Sickle cell anemia

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Sickle cell is an inherited disease and it is caused by a mutation that occurs in the beta sub units of the haemoglobin. Haemoglobin is a tetrameric protein made up of  2 alpha subunits and 2 beta subunits and it is the essential part of the blood responsible for oxygen transportation. Sickle cell is a  disease that results from a substitution of a polar amino acid known as glutamate with a non polar one valine at  position six of the beta polypeptide unit of haemoglobin. The substitution happens as a result of a change in one of the bases in the beta globin gene from adenine to thymine.

As a result of this mutation, the beta polypeptide chains become sticky in low oxygen conditions because the valine sticks out of the chain and interacts with neighbouring non-polar amino acids. The molecules stick together and finally develop into a massive fibrous polymer that causes the distortion of the red blood cells into a “c” or sickle. When the sickle-cell Hb (HbS) loses an oxygen molecule, another hydrophobic patch becomes exposed in both normal Hb and HbS which the hydrophobic Valine interacts with. This causes formation of fibrous aggregates of Hb which causes the red blood cells to distort into a sickle shape. Oxygen can not bind to the HbS fibres efficiently therefore, the sickle cell carries less oxygen than a normal Hb. The fibres of the cell cause it to become less flexible and increases tendency to clump. As opposed to the normal cell, the sickle cell carries less oxygen and it is stiff so it cannot squeeze through blood vessels easily and therefore has a high tendency of blocking arteries that supply oxygen to the cells. This could lead to organ failure and damage for example stroke, heart attack or kidney failure [1][2][3]

Newborns have a different heamoglobin structure known as hemoglobin F which consists of two alpha and two gamma subunits. This changes to 2 alpha and 2 beta subunits after around 6 months of age however this means sickle cell anemia isn't usually diagnosed until infancy. Some symptoms that may be seen in infants indicating this disease are pain and swelling in hands and feet and throughout childhood this pain can spread to the back, stomach and chest [4]

References

  1. Understanding Sickle Cell Disease by Mariam Bloom, Ph.D 1995
  2. http://sickle.bwh.harvard.edu/msh.html
  3. http://www.nejm.org/doi/full/10.1056/NEJM199008093230602
  4. Jones,P. (2008)Genes and Disease Sickle Cell Disease. New York:Infobased Publishing p86

Lubert Stryer (1995). Univeristy of Wisconsin-Madison,Biochemistry (4th edition). W.H. Freeman and Company, New York. 

https://www.chem.wisc.edu/deptfiles/genchem/netorial/modules/biomolecules/modules/protein1/sickle.htm

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