Hemoglobin C (Hbc), which is also found in people of African or Mediterranean descent, is very similar in structure to HbS. Both are caused by a change in the sixth residue of the beta chain. While valine replaces glutamic acid to form HbS, the amino acid lysine is found in this position, in HbC.
The substitution of lysine does not cause pathological changes in the hemoglobin molecule. People who are homozygous for HbC typically have red blood cells that appear unusual, but they do not sickle. These individuals have no symptoms, and they do not require treatment.
Some people have one gene for HbC and another for HbS. They have hemoglobin SC disease, which usually is much less severe than sickle cell disease.
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