BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 5. MOLECULAR DISEASES: SICKLE-CELL ANEMIA

QUESTIONS AND PROBLEMS

1. A screening program has revealed a Hemoglobin variant with abnormal electrophoretic mobility. Based on the peptide map of the tryptic digest of the hemoglobin, The amino acid substitution has occurred in the β chain. The normal N-terminal tryptic peptide (Val-Нis-Lеu-Тhr-Рго-Glu-Glu-Lys) is missing. A new tryptic peptide consisting of 6 amino acid residues has been detected, with valine as its N-terminal residue.

a) What amino acid substitution is indicated by these data?

b) How should this hemoglobin differ in electrophoretic mobility from HbA and HbS at pH 8?

2. The electrophoretic mobilities of several mutant Hemoglobins A at pH 8.6 are shown below.

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How are positions a, b, c, and d distributed among hemoglobins of the following compositions:

3. Some Mutations in the hemoglobin Gene affect all three hemoglobins A₁, A₂, and F, whereas others affect only one of them. Why?

4. When hemoglobin from an individual with the sickle-Cell trait (i.e., heterozygous for the sickle-cell gene) is subjected to starch-gel Electrophoresis, two major bands are observed. One is HbA (α₂β₂), and the other is HbS (α₂β₂). The absence of a third band with intermediate electrophoretic mobility initially suggested that the hybrid molecule α₂ββ^S does not exist. However, such a hybrid hemoglobin was subsequently detected in solution. Why is it not detected by starch-gel electrophoresis? (Hint: consider The Effect of the electric field on the equilibrium 2(α₂ββ^S) ⇄ α₂β₂ + α²β^2S.)

5. HbA inhibits The formation of long fibers from HbS and the subsequent sickling of red Blood Cells upon deoxygenation. How can this effect of HbA be explained?

6. At high Ionic strength, hemoglobin dissociates at the α₁β₂ contact site into αβ dimers. How does the dissociation tendency of oxy- and deoxy-forms of hemoglobin Kempsey compare with that of hemoglobin A?