How does hemoglobin S (HbS) behave in alkaline electrophoresis?

Hemoglobin S (HbS) is the variant hemoglobin associated with sickle cell disease. When subjected to alkaline electrophoresis, HbS exhibits distinctive migratory behavior due to its unique amino acid composition. The pH of the buffer used in alkaline electrophoresis affects the charge of the hemoglobin molecules, leading to their separation based on differences in mobility.

HbS migrates slower than normal hemoglobin A (HbA) because of its different isoelectric point and the presence of a valine instead of a glutamic acid at position 6 of the beta-globin chain. While HbS does not separate completely from other variants, it is particularly known for its relationship with hemoglobin D (HbD), as both can migrate in similar positions on the gel due to their similar charges and structures.

In this case, the correct choice indicates that HbS migrates with HbD. This is an important aspect of recognizing hemoglobin variants in an electrophoresis setting and can significantly guide the understanding and diagnosis of various hemoglobinopathies. Identifying that HbS migrates with HbD highlights the necessity of additional testing for accurate characterization and differentiation of hemoglobin types, which is essential in clinical practice.