Haemoglobin

INTRODUCTION:-

Haemoglobin is the most prevalent of the specialbloodpigmentsthat transportoxygen; it is present in all but the least complex of animals. Haemoglobin carries oxygen from thelungsorgills, where blood is oxygenated to body cells. When saturated with oxygen it is called oxyhaemoglobin and is a bright red colour. After haemoglobin releases oxygen to the bodytissues, it reverses its function and picks upcarbon dioxide, the principal product of tissuerespiration, for transport to the lungs, where it is expired. In this form, it is known as carboxyhaemoglobin and it is a purply-red colour.

The erythrocytes or red blood cells are ideally adapted for carrying oxygen. They contain haemoglobin, which gives them their red colour and is actively involved in oxygen transport. The shape of the cells means that they have a large surface area to volume ratio for thediffusionofgases, and having no nucleus means that there is the maximum amount of space available to pack in haemoglobinmolecules. In fact, each red blood cell contains around 250 million molecules of haemoglobin, giving it the capacity to carry 1,000 million molecules of oxygen. To combine properly with oxygen, the red blood cells must contain adequate haemoglobin; this, in turn, depends on the amount ofironin the body. The efficiency of haemoglobin caused by a lack of iron leads toanaemia.

GENETIC RESEARCH HISRTY :-

Oxygen carrying haemoglobin was discovered by Hunefeld in 1840. In 1951, Otto Funke published a series of articles in which he describes growing haemoglobin crystals dissolving red blood cells with pure solvent such as pure water, alcohol and ether. Hemoglobin reversible oxygenation was described by Felix Hoppe Seyler. In 1959 Max Perutz determined the molecular structure of haemoglobin by x-Ray crystallography.

HAEMOGLOBIN

Haemoglobin is found in the red blood cells of the body. Each blood cell contains approximately 280 million haemoglobin molecule.Haemoglobin is an iron containing oxygen transport metalloprotein in the red blood cells of vertebrates and tissues of some invertebrates. In mammels protein makes about 97% of the red b blood cells dry contents and 35% of water contents. Haemoglobin transports oxygen from the lungs or gills to the entire parts of the body where the body parts use the oxygen. Haemoglobin has oxygen binding capacityof between 1.36 to 1.37 of oxygen per gram haemoglobin which increases the blood oxygen capacity.

THE CHEMISTRY OF HAEMOGLOBIN

Haemoglobin is the key for the uptake of oxygen. Haemoglobin is made up of four globin poly peptide chains. Each chain has a prosthetic haem group which contains iron and gives red colour to the molecules. Haemoglobin has high affinity for oxygen .The oxygen is bound loosely to the haem group to form oxyhaemoglobin.

Haemoglobin reacts with oxygen. The first oxygen molecule attached alter the shape of the haemoglobin in such a way that it is easier for the next oxygen molecule to be taken on. This will alters the shape and makes it easier for the next oxygen molecule to be taken up, until the fourth and final oxygen molecule combines with the haemoglobin .The same process happens in reverse when oxygen dissociates from haemoglobin.The functioning of haemoglobin in the body is the effect of carbon dioxide.The process in which haemoglobin taken up and releases oxygen is affected by the proportion of carbon di oxide in the air.As the proportion of carbon dioxide increases, the haemoglomin curve moves down and to the right side.This is known as the Bohr shift. The effect of Bohr shift is that in higher partial pressure of carbon dioxide haemoglobin has higher level of oxygen to become saturated.On the other hand, carbon dioxide level in the lungs are low and so oxygen binds on to the haem group very easily.

The haemoglobin of the foetus is slightly different from the adults haemoglobin and it has a greater affinity for haemoglobin.This means that the maternal blood flows through the placenta, the foetal haemoglobin takes the oxygen from the maternal haemoglobin and carries it to the tissues and cell of the developing foetus..Haemoglobin does not only combine with oxygen and carbon dioxide. It combines so firmly with thecarbon monoxidefound incigarette smoke, faulty gas fires and car exhaust fumes that it can no longer combine with oxygen. If the carbon monoxide levels are too high this causesasphyxiationand death.Alterations in the structure of haemoglobin. The most important of these conditions issickle-cell anaemia, which involves a hereditary change in one of theamino acidsthat make up haemoglobin.

STRUCTURE OF HAEMOGLOBIN

Haemoglobin is a tetramer composed of 4 globin molecules :2 alpha globins and 2 beta globins.the alpha chain is composed of 141 amino acids and the beta globin chain is composed of 146 amino acids. Both alpha and beta globin proteins have similar parimary and secondary structures, each with 8 helical segments.Each globin chain contain one haem molecule.the haem molecule is composed of a porphyrin ring, which contains four pyrrole molecules cylindrically linked together, and an iron ion ligand bound in the center. The haem molecule is located between helix E and helix F of the globin protein. The alpha and beta subunits of the globin chains exists in the two dimmers which are bonded together strongly.

Oxygen binds to the iron ion tightly, and if two heme molecules come together in the presence of oxygen the iron atoms will oxidize and irreversibly bind to the oxygen. This irreversible binding would not be of use in the hemoglobin molecule because oxygen needs to be released in the tissues. The globin chain prevents this irreversible binding by folding the protein around the heme molecule, creating a pocket to isolate the heme molecule from other heme molecules (Perutz, 1978). Therefore, the globin molecules allow the iron atom to form loose bonds with the oxygen, and therefore, the ability to bind to oxygen and then release it into the tissues without becoming permanently oxidized in the proces Heme molecule Porphorin ring with iron atom ligand bound.

Oxygen binds to the iron ion tightly and if two heme molecules come together in the presene of oxygen the iron atoms will oxidise and binds to the oxygen.this irreversible binding would not be of use in the haemoglobin molecule because oxygen needs to be released in the tissues.The globin chains prevents the irreversible binding by folding the protein around the heme molecule, creates just like a pocket to isolate the heme molecule from other heme molecule. The globin molecule allow the iron atom to form loose bonds with the oxygen and the ability to bind to the oxygen and then release it into the tissues .

TYPES OF HAEMOGLOBIN

A hemoglobin electrophoresis test is a blood test done to check the different types of hemoglobinin the blood. Hemoglobin is the substance in red blood cells that carries oxygen. The most common types of normal hemoglobin are:

§ Hemoglobin F (fetal hemoglobin). This type is normally found in fetusesand newborn babies. Hemoglobin F is replaced by hemoglobin A (adult hemoglobin) shortly after birth; only very small amounts of hemoglobin F are made after birth. Some diseases, such as sickle cell disease, aplastic anemia, and leukemia, have abnormal types of hemoglobin and higher amounts of hemoglobin F.

§ Hemoglobin A. This is the most common type of hemoglobin found normally in adults. Some diseases, such as severe forms of thalassemia, may cause hemoglobin A levels to be low and hemoglobin F levels to be high.

§ Hemoglobin A2. This is a normal type of hemoglobin found in small amounts in adults.

More than 400 different types of abnormal hemoglobin have been found, but the most common are:

§ Hemoglobin S. This type of hemoglobin is present in sickle cell disease.

§ Hemoglobin C. This is another type of hemoglobin found in sickle cell disease.

§ Hemoglobin E. This type of hemoglobin is found in people of Southeast Asian descent.

§ Hemoglobin D. This type of hemoglobin may be present with sickle cell disease or thalassemia.

§ Hemoglobin H (heavy hemoglobin). This type of hemoglobin may be present in certain types of thalassemia.

Hemoglobin S and hemoglobin C are the most common types of abnormal hemoglobins that may be found by an electrophoresis test.

Electrophoresis uses an electrical current to separate normal and abnormal types of hemoglobin in the blood. Hemoglobin types have different electrical charges and move at different speeds.

An abnormal amount of normal hemoglobin or an abnormal type of hemoglobin in the blood may mean that a disease is present. Abnormal hemoglobin types may be present without any other symptoms, may cause mild diseases that do not have symptoms, or cause diseases that can be life-threatening. For example, hemoglobin S is found in sickle cell disease, which is a serious abnormality of the blood and causes serious problems.

HAEMOGLOBIN VARIENT

Hemoglobin variants are abnormal forms of hemoglobin. Made up of heme, an iron-containing portion, and globin, amino acid chains that form aprotein, hemoglobin (Hb or Hgb) molecules are found in all red blood cells. They bind oxygen in the lungs, carry the oxygen throughout the body, and release it to the body's cells and tissues.

Normal hemoglobin types include:

* Hb A - makes up about 95%-98% of Hb found in adults); contains two alpha (α) protein chains and two beta (β) protein chains

* Hb A2- makes up about 2%-3% of Hb; has two alpha (α) and two delta (δ) protein chains

* Hb F - makes up to 2% of Hb found in adults; has two alpha (α) and two gamma (γ) protein chains; the primary hemoglobin produced by the fetus during pregnancy; its production usually falls to a low level shortly after birth

Hemoglobin variants occur when genetic changes in the globin genes cause alterations in theamino acidsthat make up the globin protein. These changes may affect the structure of the hemoglobin, its behavior, its production rate, and/or its stability. There are fourgenesthat code for alpha globin chains and two genes that code for the beta globin chains. The most common alpha-chain-related condition is alpha thalassemia. Its severity is governed by the number of genes affected.

Beta chain hemoglobin variants are inherited in an autosomalrecessivefashion. This means that the person must have two altered gene copies, one from each parent, to have a hemoglobin variant-related disease. If one normal beta gene and one abnormal beta gene are inherited, the person is heterozygousfor the abnormal hemoglobin, acarrier. The abnormal gene can be passed on to any offspring, but it does not cause symptoms or health concerns in the carrier.

If two abnormal beta genes of different types are inherited, the person is doubly or compound heterozygous. The affected patient would typically have symptoms related to one or both of the hemoglobin variants that he or she produces. One of the abnormal beta genes would be passed on to each offspring.

OXYGEN HAEMOGLOBIN DISSOCIATION CURVE

Theoxygen-haemoglobin dissociation curve plots a graph between haemoglobinin its saturated form on the vertical axis against and oxygentension on the horizontal axis. The oxyhaemoglobin dissociation curve is an important tool for understanding how our blood carries and releases oxygen. The oxyhaemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2).

FACTORS THAT AFFECT THE DISSOCIATION CURVE

The strength with which oxygen binds to haemoglobin is affected by several factors. In effect these factors shift or reshape the oxyhaemoglobin curve . The standard curve is shifted to the right by an increase in temperature, 2,3-DPG, or pCO2, or a decrease in pH.

A rightward shift indicates that the haemoglobin has a low affinity for oxygen. This makes it more difficult for haemoglobin to bind to oxygen but it makes it easier for the haemoglobin to release oxygen bound to it. The effect of this rightward shift of the curve increases the partial pressure of oxygen in the tissues when it is most needed, during exercise, or haemorrhagic shock.

Ø VARIATION OF HYDROGEN ION CONCENTRATION

This changes the blood's pH. A decrease in pH shifts the standard curve to the right, while an increase shifts it to the left. This is known as theBohr effect.

Ø EFFECT OF CARBON DIOXIDE

Carbon dioxideaffects the curve in two ways: first, it influences intracellular pH and second, CO2accumulation causes carbamino compounds to be generated through chemical interactions, which bind to haemoglobin formingCarbaminohaemoglobin. Low levels of carbamino compounds have the effect of shifting the curve to the right, while higher levels cause a leftward shift.

Most of the CO2content is transported as bicarbonate ions. The formation of a bicarbonate ion will release a proton into the plasma. Hence, the elevated CO2 content creates a respiratory acidosis and shifts the oxygen-haemoglobin dissociation curve to the right.

Ø EFFECT OF 2,3-DPG

2,3-Disphosphoglycerate or 2,3-DPG, is an organophosphate, which is created in erythrocytes during glycolysis. High levels of 2,3-DPG shift the curve to the right, while low levels of 2,3-DPG cause a leftward shift .

Ø TEMPERATURE

Temperature does not have such a dramatic effect compared to the previous factors, buthyperthermia causes a rightward shift, while hypothermia causes a leftward shift.

Ø CARBONMONOOXIDE

Haemoglobin binds withcarbon monoxide240 times more readily than with oxygen. The presence of carbon monoxide on one of the 4 haem sites causes the oxygen on the other haem sites to bind with greater affinity. This makes it difficult for the haemoglobin to release the oxygen to the tissues and has the effect of shifting the curve to the left. With an increased level of carbon monoxide, a person can suffer from severe hypoxemia while maintaining a normal pO2.

FUNCTION OF HAEMOGLOBIN

The ability of hemoglobin to take up oxygen molecules in the lungs and then release them in the tissues is regulated by several factors both within the hemoglobin molecule itself and through external chemical factors. One of the biggest regulators of the oxygen affinity of the hemoglobin is the presence of oxygen itself. In the lungs where the oxygen levels are high, the hemoglobin has a higher affinity for oxygen and this affinity increases disproportionately with the number of molecules it already has bound to it . After the oxyhemoglobin binds one molecule of oxygen its affinity for oxygen increases until the hemoglobin is fully saturated.

Similarly the deoxyhemoglobin has a lower affinity for oxygen and this affinity decreases disproportionately with the number of molecules it already has bound . Thus, the loss of one oxygen molecule from the deoxyhemoglobin lowers the affinity for the remaining oxygen. This regulation is known as cooperativity and is essential to the functioning of the hemoglobin because it allows the oxyhemoglobin to carry the maximum amount of oxygen to the tissues and then allows the deoxyhemoglobin to release the maximum amount of oxygen into the tissues . Cooperativity is a function of the hemoglobin's unique structural characteristics, and it was found that the cooperative effects of the hemoglobin totally disappear if the hemoglobin is split in half. Hemoglobin is an allosetric protein that has more than one shape and can undergo conformational changes in its structure based on environment conditions . There are two alternative structures of hemoglobin; the relaxed structure (R) which has a greater oxygen affinity, and the tense structure (T) which has lower affinity for oxygen (given by Perutz in 1978). The change between the T and R structures is the result of a rotation of 15 degrees between the two alpha-beta dimers .This rotation changes the bonds between the side chains of the alpha-beta dimers in the F helix and therefore causes the heme molecule to change positions. In the T structure, the iron ion is pulled out of the plane of the porphyrin ring and becomes less accessible for oxygen to bind to it, thus reducing its affinity to oxygen. In the R structure the iron atom is in the plane of the porphyrin ring and is accessible to bind oxygen, thus increasing its oxygen affinity (Keates, 2004). The transformation from the T to R structure occurs when oxygen binds to the T structure under the high oxygen pressure environment in the lungs, which causes the rotation of the two dimers and shifts the remain iron atoms so that they become more accessible to oxygen. The transformation from the R to T structure occurs when oxygen is released under the low oxygen pressure environment of the tissues which causes the dimers to rotate back and shifts the iron atoms so that they become less accessible to oxygen (Keates, 2004). Thus, the cooperativity of the hemoglobin molecule can be explained by its unique structure which allows it to shift between the T and R structures in the presence or absence of oxygen.

References

Ø http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2005/Heiner/hemoglobin.html.

Ø http://health.yahoo.com/blood-diagnosis/hemoglobin-electrophoresis/healthwise--hw39098.html

Ø http://www.labtestsonline.org/understanding/analytes/hemoglobin_var/glance.html

Ø http://en.wikipedia.org/wiki/Oxygen-haemoglobin_dissociation_curve

Ø http://content.nejm.org/cgi/content/short/338/4/239