Difference Between Normal Hemoglobin and Sickle Cell Hemoglobin | Normal Hemoglobin vs Sickle Cell Hemoglobin


Key Difference – Normal Hemoglobin vs Sickle Cell Hemoglobin
 

Hemoglobin (Hgb) is the main protein molecule which provides the typical shape of the red blood cell – round shape with a narrow center. The hemoglobin molecule is made up of four sub protein molecules in which two chains are alpha globulin chains, and the other two are beta globulin chains. Iron atoms in hemoglobin and the shape of the red blood cells are important for oxygen transportation through blood. If the shape of hemoglobin is destroyed, it fails to transport oxygen through the blood. Sickle cell hemoglobin is one type of an abnormal hemoglobin molecule which causes an anemia conditions called sickle cell anemia. The key difference between normal hemoglobin and sickle cell hemoglobin is that normal hemoglobin has glutamic acid in the 6th position of the amino acid sequence of the beta globulin chain whereas sickle cell hemoglobin has Valine in the 6th position of the beta globulin chain. Normal hemoglobin and sickle cell hemoglobin only differ by a single amino acid in beta chains.

CONTENTS
1. Overview and Key Difference
2. What is Normal Hemoglobin
3. What is Sickle Cell Hemoglobin
4. Side by Side Comparison – Normal Hemoglobin vs Sickle Cell Hemoglobin
5. Summary

What is Normal Hemoglobin?

Hemoglobin is an iron-containing metalloprotein found in red blood cells. It is responsible for the transportation of oxygen from the lung to the body tissues and organs, and transportation of carbon dioxide from body tissues to the lung. It is also known as oxygen-carrying protein in the blood. It is a complex protein, which consists of four small protein subunits and four heme groups bearing iron atoms as shown in figure 01. Hemoglobin has a high affinity for oxygen. There are four oxygen binding sites located inside a hemoglobin molecule. Once hemoglobin saturates with oxygen, the blood becomes bright red in colour and is known as oxygenated blood. The second state of the hemoglobin, which lacks oxygen, is known as deoxyhemoglobin. In this state, blood bears the dark red colour.

Iron atoms embedded in the heme compound of hemoglobin mainly facilitates the oxygen and carbon dioxide transportation. Binding of oxygen molecules to Fe+2 ions changes the conformation of the hemoglobin molecule. The iron atoms in the hemoglobin also help to maintain the typical shape of the red blood cell. Therefore, iron is a vital element found in the red blood cells.

Difference Between Normal Hemoglobin and Sickle Cell Hemoglobin | Normal Hemoglobin vs Sickle Cell Hemoglobin

Figure 01: Normal Hemoglobin

What is Sickle Cell Hemoglobin?

Sickle cell anemia is a blood condition caused due to abnormal hemoglobin proteins present in the red blood cells. Sickle cell hemoglobin is a type of abnormal hemoglobin found in red blood cells. They are also known as hemoglobin S. They possess sickle or crescent shapes. They are produced as a result of sickle cell gene mutation. This mutation changes a single amino acid in the amino acid sequence of normal hemoglobin beta chain peptide. Sickle cell hemoglobin is also composed of two alpha and two beta subunits, just like normal hemoglobin.  However, there is a single amino acid difference in beta subunits due to the mutation. In normal hemoglobin, 6th position of the amino acid chain in beta chains is composed of glutamic acid. However, in sickle cell hemoglobin, 6th position is taken up by a different amino acid called valine. Though it is a single amino acid difference, it is the cause of life threatening anemia disease called sickle cell disease.

When valine is positioned at 6th, it causes the beta chain to form a protrusion which fits with beta chains of other hemoglobin molecules. These connections make sickle cell hemoglobin to aggregate each other without remaining in the solution and transport oxygen. It takes a rigid structure, and finally, red blood cells break down prematurely, which leads to anemia conditions.

Difference Between Normal Hemoglobin and Sickle Cell Hemoglobin | Normal Hemoglobin vs Sickle Cell Hemoglobin

Figure 02: Sickle Cell Hemoglobin

What is the difference between Normal Hemoglobin and Sickle Cell Hemoglobin?

Normal Hemoglobin vs Sickle Cell Hemoglobin

Normal hemoglobin is an iron-containing protein in the red blood cells, which transport oxygen and carbon dioxide through the blood. Sickle cell hemoglobin is a type of abnormal hemoglobin which causes the agglutination of the sickle shape red blood cells in the blood.
Abbreviation
Abbreviation of the normal hemoglobin is HbA. Abbreviation of the sickle cell hemoglobin is HbS.
Structure
The structure of normal hemoglobin is composed of two alpha chains and two beta chains. Structure of sickle cell hemoglobin is composed of two alpha chains and two S chains.
Shape
Normal hemoglobin is round with a narrow center. The shape of the red blood cell containing sickle cell hemoglobin is crescent or sickle shape.
6th Position of Amino Acid
The sixth position in the amino acid chain of beta globulin chain is glutamic acid. The sixth position is occupied by valine in sickle cell hemoglobins.
Result
Normal hemoglobin causes red blood cells to flow freely within blood vessels. Sickle cell hemoglobin causes blocking the flow of red blood cells within the vessels.

Summary – Normal Hemoglobin vs Sickle Cell Hemoglobin

Hemoglobin is the oxygen-transporting protein in red blood cells. It is composed of four subunits of proteins named alpha and beta chains. It is an iron-containing molecule which causes the colour and the round shape of the red blood cells. Due to mutations, the shape of the red blood cells can differ. It happens due to abnormal hemoglobin molecules in the red blood cells. Sickle cell hemoglobin is one such mutation. They change the shape of the red blood cells from round to sickle shape, which ultimately leads to the premature destructions of red blood cells. This disease condition is known as sickle cell anemia. However, the difference between normal hemoglobin and sickle cell hemoglobin a single amino acid difference in the beta chain of the hemoglobin.

Reference:
1. The Molecular Biology of Sickle Cell Anemia. N.p., n.d. Web. 28 May 2017. <http://www.nslc.wustl.edu/sicklecell/part2/molecular.html>.
2. “Sickle cell disease – Genetics Home Reference.” U.S. National Library of Medicine. National Institutes of Health, n.d. Web. 28 May 2017. <https://ghr.nlm.nih.gov/condition/sickle-cell-disease#genes>

Image Courtesy:
1. “Risk-Factors-for-Sickle-Cell-Anemia (1)2” By Diana grib – Own work (CC BY-SA 4.0) via Commons Wikimedia [Cropped]