Haemoglobin consist of 4 polypeptide chains;
2 alpha chains ( each chain consist of 141 AA)
2 beta chains( each beta chain consist of 146 AA)
In haemoglobin these four chains of protein are arranged in quaternary manner.
Normal haemoglobin Beta chain consist of; valine— Histadine—leucine— Threonine— proline— glutamic acid—- glutamic acid.
Now in sickle cell anemia the valine amino acid replaces the glutamic acid at possition number 6. Due to which the normal shape of RBC( disc like) is converted into sickle shape. The sickle shape RBC can’t carry oxygen and hence can leads to death.