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INTERACTIVE MOLECULES -- THE HEMOGLOBIN-S MOLECULE

What Causes Sickle Cell Anemia?

Sickle Cell and Hemoglobin S

Sickle cell anemia is an inherited condition. People with sickle cell anemia inherit two copies of the sickle cell gene, one from each parent. The sickle cell gene makes abnormal hemoglobin called Hemoglobin-S.

In sickle cell anemia, the abnormal hemoglobin (Hemoglobin-S) sticks together when it gives up its oxygen to the tissues. These clumps cause red blood cells to become stiff and shaped like a sickle.It takes two copies of the sickle cell gene for the body to make the abnormal hemoglobin found in sickle cell anemia

Sickle Cell Trait

.People who inherit only one copy of the sickle cell gene (from one parent) will not have sickle cell anemia. They will have sickle cell trait.

People who have sickle cell trait generally have no symptoms and lead normal lives. Like people with sickle cell anemia, however, they can pass the sickle cell gene on to their children.

How is Hemoglobin Different in the Sickle Cell?

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of the blood in vertebrates and other animals. In mammals the protein makes up about 97% of the red cells dry content, and around 35% of the total content (including water). Hemoglobin transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases the oxygen.

     
PDB FILE:2HBS
 

To view Hemoglobin S molecule using Chime 3D

PDB FILE: 2HBS

Harrington, D.J., Adachi, K., Royer Jr., W.E. (1997) The high resolution crystal structure of deoxyhemoglobin S. J.Mol.Biol. 272: 398-407

 

The Hydrophobic Pocket

The interaction between Val 6 (yellow) on one chain of one hemoglobin molecule and a hydrophobic patch formed by Phe 85 (blue) and Leu 88 (gey) on a chain of another deoxygenated hemoglobin molecule leads to hemoglobin aggregation.

 

double click for backbone structure showing pocket

 

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Sickle-cell anemia is caused by a point mutation in the -globin chain of hemoglobin, replacing the amino acid glutamic acid with the less polar amino acid valine at the sixth position of the chain. The Glu 6 Val mutation in deoxy-HbS favors a hydrophobic interaction between each strand and its neighbor. The abnormal hemoglobin in which valine has replaced glutamic acid causes the hemoglobin to become less soluble under decreasing oxygen concentrations and to polymerize into crystals that distort the red blood cells into a sickle shape.

At the molecular level the interaction between Val 6 on one chain of one hemoglobin molecule and a hydrophobic patch formed by Phe 85 and Leu 88 on a chain of another deoxygenated hemoglobin molecule leads to hemoglobin aggregation. See for image

 
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