Hemoglobin is a protein in red blood cells that transports oxygen from the lungs to the peripheral tissues of the body. it is responsible for the red color of red blood cells. Hemoglobin tightly binds oxygen from the lungs, carries it from the lungs to the peripheral tissues of the body; after unloading oxygen at the peripheral tissues, it binds carbon dioxides and returns it to the lungs to be exhaled. It is composed of two protein subunits: alpha and beta. Hemoglobin requires both subunits in order to function properly. Disorders can result from abnormalties in either subunits. Abnormal hemoglobin structure or function can result in a variety disorders including sickel cell and thalassemia.
Hemoglobins proteins are translated from mRNA which is transcriped from genes. The alpha subunit is encoded by four genes while the beta subunit is encoded by two genes. Once made, the subunits become attached to each other. Everybody has the same genes that encode for hemoglobin, therefore, hemoglobin composition is the same for everyone in the world. Sometimes, although rare, mutations occur in the genes of certain individuals. Mutated hemoglobin genes, which are passed down from parents to offspring, result in abnormal hemoglobin proteins. The children of parents with mutated hemoglobin will produce the same abnormal hemoglobin the donor parent has. The structure of proteins is very important in determining the function of the protein; therefore, abnormal hemoglobin proteins may lose some of its functions. Although a majority of mutations is harmless, some can result in sickle cell anemia and thalassemia.
Hemoglobin levels are measured by the amount of hemoglobin in grams (gm) per deciliter (dl) of blood. The normal ranges for hemoglobin values are dependent on the age and sex. Normal ranges are:
* Newborns: 17-22 gm/dl
* One (1) week of age: 15-20 gm/dl
* One (1) month of age: 11-15gm/dl
* Children: 11-13 gm/dl
* Adult women: 12-16 gm/dl
* Adult males: 14-18 gm/dl
* Women after middle age: 11.7-13.8 gm/dl
* Men after middle age: 12.4-14.9 gm/dlSignificance of low hemoglobin level
Significance of high
People that live in areas of high altitude tend to have high hemoglobin levels. Also, certain illnesses, such as cancers, drug abuse and tumors can cause high hemoglobin levels. Smokers also tend to have high hemoglobin levels.
Hemoglobin contains four polypeptide subunits: two alpha chains and two beta chains, each with 141 and 146 amino acids respectively. The “globin” in hemoglobin refers to the individual protein subunits. Each subunit is comprised of mainly alpha helices with no beta strands. Each subunit folds into eight alpha helical segments which forms a pocket that holds the heme.
Figure 2: Hemoglobin
structure displaying heme group with iron attached
3: Hemoglobin structure
displaying bonds between protein subunits
A heme molecule is a cyclic molecule that consists of nitrogen, carbon and hydrogen atoms with a Fe2+ ion located in the center. Within the molecule, four nitrogen molecules hold the iron in the center. The iron ion also bonds with a histidine side chain from one of the subunits that form the pocket. The iron ion bonds to histidine 87 in the alpha chain and histidine 92 in the beta chain. The histidine 87 and histidine 92 are both part of the F helix in each subunit.
Hemoglobin’s ability to release oxygen is influenced by its environment, pH and CO2 levels. Generally, the oxygen-poor environment of the peripheral tissues has a lower pH than the oxygen-rich environment of the lungs. The acidic environment of the peripheral tissues results from the reaction between water and CO2, which forms bicarbonate and a proton.
Figure 4: Oxygen transport by hemoglobin within red blood cells Figure 5: Oxygen tranport by hemoglobin displaying iron and oxygen bonds
Anemia refers to the shortage of red blood cells or hemoglobin. The shortage of hemoglobin affects the delivery of oxygen to peripheral tisses. anemia is the most frequent blood disorder. The symptoms of anemia include fatigue, poor concentration, pallor and malaise. Severe anemia can result in palpitations and heart failure.
The size of red blood cells determine the classification of anemia. Microcytic anemia results from smaller than average red blood cells; normocytic anemia are average sized red blood cells; and macrocytic anemia results from larger than average red blood cells. Microcytic is the most frequent form of anemia. Individuals afflicted with microcytic usually appear pale. Iron deficiency is one of the causes of anemia since iron is a vital part of hemoglobin function. One of the common causes of iron deficiency is the monthly loss of blood during the menstrual cycle. Another common cause of iron deficiency is parasitic infestation from blood sucking helminthes such as hookworms. Normocytic anemia occurs when the red blood cells are a normal size but due to blood loss or another condition, hemoglobin levels are reduced. Some of the conditions that lead to normocytic anemia is acute blood loss, chronic anemia, and bone marrow failure. Macrocytic anemia can be cause by several factors. Lack of folic acid or vitamin B12 and lack of sufficient parietal calls can cause macrocytic anemia. Another condition that contributes to macrocytic anemia is alcoholism and prescription medicine that inhibit DNA replication.
Thalassemia is an inherited
disorder associated with red blood cells. It is a recessive trait
both parents are carriers. In thalassemia, a mutation occurs in the
which results in the decreased rate of normal globin subunit synthesis.
abnormalty in the chains of the protein. Bone marrow transplants and
transfusions are methods used to combat thalassemia. About 16% of
cases are in
Figure 6: Inheritance of recessive disorders
The alpha globin are encoded by four genes, two from each parent. The amount of mutation in each gene affects the severity of thalassemia. If all four alpha globin genes are affected, a fetus cannot live outside the uterus. The condition results in edematous where there is an increase in the interstitial fluid of certain organs. In the case where three of the four alpha globin genes are defective, hemoglobin H disease is the result. If two of the four genes of the alpha globin genes are affected, alpha thalassemia trait, type 1 results. Alpha thalassemia, type 1 is a mild condition where oxygen delivery is relatively normal. If one of the four genes of the alpha globlin genes are affected, alpha thalassemia trait, type 2 results. In alpha thalassemia trait, type 2, there is normal hemoglobin production and function. People with alpha thalassemia trait, type 2 are considered alpha thalassemia carriers.
Sickle-cell disease is a hemoglobin disorder caused by abnormally shaped red blood cells. The abnormally shaped red blood cells can aggregate within blood vessels which can lead to several complications. Sickle-cell anemia is a recessive trait disease that occurs more frequently with people of African descent .
Figure 7: 1.Normal red blood cells; 2.Sickled red blood cells
There are several conditions associated with sickle-cell disease. One of them is called Vaso-occlusive crises. vaso-occlusive crisis are periodic painful episodes experienced by individuals with sickle-cell anemia. In vaso-occlusivecrisis, aggregate abnormal red blood cells obstruct blood flow to organ tissues. This condition can lead to pain and eventual organ damage. The spleen, the organ that clears the blood can also be affected when an individual experiences vaso-occlusive crises. Splenic sequestration crisses, the enlargement of the spleen can result. It is a painful condition where the abdomen becomes hard and swollen. Blood transfusions can help with splenic sequestration crisis. The bones can also have vaso-occlusive damage. Common conditions associated with vaso-occlusive crisis is aplenia ( abnormal spleen function), acute chest crisis ( fever, hard breathing, chest pain and pulmonary infiltrate), and bone ischemia (bone damage as a result of insufficient blood supply).
Another condition associated with sickle cell anemia is called aplastic crises. Aplastic crises, a life-threatening condition, is the worsening of an individual’s sickle cell anemia. Often, it is triggered by parvovirus B19 which stops red blood cell production for a couple of days. A blood transfusion can help lesson the symptoms associated with aplastic crisis.
(4): "Anemia." Wikipedia. 26 April 2007.< http://en.wikipedia.org/wiki/Anemia >
(5): "Thalassemia." Wikipedia.