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Haemoglobin is a protein that carries oxygen in red blood cells. It contains two a-globin subunits and two ß-globin subunits, which are identical and symmetrically arranged.
A heme molecule the site that binds oxygen (O2 ) is contained in each of these four polypeptide chains thus contains four molecules of oxygen.
To understand the structure of haemoglobin we need to understand proteins. Proteins are functional they provide structural support, transport, protection, defence, regulation and movement. Proteins can range in various sizes.
A protein molecule is a long chain of amino acids linked through covalent peptide bonds, thus proteins are known as polypeptides.
Amino acids are the backbone of proteins they contain a carbon atom called a carbon. Amino acids are both acids and basses, as at common PH values found in cells the carboxyl group loses a hydrogen atom and the amino group gains one.
Attached to the a carbon atom a hydrogen atom is a side chain or R group. The side chains of amino acids are a functional groups they determine three dimensional structures.
In polymerized amino acids the carboxyl group reacts with amino group of another that forms a peptide bond (a peptide linkage).
Proteins are classed in four structures, a primary structure, a secondary structure, a tertiary structure and a quaternary structure.
The primary structure of protein is the precise sequence of amino acids in a polypeptide chain. The polypeptide chain is made by repeating a sequence -N-C-C-, the N atom from amino group, a carbon atom, and the C atom from the carboxyl group. The amino acids sequence dictates how the protein structure twists and folds distinguishing its structure. The other levels of protein structure are derived from primary structure.
The secondary structure is determined b the hydrogen bonding between the a Helix and ß sheet of the amino acids.
The a (alpha) helix is coiled right-handed, this coiling occurs between the hydrogen bond formed of N-H from one amino acid and the oxygen of the C=O of another. This stabalizes the coil when repeated this secondary structure is common in proteins called Keratins.
The ß (beta) pleated sheet is formed when two or more polypeptide chains extend and are aligned. This happens by the hydrogen bonds between the N-H groups and the C=O groups.
The tertiary structure is the three dimensional organisation of polypeptide chains at specific points. The structure is determined between the R groups- the amino acid chains.
The quaternary structure is made up of subunits. Two or more polypeptide chains and the way they bind together and interact.
Haemoglobin is a quaternary structure. Haemoglobin is formed by hydrogen bonds, hydrophobic interactions, van der Waals forces, and ionic bonds all hold the subunits together.
Oxygen is carried in red blood cells and this is the function of haemoglobin. The structure changes when the subunits change their relative position slightly. As haemoglobin binds one O2 molecule broken ionic bonds expose side chains enhancing binding of additional O2 molecules.
When O2 molecules are release into the cells of the body the quaternary structure changes again.
Oxygen is attached to a heme molecule which is attached to four polypeptide chains. So each molecule of haemoglobin carries four oxygen molecules.
The four heme groups in a molecule of haemoglobin each have a central iron atom. The red colour of blood is given by the heme in haemoglobin.
By binding to oxygen gas through the iron atoms haemoglobin in the lungs can pick up oxygen and release it in the tissues.
As the chemical structure of polypeptide chain that form haemoglobin molecule varies there is more than one form of haemoglobin.
Adult haemoglobin has two polypeptide chains 2 a globin chains and 2 ß globin chains. The human foetus before birth has a different kind of haemoglobin, 2 a globin and 2 ? globin chains.
The difference between the functions of adult haemoglobin and foetal haemoglobin is that foetal haemoglobin has a higher affinity to O2.
The foetal haemoglobin can pick up O2 more easily than the mothers haemoglobin.
Now to summarize what haemoglobin is. Haemoglobin is a protein that carries oxygen in red blood cells from the lungs to tissues in the body and returns carbon dioxide from the tissues to the lungs. . Haemoglobin contains two a-globin subunits and two ß-globin subunits, which are identical and symmetrically arranged.
The alpha chains are 141 amino acids long and the beta chains are 146 amino acid s long. Haemoglobin is a quaternary structure.
There are numerous diseases linked to haemoglobin, the common ones are anaemia, sickle cell and thalassemia. Some of these diseases are inherited.
Let us take thalassaemia for instance Alpha thalassemia and beta thalassemia major are inherited. There are over 700 people with thalassemia disorders and approximately 214,000 healthy carriers in the UK alone (Nice, 2008).
Beta thalassemia major requires blood transfusions every 4-6 weeks and iron chalation to prevent further illness as this form of thalassemia can be life threatening.
Due to the nature and severity of the disorders pregnant women are screened for sickle cell and thalassaemia disorders as well as other haemoglobin variants.
People who have the genetic disease beta thalassaemia, there ß-globin subunit of haemoglobin is inadequate. Due to this inadequate amount of red blood cells these people suffer from severe anemia.
Thalassaemia occurs when the particular condition is present in both the parent; a child receives two of the same traits genes from each parent. The defective production of the alpha chain of haemoglobin is called alpha thalassaemia and is mainly found in Africa, the Middle East and the Far East.
There is a 25% chance that a child will receive two trait genes from two carriers and developing the disease, and a 50% chance of being a carrier. Most lead a completely healthy and normal life.
When two alpha genes are inherited from the parents the genotypes for the normal alpha gene is written Alpha Alpha / Alpha Alpha. In the silent carrier state there is a deletion of one of the alpha-globin gene producing the genotype - Alpha / Alpha - Alpha.
The alpha thalassaemia trait occurs when there is a deletion of two alpha globin, and this is termed mild hypochromic anaemia and results from a loss of both pairs of chromosomes -- -- / Alpha Alpha genotype. This trait is found mainly in Asia especially in South East Asia.
Alpha thalassaemia also occurs when there is a loss of one gene from each chromosome giving the genotype -- Alpha /-- Alpha, this trait is mainly found amongst black Africans.