixth codon of the ß gene. This change encodes Valine instead of Glutamic acid in the sixth position on the ß-globin molecule. The charge at this site is altered and allows for polymerization of hemoglobin under conditions of hypoxia. These properties are responsible for the profound clinical expressions of the Sickling syndromes. The mutant β globin chain is designated as βs, and the resulting hemoglobin is referred as HbS.

The substitution of the nonpolar Valine for the charged Glutamic acid results in decreased mobility of the HbS in the electric field as compared to HbA. This altered mobility is due to the presence of less negative charge on the two β –globin chains, thus rendering HbS less negative than HbA- Figure . The diagnosis is confirmed by high-pressure liquid chromatography (HPLC).

Hb electrophoresis

Figure- Hb electrophoresis

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