The substitution of valine for glutamate at position 6 of the ß chains of hemogl

Question

The substitution of valine for glutamate at position 6 of the ß chains of hemoglobin places a nonpolar residue on the outside of hemoglobin S, the version of hemoglobin that is responsible for the sickle-cell anemia. The oxygen affinity and allosteric properties of hemoglobin are virtually unaffected by this change. However, the valine sidechain of hemoglobin S interacts with a complementary sticky patch (formed by phenylalanine ß85 and leucine ß88) on another hemoglobin molecule -- a patch that is exposed in deoxygenated but not in oxygenated hemoglobin. a) what is the chemical basis for the interaction between the hemoglobin molecules? b) What would be the effect of this interaction? The substitution of valine for glutamate at position 6 of the ß chains of hemoglobin places a nonpolar residue on the outside of hemoglobin S, the version of hemoglobin that is responsible for the sickle-cell anemia. The oxygen affinity and allosteric properties of hemoglobin are virtually unaffected by this change. However, the valine sidechain of hemoglobin S interacts with a complementary sticky patch (formed by phenylalanine ß85 and leucine ß88) on another hemoglobin molecule -- a patch that is exposed in deoxygenated but not in oxygenated hemoglobin. a) what is the chemical basis for the interaction between the hemoglobin molecules? b) What would be the effect of this interaction?

Explanation / Answer

This mutation is the one that occurs in sickle-cell anemia. This alteration markedly reduces the solubility of the deoxygenated, but not the oxygenated, form of hemoglobin because the hemoglobin molecules interact with one another until a complex forms that is large enough that it cannot be solubilized. This complex precipitates and deforms the cell. Thus, sickling occurs when there is a high concentration of the deoxygenated form of hemoglobin S.

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