Why does sickle cell hemoglobin migrate slower than normal during gel electrophoresis?

During electrophoresis, molecules are separated based on their charge and size. Sickle cell hemoglobin (HbS) differs from normal hemoglobin (HbA) due to a mutation in the beta-globin gene, resulting in the substitution of a single amino acid (glutamic acid to valine) in the beta-globin chain. This change affects the overall shape and charge of the hemoglobin molecule.

In HbS, the valine substitution causes the molecule to become less soluble and more prone to polymerization. When deoxygenated, HbS molecules tend to aggregate and form elongated, rigid polymers that can distort red blood cells, leading to the characteristic sickle shape.

During gel electrophoresis, HbS migrates slower compared to HbA due to several factors:

Size and shape: The polymerized HbS molecules are larger and have a more irregular shape compared to HbA. Larger molecules generally migrate slower through the gel matrix. Additionally, the elongated and distorted shape of HbS polymers hinders their movement through the gel.

Charge: The mutation in HbS alters the overall charge of the molecule. Compared to HbA, HbS has a reduced net negative charge. The gel matrix used in electrophoresis typically has a negative charge, which attracts positively charged molecules. Since HbS has a weaker negative charge, it experiences less electrostatic attraction toward the negative pole, resulting in slower migration.

Interactions with the gel matrix: The HbS polymers can interact with the gel matrix more extensively than HbA. This interaction can create additional resistance to the movement of HbS molecules, further slowing down their migration.

As a result of these factors, HbS migrates slower than HbA during gel electrophoresis, producing distinct bands that can be visualized and used to identify and differentiate between individuals with sickle cell trait or sickle cell disease from those with normal hemoglobin.