Bloods get their distinctive red color from the hemoglobin in their red blood cells. Hemoglobin is a protein that contains iron, and it is the iron that gives blood its red hue. The main purpose of hemoglobin is to carry oxygen from the lungs to tissues throughout the body. As blood circulates through the lungs, the hemoglobin picks up oxygen, which temporarily changes its color from dark red to bright red. After delivering oxygen to the body’s cells, the hemoglobin turns back to dark red.
What is the function of hemoglobin?
Hemoglobin, or Hb, is a protein molecule found in red blood cells that is responsible for transporting oxygen from the lungs to tissues throughout the body. It makes up about 35% of the total content of red blood cells and gives blood its distinctive red color. The main role of hemoglobin is binding to oxygen in the lungs, where oxygen levels are high, and then releasing the oxygen in parts of the body where oxygen levels are low. When hemoglobin binds to oxygen, it forms a bright red oxyhemoglobin molecule. As it releases oxygen, the oxyhemoglobin turns back into darker red deoxyhemoglobin. This allows hemoglobin to efficiently deliver oxygen to cells throughout the body.
How does hemoglobin bind and release oxygen?
Hemoglobin is able to bind and release oxygen thanks to its unique molecular structure. Each hemoglobin molecule contains four subunits – two alpha subunits and two beta subunits – and each subunit has an iron atom bound to it. The iron atoms are located in “heme” groups within the subunits and are essential for oxygen transport. In the oxygen-rich environment of the lungs, the iron atoms readily bind to oxygen, forming oxyhemoglobin. The oxyhemoglobin then travels in the bloodstream to tissues with lower oxygen levels. Here, the iron atoms release the oxygen they are carrying, as the environment favors deoxyhemoglobin over oxyhemoglobin. This allows oxygen to diffuse into the cells where it is needed for energy production. After releasing oxygen, the hemoglobin picks up carbon dioxide waste and transports it back to the lungs to be exhaled.
What gives hemoglobin its red color?
The iron atoms in hemoglobin are central to its ability to bind oxygen, but they also give hemoglobin its characteristic red color. Iron atoms have unique light absorbing properties that cause hemoglobin to appear deep red when seen in large quantities in blood. The iron-containing heme groups are planar rings known as porphyrins. These porphyrin rings absorb light in the green range of wavelengths, causing them to appear red. When packed together in red blood cells, the millions of hemoglobin molecules give blood a bright red color. The more oxygen-rich oxyhemoglobin is present, the brighter the red color. As the oxyhemoglobin releases oxygen and transitions to deoxyhemoglobin, the color becomes darker. Veins, where deoxygenated blood is returning to the lungs, have a darker red color compared to oxygen-rich arteries.
What other proteins are involved in oxygen transport?
While hemoglobin is the most abundant protein involved in oxygen transport, there are a few other important players that help facilitate the movement of oxygen in blood:
- Myoglobin – Stores oxygen in muscle cells
- Hemocyanin – Transports oxygen in some invertebrates
- Chlorocruorin – Transports oxygen in some marine invertebrates
Myoglobin contains heme groups similar to hemoglobin. It acts as an oxygen storage unit within muscle tissue, releasing oxygen during muscle contraction. Hemocyanin and chlorocruorin also utilize copper or iron atoms to bind oxygen, but are not as efficient as hemoglobin and found in more primitive organism. All of these oxygen transport proteins utilize metal atoms to temporarily and reversibly bind molecular oxygen.
How does the structure of hemoglobin facilitate its function?
Hemoglobin has a quaternary structure made up of four subunits that allows it to efficiently bind and release oxygen molecules. Each hemoglobin molecule contains:
- 2 alpha subunits
- 2 beta subunits
The subunits assemble together in a tetrahedral arrangement, forming a protein with four oxygen binding sites. Human adults have a type of hemoglobin called HbA, with 141 amino acids in the alpha subunits and 146 amino acids in the beta subunits. The subunits are connected by hydrophobic interactions and salt bridges, which keep the molecule stable. Within each subunit is a heme group with a central iron atom capable of binding one oxygen molecule. The iron atoms can rapidly switch between two different conformations:
- Tense (T) – Oxygen is bound
- Relaxed (R) – Oxygen is released
This transition between the T and R states allows hemoglobin to efficiently uptake and deliver oxygen.
How does the hemoglobin tetramer change shape when binding oxygen?
When oxygen binds to the iron atoms of the heme groups, it causes some significant shape changes in the hemoglobin protein:
- The subunits rotate 15 degrees compared to their relaxed position
- The tetramer transitions from a low affinity “T” tense structure to a high affinity “R” relaxed structure
- These changes stabilize the oxygen-bound form of hemoglobin
The conformational changes are communicated between subunits via the contact points between them. As one heme group binds oxygen, it destabilizes the structure and triggers the other subunits to transition into the high affinity “R” form. This cooperativity allows the subunits to easily bind oxygen in the lungs and deliver it around the body.
What are the different types of hemoglobin?
There are several different types of hemoglobin that serve various purposes at different life stages:
| Type | Time Period | subunit composition |
|---|---|---|
| HbA | Adult | 2 alpha, 2 beta |
| HbF (fetal hemoglobin) | Fetus | 2 alpha, 2 gamma |
| HbA2 | Adult | 2 alpha, 2 delta |
HbA makes up about 97% of adult hemoglobin. HbA2 makes up around 2-3% of hemoglobin in adults. HbF is the main hemoglobin while a baby is developing in the womb. At birth, HbF makes up about 80% of total hemoglobin, but declines as HbA takes over through infancy. These different hemoglobin types have slightly different oxygen affinities that allow them to function optimally at specific life stages.
What is hemoglobin S and how does it impact sickle cell disease?
Hemoglobin S is an abnormal form of adult hemoglobin that contains a mutation in the beta globin subunit. In hemoglobin S, the 17th amino acid in the beta chain is valine instead of glutamic acid. This small change leads to the hemoglobin forming long polymers that distort red blood cells into a sickled shape. The sickled red blood cells get stuck in small blood vessels and block blood flow. This can cause severe pain, infections, and organ damage. Hemoglobin S is found in people who inherit one sickle cell gene and one normal gene, giving them sickle cell trait (AS), as well as those who inherit two sickle cell genes (SS), giving them sickle cell anemia.
What controls hemoglobin production?
Hemoglobin production is carefully regulated by the body to ensure sufficient amounts are present to transport oxygen, while avoiding potentially toxic excess. There are several control mechanisms:
- The kidneys secrete the hormone erythropoietin when low oxygen is detected, which stimulates red blood cell production in the bone marrow.
- The availability of iron, vitamin B12, and folic acid impact the synthesis of heme and globin components of hemoglobin.
- The oxygen levels of blood itself provide feedback – more hemoglobin leads to more oxygen, which reduces further hemoglobin production.
In addition, the gene transcription factors that control hemoglobin subunit production change throughout development. The different globin genes are sequentially turned on and off as the embryo transitions to a fetus and then baby.
How is hemoglobin breakdown regulated?
The breakdown of hemoglobin is carefully balanced with its production. Red blood cells contain an enzyme called heme oxygenase that degrades hemoglobin into its components – iron, biliverdin, and carbon monoxide. When red blood cells reach the end of their lifespan (about 120 days), they are removed from circulation by phagocytic cells, which break them down and allow recycling of hemoglobin iron. This heme catabolism produces bilirubin, which is removed by the liver. Problems with bilirubin production or excretion can lead to jaundice. The body has a number of built in mechanisms for regulating this hemoglobin recycling system.
What health problems are related to hemoglobin?
Hemoglobin levels and function are tightly linked to human health. Some key diseases and conditions related to hemoglobin include:
- Anemia – Low hemoglobin levels. Can be caused by iron deficiency, complications of pregnancy, vitamin deficiencies, and diseases that affect red blood cell production or survival.
- Polycythemia – High hemoglobin levels, which thickens the blood. Often caused by low oxygen levels or bone marrow disorders.
- Sickle cell disease – Caused by mutated hemoglobin S. Leads to sticky, crescent-shaped red blood cells that block blood vessels.
- Thalassemias – Genetic disorders that reduce globin production, leading to low hemoglobin and anemia.
- Hemochromatosis – Too much iron absorption, which can damage organs as it builds up.
Monitoring hemoglobin levels and signs of its dysfunction can provide great insight into overall health and detect underlying issues. Anemia, defined as low hemoglobin, is one of the most common blood disorders.
How do doctors test hemoglobin levels and function?
Some common ways to test hemoglobin include:
- Complete blood count (CBC) – Measures hemoglobin concentration and hematocrit.
- Hemoglobin electrophoresis – Separates out different hemoglobin types like HbA, HbS, and HbF.
- Iron tests – Low iron can impair hemoglobin production.
- Genetic testing – Can detect mutations leading to abnormal hemoglobins like sickle cell.
- Heinz bodies – Abnormal precipitates that form on red blood cells with unstable hemoglobin variants.
These tests help diagnose the wide range of conditions that affect hemoglobin synthesis, breakdown, and function.
Conclusion
In summary, hemoglobin is an extraordinary protein that gives blood its red color and enables it to deliver life-sustaining oxygen throughout the body. Its unique structure allows it to efficiently bind and release oxygen to meet the dynamic needs of tissues and organs. Hemoglobin production and degradation are carefully regulated, and abnormalities can lead to diseases like anemia, polycythemia, and sickle cell. Understanding the essential role this protein plays in oxygen transport and the factors that influence its function provides great insight into red blood cell biology and many aspects of human health and disease.