Hemoglobin is a protein produced in the bone marrow that is stored in erythrocytes which carry oxygen throughout the body. The main function of erythrocytes is the transportation of oxygen to the body's cells to enable cellular respiration to occur. 97% of the oxygen carried by the blood from the lungs is carried through hemoglobin, whilst the remaining 3% is dissolved in the plasma. Hemoglobin allows blood to move 30 to 100 times more oxygen than that dissolved in the plasma. In the lungs, hemoglobin physically bonds with oxygen due to the high levels of present oxygen within the organ.
The Structure of Hemoglobin
Hemoglobin consists of a heme molecule and a globin protein. The globin is an example of a globular protein present in a quaternary polypeptide structure. Globular proteins are multiple chains of polypeptides that are soluble in water with a spherical shape. The spherical structure of the globular proteins enables them to carry out synthesis, transportation, and metabolism within cells. Hemoglobin consists of 4 polypeptide subunits: 2 alpha chains and 2 beta chains. The genes for hemoglobin are found in chromosomes 11 and 16. DNA helicase unwinds the DNA double helix structure to enable the production of an mRNA as the first step of transcription. During the process of translation, the mRNA determines the sequence of the amino acids of the globin protein. The end result of the protein synthesis is 4 chains of proteins are produced, which can each hold one iron atom. Each iron atom binds with one molecule of oxygen. This increases the efficiency of oxygen transportation, as one hemoglobin can hold 4 oxygen particles in the area of the body for essential cellular respiration.
Protein Synthesis of Hemoglobin
The heme portion of the hemoglobin synthesis occurs in the mitochondria of the immature erythrocytes. δ-aminolevulinic acid (ALA) is formed as a result of the condensation of glycine and succinyl-CoA in the mitochondria. ALA then once produced enters the cytoplasm, in which porphobilinogen is formed through a chain of chemical reactions to hence produce coproporphyrinogen. Coproporphyrinogen re-enters the mitochondria where it undergoes additional reactions to become protoporphyrin. This then combines with an iron molecule to form heme. Globin synthesis of alpha and beta chains and their structure is given by the genes found on chromosomes 16 and 11. The 4 polypeptide globin chains are in pairs, exhibiting the formation of a tetrameric molecule in which the globin chain is covalently bonded to a heme molecule. The tetramer is an ellipsoid structure whilst being 550 nm in diameter. A normal alpha chain is composed of 141 amino acids, in comparison to a beta chain which consists of 146 amino acids.
Variations of Hemoglobin
There are 3 types of normal hemoglobin variants: hemoglobin A (HbA), hemoglobin A2 (HbA2), and fetal hemoglobin (HbF). HbA, known as adult hemoglobin, is the most common human hemoglobin tetramer. It consists of 2 alpha chains and 2 beta chains. HbA2 is a normal variant of hemoglobin A that consists of 2 alpha and 2 delta chains. HbF is the main oxygen transport protein in the human fetus during the last seven months of development in the uterus and persists in the newborn until roughly 2-4 months old. HbF consists of 2 alpha and 2 gamma chains. The level of HbF drops after 1-2 years of the child’s birth and reaches the adult level. HbF levels can become elevated in several congenital conditions. Its levels can remain the same or become increased in beta-thalassemia and sickle cell anemia.
There are 5 common hemoglobin variants: hemoglobin S (HbS), hemoglobin C (HbC), hemoglobin E (HbE), hemoglobin H (HbH), and hemoglobin Barts. Although, prior to understanding the different variants of hemoglobin, a distinguishment between signs and symptoms must be established. A sign may be defined as evidence of a condition as discovered or elicited by a practitioner while examination of the patient. However, symptoms are things that a patient complains about regards of. This is helpful in understanding why the detection of certain variants of hemoglobin is difficult as some conditions are asymptomatic. Asymptomatic is the lack of presence of symptoms of a condition in an infected individual.
HbS is a mutation occurring on the beta chains. HbS is a primary hemoglobin in people who have sickle cell anemia. A heterozygous gene mutation of sickle cell is called sickle cell trait and is usually asymptomatic unless combined with another hemoglobin mutation. A homozygous mutation in the cell genes results in a sickle cell disease. The presence of HbS makes the red blood cells deformed, resulting in a sickle shape when exposed to a low-oxygen environment. A low-oxygen environment example includes intense exercise and sickness. Sickle cells block the small blood vessels during the time of low oxygen, causing pain and impaired circulation. The cells result in a decreased cell lifespan this is called a sickling crisis. The patient who suffers from it will usually encounter symptoms such as abdominal pains, back pain, and knee pain. In individuals with increased levels of hemoglobin F in whilst having sickle cell anemia the condition may be reduced and become milder as the affinity of hemoglobin F inhibits sickling of the red blood cells.
HbC is a mutation occurring on the beta chains, similar to that of HbS. A heterozygous mutation of a gene for HbC is the most common form of HbC. However, a homozygous gene mutation for HbC is rare and relatively mild. It usually causes a minor amount of hemolytic anemia and a mild to moderate enlargement of the spleen. Hemolytic anemia is a condition in which red blood cells are destroyed and removed from the bloodstream before their normal lifespan is over.
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HbE is one of the most popular beta chain variants in the world. People who are homozygous for HbE generally have mild hemolytic anemia. The individual will possess microcytic red blood cells and mild enlargement of the spleen. A heterozygous mutation for HbE is usually asymptomatic unless is combined with another hemoglobin abnormality such as beta thalassemia trait. HbH is a type of Alpha Thalassemia caused by impaired production of alpha globins. It is composed of 4 beta chains and is produced in response to a severe shortage of alpha chain production. The tretrema of the 4 beta chains do not function normally. It has an increased affinity to oxygen, holding on to it instead of releasing it to the tissues.
Hemoglobin Barts is an abnormal type of hemoglobin that consists of 4 gamma globulins. It is produced as a result of a shortage of alpha chain production. It has an extremely high affinity for oxygen, resulting in almost no oxygen delivery to the tissues.
There are many other variants, some of which are silent, causing no symptoms or signs, while others affect the functionality and stability of the hemoglobin molecule. Examples are hemoglobin D, hemoglobin G, hemoglobin J, hemoglobin M, and hemoglobin constant spring. The most common example of silent variants is that of hemoglobin constant spring. Hemoglobin constant spring is a variant of hemoglobin in which a mutation in the alpha globin gene produces an alpha globin chain that is abnormally long. It is the most common non-deletional alpha-thalassemia mutation associated with hemoglobin H disease.
The Effect of Hemoglobin on the Human Body
The normal concentration of HbA is 15.5 grams per deciliter of blood in males. In females, the normal concentration is 14 grams per deciliter of blood. Females tend to have lower hemoglobin concentration due to their menstruation. There is a variation of the concentration of hemoglobin within populations varying from low to increased hemoglobin count.
Low hemoglobin count is called anemia and is defined as hemoglobin less than 13.5 grams per deciliter for men and less than 12 grams per deciliter for women. Anemia is caused by a variety of conditions that a person may be affected by. A mild decrease in hemoglobin count can be asymptomatic. Symptoms of low hemoglobin count are tiredness, pallor, shortness of breath, and palpitations. Untreated low hemoglobin count can be serious leading to serious problems like heart failure.
Increased hemoglobin concentration is called polycythemia and is defined as hemoglobin greater than 18.5 grams per deciliter for men and greater than 16.5 grams per deciliter for women. The types of polycythemia can be primary, which means that there is no obvious cause, and it is called polycythemia vera, or secondary to decreased oxygen concentration, such as high altitude or chronic lung conditions. High hemoglobin concentration can be as serious as low hemoglobin concentration as it can lead to blood clots due to increased blood viscosity, causing strokes and heart attacks. Polycythemia can be treated with low-dose aspirin to decrease the viscosity of the blood.
Conclusion
In conclusion, the analysis of the function of hemoglobin aims to highlight the importance of its variants’ effects on the functioning of the body. The investigation displays the importance of protein synthesis from the perspective of cellular respiration. High or low hemoglobin count contributes to serious conditions in individuals who have it. The hemoglobin molecule is constructed by a heme and a globin protein, which is synthesized separately and then combined in the mitochondria. Protein synthesis produces 2 beta chains and 2 alpha chains, which each hold an iron molecule to enable the transportation of oxygen to the body for sufficient cellular respiration to occur.
Bibliography
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- DAVID W. E. SMITH & M.D. (1980). The Molecular Biology of Mammalian Hemoglobin Synthesis.
- Clancy, S. (2008). Genetic Mutation.
- Bernard G. Forget and H. Franklin Bunn. (2013). Classification of the Disorders of Hemoglobin.
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- Ou Z., Li Q., Liu W. & Sun X. (2011). Elevated Hemoglobin A2 as a Marker for β-Thalassemia Trait in Pregnant Women.
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