Hemoglobin
  
 

Introduction(1)

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.


   hemoglobinmypolpic hemoglobin chains hemoglobin spheres
  Figure 1: Hemoglobin sturctures
made using PyMol and PDB file 101N  

Typical Hemoglobin Values(2):

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/dl

Significance of low hemoglobin level
People with low hemoglobin levels are anemic. Anemia can be caused by several factors such as vitamin deficiency (deficiency in iron, vitamin B12 or folate), blood loss, problems with bone marrow or abnormal hemoglobin which causes sickle cell anemia. 

Significance of high hemoglobin levels
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 Structure(3)

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.

                   hememolecule                    stablehemestructure

                   Figure 2: Hemoglobin structure displaying heme group with iron attached                                              Figure 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.

Bohr Effect(3)

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.

 CO2 + H20 -----------------> HCO3- + H+

 The acidic nature of the tissues enables the hemoglobin to function properly. Hemoglogin binds to two protons as  four oxygen molecules are released.  Protons aid in transferring oxygen from hemoglobin to tissues by lessening the attraction of hemoglobin for oxygen. This is called the Bohr effect and it is important in making the right side of the equation more favorable. The Bohr effect is important in removing CO2 from blood. HCO3 is more soluble in blood than CO2, therefore, it can bind to hemoglobin and be transported back to the lungs.
                                       oxytransport                                      oxytrans
                                       Figure 4:  Oxygen transport  by hemoglobin  within red blood cells                                          Figure 5: Oxygen tranport by hemoglobin displaying iron and oxygen bonds

 The Bohr effect also works in the opposite way. In the lungs, high oxygen concentration lessens the attraction of hemoglobins for protons. Hemoglobins release protons which makes the shifts the reaction to the left. The insoluble CO2 gas that forms as a result leaves the lungs. Without protons, hemoglobin once again has a strong attraction to oxygen. The cycle continues.

 Hemoglobin Disorders

Anemia(4)

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(5)

Thalassemia is an inherited disorder associated with red blood cells. It is a recessive trait disease where both parents are carriers. In thalassemia, a mutation occurs in the hemoglobin gene which results in the decreased rate of normal globin subunit synthesis. an abnormalty in the chains of the protein. Bone marrow transplants and blood transfusions are methods used to combat thalassemia. About 16% of thalassemia cases are in Cyprus, 3-14% in Thailand, 3-8% are in India, Bangladesh, Pakistan and China. There are two forms of thalassemias: ß thalassemia and α thalassemia. An abnormal ß globin results in ß thalassemia while an abnormalty in the α globin subunit results in α thalassemia.

recessivedisorder
Figure 6: Inheritance of recessive disorders

Alpha (α) thalassemias

HBA1 and HBA2 genes are involved in alpha thalassemias. Alpha thalassemia results in a decrease in the production of alpha  globin resulting in an excess β globin. Alpha thalassemia results in an unbalance tetramer called hemoglobin H that results in abnormal oxygen delivery.

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. 

Beta (β) thalassemias

Beta thalassemia results from a mutation of the HBB gene which is inherited recessively. The abnormal hemoglobin has an excess of alpha globin chains but does not form into tetramers. Beta thalassemia results in the binding of hemoglobin to the membrane to the red blood cell which causes membrane damage. At high concentrations, aggregate red blood cells with damaged membranes become toxic. As with alpha thalassemia, the severity of beta thalassemia depends on the amount of mutation in the gene.

Two genes encoded for beta globin, one from each parent. If both genes contain thalassemia mutation, the resulting condition is called beta thalassmia major or Cooley’s anemia. Beta thalassmia major is a serious condition where afflicted individuals have a life span of twenty years. A bone marrow transplant could cure beta thalassmia major. Frequent blood transfusions are another method to treat beta thalassmia major. A mutation in only one of the beta globin genes results in beta thalassmia minor. Beta thalassmia minor results in mild anemia and micocytosis which results in fatigue and weakness. Another condition resulting from a mutation in the beta globin genes is thalassemia intermedia, a condition between beta thalassemia major and minor. In afflicted individuals, blood transfusions helps control thalassemia intermedia.

Sickle-cell disease(6)

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 .

sickle cell
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.

 

References:

(1): Bridges, Kennet R. M.D.. “An Overview of Hemoglobin.” Information Center for Sickle-cell and Thalassemia Disorder.  < http://sickle.bwh.harvard.edu/hemoglobin.html >

(2): MedicineNet. “Hemoglobin.” MedicineNet. Inc. 9/5/2005. WebMD. 4/29/07.//www.medicinenet.com/hemoglobin/article.htm>

(3): James Madison University. CISAT Sharepoint College of Intergrated Science and Technology Sharepoint. <https://sharepoint.cisat.jmu.edu/isat/klevicca/Web/isat454/hemoglobin_essay.htm>

(4): "Anemia." Wikipedia. 26 April 2007.< http://en.wikipedia.org/wiki/Anemia >

(5): "Thalassemia." Wikipedia. 28 April 2007. < http://en.wikipedia.org/wiki/Anemia >

(6): "Sickle-cell Disease." Wikipedia. 27 April 2007. < http://en.wikipedia.org/wiki/Sickle-cell_disease >


Acknowledgements:
Dr. Amy Medlock - Thank you, I learned a lot.