Haemoglobin is an iron binding metalloprotein that is found in the red blood cells of almost all vertebrates and some invertebrates. Over 300 million haemoglobin proteins are found in each red blood cell and the function of haemoglobin is to transport oxygen from respiratory organs- the lungs or gills, to the tissues and organs. As haemoglobin is able to bind to 1.34ml of oxygen per gram of the protein it allows the total blood oxygen carrying capacity of the blood to increase by over seventy times compared to that of the amount of oxygen that can dissolve in the blood. Haemoglobin, in red blood cells, is transported in the blood through blood vessels and once it reaches the tissues and releases its oxygen it is then able to transport carbon dioxide (CO2) from the tissues and organs to the respiratory organs.
Haemoglobin has an oligomeric quaternary structure composed of four subunits. These four subunits are two alpha chains (145 amino acid residues) and two 1beta chains (146 residues) that usually form alpha helices. The helices are connected by short non helical sections and are stabilised by hydrogen bonds. In the centre of each subunit there is a heme group that contains an iron atom that is held in the centre of a heterocyclic, porphryin ring. It is here that oxygen binding takes place.
Cooperativity is seen in the protein with haemoglobin being able to alternate between two structures; the relaxed or R state, and the tense or T state. The two states differ in their oxygen binding affinity with the R state having a greater affinity than the T state. This greater affinity is the result of the location and plane of the iron atom. In the R state the iron ion is in the plane of the porphrin ring and is therefore accessible to oxygen. This state allows oxygen to bind and therefore has a greater affinity for oxygen. Whereas in the T state the iron ion is pulled out of the plane of the porphrin ring and oxygen can bind less easily, therefore resulting in a lower affinity for oxygen.
Haemoglobin can alternate between the R and T states as the result of a rotation of 15 degrees between the dimers. The rotation causes the bonds between the two dimers to alter and forms a strain, this results in the central heme group of the dimers to change position. This transformation occurs when haemoglobin reaches the lungs, a high oxygen environment, and oxygen binds to the T state which results in the rotation of the dimers, allowing oxygen to bind more easily to the other heme groups. The R state is then regained when the bound oxygen is released when haemoglobin reaches the tissues, a low oxygen environment. The release of the oxygen from one of the heme groups causes the dimers to rotate back 15 degrees and results in the iron ions shifting and becoming less accessible to oxygen. This makes it easier for haemoglobin to release the other oxygen atoms bound to the three other heme groups. This is important because it means that haemoglobin still retains oxygen which can be released when it reaches the more peripheral tissues.
Binding of oxygen is a cooperative process. This means that the oxygen binding affinity of haemoglobin is increased as saturation of the haemoglobin molecule increases. This occurs because when the first oxygen binds to one of haemoglobins domains it results in a sterical conformational change of the whole protein and makes it easier for the next oxygen to bind as the other subunits have an increased affinity for oxygen. As a result of this cooperative binding process the oxygen binding curve of haemoglobin is sigmoidal or s shaped.
The oxygen affinity of haemoglobin can be regulated by physiological conditions. These conditions include pH, CO2 concentration and 2,3-diphosphoglutarate (DPG). These examples strengthen the bonds between the alpha units and prevent rotation of the alpha units from the T state to the R state, thus reducing the oxygen binding affinity of haemoglobin.
DPG is an allosteric effector molecule that can bind to haemoglobin and reduce its affinity for oxygen. The DPG can bind to haemoglobin only when it is in the T state. As a result when DPG is present it lowers the oxygen affinity of haemoglobin by preventing haemoglobin from changing from the T state to the R state.
CO2 is released from the tissues into the blood as a byproduct of respiration. The CO2 causes the pH of the blood to drop, acidifying it as a result of an increase in the concentration of hydrogen ions in the blood. This pH decrease causes haemoglobins affinity for oxygen to decrease and results in an effect known as the Bohr shift.
The Bohr shift was first described by Christian Bohr in 1904. The shift describes haemoglobins inverse oxygen binding affinity as a result of pH and the concentration of CO2 in the blood. Or in other words the Bohr shift describes how an increase in blood CO2 concentration or a decrease in pH results in the loss of oxygen from haemoglobin and also how a decrease in blood CO2 concentration or an increase in pH results in the uptake of oxygen by haemoglobin. However in addition as CO2 can react with water, and as water is present in blood, an increase in CO2 in the blood also casues a decrease in blood pH.
When the blood pH decreases the T state of haemoglobin dominates, this occurs because the T state binds hydrogen more readily than the R state. This effect also occurs when CO2 is released from the tissues into the blood because CO2 causes the blood pH to decrease. Additionally the CO2 binds to the haemoglobin molecules, but unlike oxygen, CO2 binds to the N- terminus of the alpha chains. The CO2 binds more strongly to haemoglobin when the protein is in the T state. As a result the release of oxygen into the tissues by the T state allows, and facilitates the uptake of CO2 by haemoglobin.
When haemoglobin reaches the lungs oxygen binds and this uptake causes the haemoglobin to change from the T state to the R state. This conformational change results in the release of CO2 from haemoglobin into the lungs due to CO2's reduced affinity for haemoglobin when it is in the R state.
There are many different forms of haemoglobin that are all approximately 64,000 daltons in molecular weight but differ in their chain composition. In adults the most common type of haemoglobin is haemoglobin A or adult haemoglobin. This form of the protein is composed of two 141 amino acid long alpha chains and two 146 amino acid long beta chains, forming an alpha2 beta2 tetramer.
In the human foetus and in infants up to about 6 months of age the most common form of haemoglobin is composed of two alpha and two gamma chains. This haemoglobin is called foetal haemoglobin and functionally it differs from adult haemoglobin because it can bind oxygen with a greater affinity. On the oxygen dissociation curve this is shown as a shift in the s shaped curve to the left. This characteristic allows the human foetus to better access oxygen from the mothers blood stream.
This effect is enhanced by foetal haemoglobin not binding to DPG. Normally DPG interacts with adult haemoglobin reducing its affinity for oxygen, however it doesn't interact with foetal haemoglobin and therefore does not affect its affinity for oxygen. The lack of interaction is due to the presence of the gamma chains in the foetal haemoglobin. However the gamma chains that cause the changes in the foetal haemoglobin are gradually replaced by beta chains as the infant grows, and all gamma chains are replaced by the time the infant is 6 months old.
Haemoglobinopathies are inherited disorders that cause alterations to the function of haemoglobin.
The most common haemogloninopathy is sickle cell anaemia which is an autosomal recessive disease that is the result of a base change of alanine (A) to thymine (T) in the second nucleotide of the sixth codon of the beta globin gene. The mutation results in the substitution of the amino acid valine for glutamine in the beta chain. This change alters the structure of the haemolgobin chains and results in the formation of a tetramer that is unstable when the haemoglobin is deoxygenated. This unstable tetramer polymerizes and forms rod like structures when the saturation of the unstable haemoglobin molecule falls below 85% and this results in the red blood cells forming sickle shapes. The sickle red blood cells can then get stuck in blood vessels and cause clots. In addition the altered structure of the haemoglobin and the red blood cells results in less oxygen binding capacity and therefore reduced oxygen transport.
Although sickle cell anaemia is a severe disorder that can impair life quality and reduce life expectancy in individuals who are homozygous for the sickle cell trait and are thus affected, it has also been found that individuals who are heterozygous for the sickle cell trait have resistance to marlaria. This resistance is due to the fact that the sickle red blood cells do not allow the malarial parasites to complete their life cycle in their human hosts. As a result the sickle cell gene is found more frequently in parts of the world where malaria is common.
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