Hemoglobin: A Globular Protein

Hemoglobin is a well known example of a globular protein. These proteins fulfill a great diversity of functions. For example, enzymes, antibodies and transport proteins are all globular proteins.

These proteins, as the name already suggests, are shaped as a globe. And it just so happens that globes are quite flexible. Globular proteins can change their shape, so that they fit in very small spaces (convenient for an antibody chasing a virus), can pass through cellular membranes (comes in handy for a transport protein) and can be involved with chemical reactions at cellular level (as enzymes are supposed to do).

Transporting Oxygen

Hemoglobin is in fact a transport protein that can be found in red blood cells. It transports oxygen through the body. A hemoglobin molecule roughly has the shape of a three dimensional four-leaf clover, without the stem. Each leaf represents a certain protein chain. In the middle of the clover, connected to each chain, one can find a heme group. Exactly in the centre of this heme group a, iron atom can be found.

When gas is exchanged between the lungs and the blood cells, the iron binds to the oxygen. After this, the hemoglobin molecule releases the iron-oxygen complex, so that the oxygen can pass through the cellular membrane and can find its way to all the cells in the body. The iron atom and the hemoglobin molecule are not just used once, they transport carbon dioxide back to the lungs and release it there so that it can be exhaled. For its entire lifetime, the hemoglobin molecule remains in the same cell. When that cell is going to die, the iron will be recycled by another red blood cell that will bind it to a hemoglobin molecule, or it will be excreted (and is responsible for the color of the feces).

Sicle Cell Anemia

Like said before, hemoglobin molecules are quite flexible and can change shape. People with sickle cell anemia, a hereditary disease, possess red blood cells with a sickle form (meaning thin and bent) instead of a globelike form, as normal red blood cells. This change in shape occurs after faulty hemoglobin molecules lose their oxygen and polymerize: they stick together and for larger molecules. Since these blood cells have a different shape, they move less easily through the veins and arteries. When blood and oxygen have difficulty to reach certain areas of the body, the consequences are a lot of pain and tissue damage. Sickle cell anemia is caused by as little as one mutated amino acid in the protein chain that constitutes hemoglobin.