Explain the characteristic of sickle-cell hemoglobin that makes it different from normal hemoglobin.

Hemoglobin is a protein found in red blood cells. It is responsible for binding oxygen in the lungs, carrying it throughout the body, and releasing it where needed. Hemoglobin consists of four subunits, each containing an iron-containing heme group that can bind one oxygen molecule.

Sickle-cell hemoglobin (HbS) is a variant of normal hemoglobin (HbA). The primary difference between the two is a single amino acid substitution: in HbS, the sixth amino acid in the β-globin chain is changed from glutamic acid to valine. This small change in the protein structure leads to significant consequences in the function and behavior of the hemoglobin molecule.

The substitution of valine in place of glutamic acid in HbS causes a hydrophobic patch to form on the surface of the hemoglobin molecule. When HbS loses its bound oxygen (which happens when it delivers oxygen to tissues), the hydrophobic patch interacts with a complementary site on an adjacent hemoglobin molecule. This interaction leads to the aggregation of HbS molecules, forming long fibers which distort the red blood cell into a stiff, sickle shape. In contrast, HbA does not exhibit this aggregation behavior and allows red blood cells to maintain their flexible, biconcave shape.

The sickle shape of red blood cells containing HbS leads to several problems. Firstly, the sickled cells become less flexible and can get stuck in small blood vessels, causing blockage and reducing blood flow. Secondly, the sickled cells are more fragile and break apart easily, leading to an overall decrease in red blood cell count or anemia. Finally, these blockages and reduced oxygen delivery to tissues can cause pain, organ damage, and increased susceptibility to infections. In summary, the characteristic of sickle-cell hemoglobin that makes it different from normal hemoglobin is the single amino acid substitution, which leads to aggregation and deformation of red blood cells, ultimately resulting in several health complications.