Hemoglobin (Hb) is a metalloprotein found in red blood cells. Red blood cells transport Oxygen throughout the body. All vertebrates except fish, have hemoglobin in the red blood cells as the oxygen carrier. Hemoglobin makes up the 96% of dry weight of the red blood cell and contains iron. All human bodies contain hemoglobin. The normal hemoglobin level of a normal male adult is 13.8 – 17.2 g/dL. Adult female (non-pregnant) should have 12.1 – 15.1 g/dL of hemoglobin.
This article will look at,
1. What is the structure of Hemoglobin
2. What is the Function of Hemoglobin in the Human Body
What is the Structure of Hemoglobin
Hemoglobin is a multi-subunit globular protein, which has a quaternary structure – four globin subunits are arranged in a tetrahedral structure. Each globular protein subunit contains a protein chain which is associated with non-protein, prosthetic heme group. The alpha-helix structure of the globin proteins creates a pocket which binds the heme group. Globin proteins are synthesized by ribozymes in the cytosol. The Heme part is synthesized in the mitochondria. A charged iron atom is held in the porphyrin ring by covalent binding of iron with four nitrogen atoms in the same plane. These N atoms belong to the imidazole ring of the F8 histidine residue of each of the four globin subunits. In hemoglobin, iron exists as Fe2+.
The human body contains three hemoglobin types: Hemoglobin A, Hemoglobin A2, and Hemoglobin F. Hemoglobin A is the most common type. Hemoglobin A is encoded by HBA1, HBA2, and HBB genes. The four subunits of Hemoglobin A consist of two α and two β subunits (α2β2). Hemoglobin A2 and Hemoglobin F are rare, and they consist of two α and two δ subunits and two α and two γ subunits respectively. In infants, the hemoglobin type is Hb F (α2γ2).
Figure 1: Structure of Hemoglobin
What is the Function of Hemoglobin in the Human Body
- Hemoglobin is an oxygen carrier.
- Hemoglobin is a carbon dioxide carrier.
- Hemoglobin gives the red color to blood.
- Hemoglobin maintains the shape of the red blood cells.
- Hemoglobin acts as a buffer.
- Hemoglobin interacts with other ligands.
- Hemoglobin degradation accumulates physiologically active catabolites.
Oxygen Carrier
The major function of hemoglobin is the transportation of oxygen from lungs to all the tissues of the body. The oxygen binding capacity of hemoglobin is 1.34 mL O2 per gram. Each globin subunit of the hemoglobin molecule can bind with one Fe2+ ion. The affinity of hemoglobin towards oxygen is gained by the Fe2+ ion. Each Fe2+ can bind with one oxygen molecule. The binding of oxygen oxidizes Fe2+ into Fe3+. One atom of the oxygen molecule, which binds to Fe2+ becomes a superoxide, where the other oxygen atom protrudes at an angle. The oxygen-bound hemoglobin is referred to as oxyhemoglobin. When blood reaches an oxygen deficient tissue, oxygen is dissociated from hemoglobin and diffused into the tissue. The O2 is the terminal electron acceptor in the process called oxidative phosphorylation in the production of ATP. The removal of O2 turns the iron into its reduced form. The oxygen-unbound hemoglobin is referred to as deoxyhemoglobin. Oxidation of Fe2+ into Fe3+ creates methemoglobin which cannot bind with O2.
Carbon Dioxide Carrier
Hemoglobin also transports carbon dioxide from tissues to lungs. 80% of the carbon dioxide is transported via plasma. Carbon dioxide does not compete with the oxygen-binding site of hemoglobin. It binds to the protein structure other than iron-binding position. The carbon dioxide bound hemoglobin is referred to as carbaminohemoglobin.
Influence on Red Blood Cells
Hemoglobin gives a red color to red blood cells by Fe2+ ions. With red blood cells, blood reaches to its unique red color. Plasma, without red blood cells, has a pale yellow color. The shape of the red blood cells is maintained by hemoglobin. Red blood cells are biconcave disks which are flattened and depressed in the center. They have a dumbbell-shaped cross section. Hemoglobin gene also consists of various alleles. Most mutants may cause no disease. But some mutants may cause hereditary diseases like hemoglobinpathesis.
Figure 2: Red blood cells
Buffering Action
Hemoglobin maintains the blood pH at 7.4. Accumulation of carbon dioxide in the blood decreases the pH from 7.4. The change of the pH can be reversed by ventilation. Due to this buffering action of hemoglobin, all the enzymatic reactions in the body, which prefers this pH, can take place without any disturbance.
Interaction with Ligands
Hemoglobins also bind to other ligands such as carbon monoxide, nitrogen oxide, cyanide, sulfur monoxide, sulfide, and hydrogen sulfide. Binding of carbon monoxide may sometimes be lethal because the binding is irreversible. Hemoglobin can also transport drugs to their site of action.
Production of Physiological Active Catabolites
Aging and defects in the cell can kill the red blood cells, accumulating various physiologically active catabolites. Hemoglobin of the dead red blood cells is cleared from the circulation by the hemoglibin transporter, CD163. Heme degradation, which occurs in monocytes and macrophages, is a natural source of the carbon monoxide generation. Bilirubin is the final product of heme degradation. It is secreted as bile into intestine. Bilirubin is converted into urobilinogen which is found in feces, giving the unique yellow color. On the other hand, iron, which is removed from heme is converted to ferritin and stored in tissues for the later use.
Hemoglobin can also be found in other cells of the body than red blood cells. Other hemoglobin carrying cells are macrophages, alveolar cells in lungs and mesangial cells in the kidney. Hemoglobin functions as a regulator of iron metabolism and an antioxidant in these cells.
Reference:
1. “Hemoglobin”. Wikipedia, the free encyclopedia. 2017. Accessed 15 Feb. 2017
2. Davis C. P. and Shiel W. C. “Hemoglobin”. MedicineNet, 2015.htm. Accessed 15 Feb. 2017
3. “Structure and Functions of hemoglobin”. All Medical stuff, 2017. Accessed 15 Feb.2017
Image Courtesy:
1. “1904 Hemoglobin” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia
2. “Red blood cells” By Jessica Polka – Own work (CC BY-SA 4.0) via Commons Wikimedia
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