Transport of Oxygen
Oxygen is primarily transported in red blood cells bound to hemoglobin molecules.
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Oxygen is also dissolved directly in the bloodstream.
This dissolved fraction contributes little to the total amount of oxygen carried in the bloodstream.
Henry’s Law states that the dissolved fraction is proportional to the atmospheric pO2, but the solubility of oxygen is so low that only 3ml O2/L of blood is dissolved at atmospheric oxygen tension.
Hemoglobin carries 98% of the oxygen in the blood in the protein-bound form, approximately 197 ml/L.
It is important to differentiate between pO2 (mm Hg, the dissolved fraction), oxygen saturation (% of hemoglobin occupied), and O2 content (expressed as a volume percentage).
Arterial oxygen content is approximately 20 g/dL, the venous oxygen content is 15 g/dL, and dissolved oxygen contributes 0.1 g/dL in each case (but is continuously replenished from the hemoglobin bound pool).[1][2][3]
+++++++++++ Oxygen is bound to hemoglobin, a tetramer of 2 alpha and 2 beta subunits. Each subunit can carry one molecule of oxygen, and a complete hemoglobin tetramer can carry four molecules. In the completely unbound state, hemoglobin predominates in the T (tense) form. The T form requires a higher pO2 to bind an oxygen atom, like that found in the oxygen-rich pulmonary capillary beds. The subsequent oxygen molecules can bind to hemoglobin more favorably. This is because binding oxygen-binding induces a conformational change in the other subunits towards the R (relaxed) form. This interaction between the hemoglobin subunits is termed cooperativity. The R form does not require a high pO2 to allow oxygen binding. By the time blood exits the pulmonary circulation, hemoglobin is 100% saturated with oxygen (four molecules bound). In normal lungs, hemoglobin molecules become close to 100% saturated with oxygen long before the end of the capillary bed, about a third of the way along. This allows effective oxygenation of the blood fully even during times of increased oxygen consumption (heavy exercise). Entering the systemic circulation, oxygen-rich hemoglobin is in the R form. At the lower pO2 in the peripheral tissues, oxygen begins to unbind. With less oxygen bound, and at a lower pO2, the T state becomes more favorable, which facilitates unloading oxygen atoms two through four. Throughout the bloodstream, at different pO2 levels, there is a continuum between the T state (unloading, requiring high O2 to bind) and the R state (loading, requiring low O2 to unbind). At rest, most of the oxygen-binding sites on hemoglobin are occupied. Of note, myoglobin, as a single subunit oxygen-carrying protein, does not exhibit cooperativity. It behaves like a single T state hemoglobin. This helps pull oxygen into skeletal muscle.[4][5][6][7]
Plotting oxygen tension (x) vs saturation (y) reveals a sigmoid curve that describes visually how oxygen binds to hemoglobin. At higher oxygen tension, for example during pulmonary circulation, the oxygen dissociation curve plateaus. At lower oxygen tension, the slope of the oxygen dissociation curve is steeper.
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The oxygen–hemoglobin dissociation curve is a curve that plots the proportion of hemoglobin in its saturated (oxygen-laden) form on the vertical axis against the prevailing oxygen tension on the horizontal axis.
This curve is an important tool for understanding how blood carries and releases oxygen
Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.
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The quaternary structure of HbA allows it to bind oxygen with positive cooperativity, giving a sigmoidal oxygen dissociation curve.
A single hemoglobin molecule can bind up to four oxygen molecules.
The four globin subunits work cooperatively in hemoglobin, in which the binding of oxygen to one subunit of the tetramer increases the affinity of the other subunits for oxygen.
The first oxygen binds with low affinity, but this leads to a transition from T to R form.
The second through fourth oxygen molecules bind with increasing affinity, which leads to a sigmoidal oxygen-binding curve for hemoglobin).
This interaction is reversible.
The O 2 -binding curve shifts right with increase in CO 2 , acidosis (low pH), increase in 2,3-BPG, exercise, and temperature.
Deoxyhemoglobin (T form) preferentially binds hydrogen ion (H + ), 2,3-bisphosphoglycerate (2,3-BPG), and CO 2 . This leads to stabilization of the T state, decreased affinity for oxygen, and a rightward shift in the oxygen saturation curve. Exercise and increase in temperature can also cause a rightward shift. Conversely, a leftward shift in the oxygen saturation curve occurs in the presence of a decrease in CO 2 , alkalosis (high pH, low hydrogen ion concentration), and decrease in 2,3-BPG .
CO poisoning causes stabilization of the R state, a leftward shift in O 2 saturation curve, and a saturation curve for hemoglobin that resembles the curve for myoglobin.
Carbon monoxide (CO) binds to hemoglobin and forms carboxyhemoglobin, which has a high affinity for CO and displaces O 2 . This leads to stabilization of the R state, a leftward shift of the oxygen saturation curve, and an oxygen saturation curve for hemoglobin that resembles the curve for myoglobin.
Fetal hemoglobin is α 2 γ 2 . Sickle cell disease (SSD) is α 2 β S 2 .
(1) Carry Oxygen
Hemoglobin binds and release oxygen as a function of the surrounding concentration, or partial pressure of oxygen. [make it simpler)
[image
A plot of percent saturation of hemoglobin with oxygen versus the partial pressure of oxygen is called the oxygen dissociation curve.
The quaternary structure of HbA allows it to bind oxygenwith positive cooperativity, giving a sigmoidal oxygen dissociation curve. Essentially, binding of oxygen to one HbA subunit increases the affinity of binding to other subunits in the tetramer, thereby shifting the equilibrium between oxy and deoxy forms. The net effect of this cooperativity is that HbA releases oxygen, The quaternary structure of HbA also allows it to respond to 2,3-bisphosphoglycerate, carbon dioxide, and hydrogen ion, all of which are heterotropic negative allosteric effectors of oxygen binding.
Hemoglobin synthesis starts with heme and globin synthesis.
Heme Synthesis
When red cells reach the end of their life due to aging or defects, they are broken down in the spleen.
Macrophages separate hemoglobin molecules into proteins (globulin) and heme.
Heme
The major final product of heme degradation is bilirubin.
Bilirubin is the product of hemoglobin breakdown (heme metabolism) formed during the elimination of senescent red blood cells.
Cycling of bilirubin and its products through the liver, intestines, portal and systemic circulations.
Oxidation of the heme generates biliverdin, which is metabolized into unconjugated bilirubin, and then bound to albumin. and transported to the liver.
- The unconjugated bilirubin-albumin complex reaches the hepatocyte; bilirubin dissociates from albumin and then enters the hepatocyte.
- Conjugation: Unconjugated bilirubin and glucuronic acid combine to make conjugated bilirubin.
- Excretion: The hepatocyte excretes conjugated bilirubin into the bile.
- The rate-limiting step of bilirubin metabolism in the liver
- If excretion is impaired, conjugated bilirubin enters the hepatic sinusoids and then into the bloodstream.
- Conjugated bilirubin in the bile is transported through the biliary ducts into the duodenum; it is not reabsorbed by the intestine.
- Can be excreted unchanged in the stool
- Can be converted to urobilinogen by colonic bacteria
- Urobilinogen can be reabsorbed, entering the portal circulation.
- Some is taken up by the liver and re-excreted into the bile.
- Some bypasses the liver and is excreted by the kidney, thus appearing in the urine in small amounts.
- Can be converted in the bowel to stercobilin rendering the stool brown.
- Unconjugated bilirubin is not found in the urine because it is bound to albumin and cannot be filtered by the glomeruli.
- Conjugated bilirubin is filtered and excreted in the urine when there is conjugated hyperbilirubinemia.
See: Animation
Iron is removed from heme and salvaged for later use, it is stored as hemosiderin or ferritin in tissues and transported in plasma by beta globulins as transferrins.
When the porphyrin ring is broken up, the fragments are normally secreted as a yellow pigment called bilirubin, which is secreted into the intestines as bile. Intestines metabolise bilirubin into urobilinogen. Urobilinogen leaves the body in faeces, in a pigment called stercobilin. Globulin is metabolised into amino acids that are then released into circulation.
Splitting of the four pyrrole ring of heme produces biliverdin, which is then reduced to bilirubin.
This type of bilirubin is unconjugated, and insoluble in water. As a result, it must be attached to albumin for transport to the liver for excretion.
[The hemoglobin molecule is broken up, and the iron gets recycled. This process also produces one molecule of carbon monoxide for every molecule of heme degraded.
Heme degradation is one of the few natural sources of carbon monoxide in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air.
[Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage.]
Study Question
A 20-year-old exchange male student from Greece presents to the office for evaluation of glucose-6-phosphate dehydrogenase deficiency. 2 days ago, he noticed yellow discoloration of his conjunctiva. His family history is significant for hemolytic anemia due to glucose-6-phosphate dehydrogenase deficiency. Which of these conditions is most likely associated with hemolytic anemia?
A-Increased plasma unconjugated bilirubin
B-Increased bilirubin in urine
C-Increased urobilinogen in urine
D-Clay colored stool
E-Dark brown urine
The correct answer is A
Hemolysis of RBCs results in a large amount of bilirubin delivered to the liver. The ability of the liver to conjugate all the bilirubin to glucuronic acid fails and unconjugated bilirubin increases in the plasma causing jaundice. Unconjugated bilirubin is unlikely to pass into the urine because it is bound to albumin and has low solubility in plasma. Clay colored stool occurs due to absence of bile pigments in the colon, which is not associated with hemolytic anemia.
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Hemoglobin has two primary forms. Hemoglobin that is desaturated with oxygen is deoxyhemoglobin (T, tense form), [which has a low oxygen affinity and little available movement.] ?Hemoglobin that is saturated with oxygen is oxyhemoglobin (R, relaxed form), which has a high oxygen affinity and more available movement. ]
The quaternary structure of HbA allows it to bind oxygen with positive cooperativity, giving a sigmoidal oxygen dissociation curve.
A single hemoglobin molecule can bind up to four oxygen molecules. The four globin subunits work cooperatively in hemoglobin, in which the binding of oxygen to one subunit of the tetramer increases the affinity of the other subunits for oxygen. The first oxygen binds with low affinity, but this leads to a transition from T to R form. The second through fourth oxygen molecules bind with increasing affinity, which leads to a sigmoidal oxygen-binding curve for hemoglobin).
The O 2 -binding curve shifts right with increase in CO 2 , acidosis (low pH), increase in 2,3-BPG, exercise, and temperature.
Deoxyhemoglobin (T form) preferentially binds hydrogen ion (H + ), 2,3-bisphosphoglycerate (2,3-BPG), and CO 2 . This leads to stabilization of the T state, decreased affinity for oxygen, and a rightward shift in the oxygen saturation curve. Exercise and increase in temperature can also cause a rightward shift. Conversely, a leftward shift in the oxygen saturation curve occurs in the presence of a decrease in CO 2 , alkalosis (high pH, low hydrogen ion concentration), and decrease in 2,3-BPG .
CO poisoning causes stabilization of the R state, a leftward shift in O 2 saturation curve, and a saturation curve for hemoglobin that resembles the curve for myoglobin.
Carbon monoxide (CO) binds to hemoglobin and forms carboxyhemoglobin, which has a high affinity for CO and displaces O 2 . This leads to stabilization of the R state, a leftward shift of the oxygen saturation curve, and an oxygen saturation curve for hemoglobin that resembles the curve for myoglobin.
Fetal hemoglobin is α 2 γ 2 . Sickle cell disease (SSD) is α 2 β S 2 .
Hemoglobin Degradation and Metabolism
When red cells reach the end of their life due to aging or defects, they are broken down in the spleen. Macrophages separate hemoglobin molecules into proteins (globulin) and heme.
Splitting of the four pyrrole ring of heme produces biliverdin, which is then reduced to bilirubin. This type of bilirubin is unconjugated, and insoluble in water. As a result, it must be attached to albumin for transport to the liver for excretion.The hemoglobin molecule is broken up, and the iron gets recycled. This process also produces one molecule of carbon monoxide for every molecule of heme degraded. Heme degradation is one of the few natural sources of carbon monoxide in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air.
The major final product of heme degradation is bilirubin.
[Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage.]
Iron is removed from heme and salvaged for later use, it is stored as hemosiderin or ferritin in tissues and transported in plasma by beta globulins as transferrins.
When the porphyrin ring is broken up, the fragments are normally secreted as a yellow pigment called bilirubin, which is secreted into the intestines as bile. Intestines metabolise bilirubin into urobilinogen. Urobilinogen leaves the body in faeces, in a pigment called stercobilin. Globulin is metabolised into amino acids that are then released into circulation.
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Oxidation of the heme moiety of Hgb generates biliverdin, which is metabolized into unconjugated bilirubin, and then bound to albumin.
-
There are 3 steps in bilirubin metabolism in the liver:
-
Uptake: The unconjugated bilirubin-albumin complex reaches the hepatocyte; bilirubin dissociates from albumin and then enters the hepatocyte.
-
Conjugation: Unconjugated bilirubin and glucuronic acid combine to make conjugated bilirubin.
-
Excretion: The hepatocyte excretes conjugated bilirubin into the bile.
-
The rate-limiting step of bilirubin metabolism in the liver
-
If excretion is impaired, conjugated bilirubin enters the hepatic sinusoids and then into the bloodstream.
-
-
- Conjugated bilirubin in the bile is transported through the biliary ducts into the duodenum; it is not reabsorbed by the intestine.
- Can be excreted unchanged in the stool
- Can be converted to urobilinogen by colonic bacteria
- Urobilinogen can be reabsorbed, entering the portal circulation.
- Some is taken up by the liver and re-excreted into the bile.
- Some bypasses the liver and is excreted by the kidney, thus appearing in the urine in small amounts.
- Can be converted in the bowel to stercobilin rendering the stool brown.
- Unconjugated bilirubin is not found in the urine because it is bound to albumin and cannot be filtered by the glomeruli.
- Conjugated bilirubin is filtered and excreted in the urine when there is conjugated hyperbilirubinemia.
See: Animation
The hemoglobin molecule is made up of four polypeptide chains: two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each.
The alpha and beta chains have different sequences of amino acids, but fold up to form similar three-dimensional structures. The four chains are held together by noncovalent interactions. There are four binding sites for oxygen on the hemoglobin molecule, because each chain contains one heme group < >. In the alpha chain, the 87th residue is histidine F8 < >and in the beta chain the 92nd residue is histidine F8 >. A heme group is attached to each of the four histidines. The heme consists of an organic part and an iron atom < >. The iron atom in heme binds to the four nitrogens in the center of the protoporphyrin ring. The hemoglobin molecule is nearly spherical, with a diameter of 55 angstroms . The four chains are packed together to form a tetramer. The heme groups are located in crevices near the exterior of the molecule, one in each subunit. Each alpha chain is in contact with both beta chains< >. However, there are few interactionsbetween the two alpha chains or between the two beta chains >.
Each polypeptide chain is made up of eight or nine alpha-helical segments < >and an equal number of nonhelical ones placed at the corners between them and at the ends of the chain. The helices are named A-H, starting from the amino acid terminus, and the nonhelical segments that lie between the helices are named AB, BC, CD, etc. The nonhelical segments at the ends of the chain are called NA at the amino terminus and HC at the carboxyl terminus.
To form the tetramer < >, each of the subunits is joined to its partner around a twofold symmetry axis, so that a rotation of 180 degrees brings one subunit into congruence with its partner. One pair of chains is then inverted and placed on top of the other pair so that the four chains lie at the corners of a tetrahedron. The four subunits are held together mainly by nonpolar interactions and hydrogen bonds. There are no covalent bonds between subunits. The twofold symmetry axis that relates the pairs of alpha and beta chains runs through a water-filled cavity >at the center of the molecule. This cavity widens upon transition form the R structure to the T structure to form a receptor site for the allosteric effector DPG (2,3 diphosphoglycerate) between the two beta chains. The heme group is wedged into a pocket of the globin with its hydrocarbon side chains interior and its polar propionate side chains exterior.
There are nine positions in the amino acid sequence that contain the same amino acid in all or nearly all species studied thus far. These conserved positions are especially important for the function of the hemoglobin molecule. Several of them, such as histidines F8 (His87)< > and E7 (His63)< >, are directly involved in the oxygen-binding site< > . Phenylalanine CD1 (Phe43) < > and leucine F4 (Leu83) < > are also in direct contact with the heme group< >. Tyrosine HC2 (Tyr140) < >stabilizes the molecule by forming a hydrogen bond between the H< > and F helices< >. Glycine B6 (Gly25)< >is conserved because of its small size: a side chain larger than a hydrogen atom would not allow theB< > and E helices< > to approach each other as closely as they do. Proline C2 (Pro37)< > is important because it terminates the C helix. Threonine C4 (Thr39) and lysine H10 (Lys127) are also conserved residues, but their roles are uncertain.
Heme Synthesis
Heme biosynthesis involves eight enzymatic steps in the conversion of glycine and succinyl-CoA to heme.
The heme biosynthetic pathway showing the eight enzymes and their substrates and products. Four of the enzymes are localized in the mitochondria and four in the cytosol.These eight enzymes are encoded by nine genes, as the first enzyme in the pathway, 5′-aminolevulinate synthase (ALA synthase), has two genes that encode unique housekeeping (ALAS1) and erythroid-specific (ALAS2) isozymes. The first and last three enzymes in the pathway are located in the mitochondrion, whereas the other four are in the cytosol.
Heme is required for a variety of hemoproteins such as hemoglobin, myoglobin, respiratory cytochromes, and the cytochrome P450 enzymes (CYPs). Hemoglobin synthesis in erythroid precursor cells accounts for approximately 85% of daily heme synthesis in humans.
Hepatocytes account for most of the rest, primarily for the synthesis of CYPs, which are especially abundant in the liver endoplasmic reticulum, and turn over more rapidly than many other hemoproteins, such as the mitochondrial respiratory cytochromes. Pathway intermediates are the porphyrin precursors, ALA and PBG, and porphyrins (mostly in their reduced forms, known as porphyrinogens).
Heme to hemoglobin
Hemoglobin A, the most common hemoglobin in adults, is composed of two α-globin subunits and two β-globin subunits (α 2 β 2 ).
Each globin protein subunit is composed of an α- or β-protein chain plus a heme group. Heme, in turn, is composed of an Fe 2 + held in a protoporphyrin IX ring.
Must See Reference: http://www.buzzle.com/articles/structure-of-hemoglobin.html
Hemoglobin has two primary forms. Hemoglobin that is desaturated with oxygen is deoxyhemoglobin (T, tense form), which has a low oxygen affinity and little available movement. Hemoglobin that is saturated with oxygen is oxyhemoglobin (R, relaxed form), which has a high oxygen affinity and more available movement.
The hemoglobin O 2 -binding curve shifts right with increase in CO 2 , acidosis (low pH), increase in 2,3-BPG, exercise, and temperature.
A single hemoglobin molecule can bind up to four oxygen molecules. The four globin subunits work cooperatively in hemoglobin, in which the binding of oxygen to one subunit of the tetramer increases the affinity of the other subunits for oxygen. The first oxygen binds with low affinity, but this leads to a transition from T to R form. The second through fourth oxygen molecules bind with increasing affinity, which leads to a sigmoidal oxygen-binding curve for hemoglobin).
Deoxyhemoglobin (T form) preferentially binds hydrogen ion (H + ), 2,3-bisphosphoglycerate (2,3-BPG), and CO 2 . This leads to stabilization of the T state, decreased affinity for oxygen, and a rightward shift in the oxygen saturation curve. Exercise and increase in temperature can also cause a rightward shift. Conversely, a leftward shift in the oxygen saturation curve occurs in the presence of a decrease in CO 2 , alkalosis (high pH, low hydrogen ion concentration), and decrease in 2,3-BPG .
CO poisoning causes stabilization of the R state, a leftward shift in O 2 saturation curve, and a saturation curve for hemoglobin that resembles the curve for myoglobin.
Carbon monoxide (CO) binds to hemoglobin and forms carboxyhemoglobin, which has a high affinity for CO and displaces O 2 . This leads to stabilization of the R state, a leftward shift of the oxygen saturation curve, and an oxygen saturation curve for hemoglobin that resembles the curve for myoglobin.
Fetal hemoglobin is α 2 γ 2 . Sickle cell disease (SSD) is α 2 β S 2 .
Hemoglobin Degradation
When red cells reach the end of their life due to aging or defects, they are broken down in the spleen. The hemoglobin molecule is broken up, and the iron gets recycled. This process also produces one molecule of carbon monoxide for every molecule of heme degraded.
Heme degradation is one of the few natural sources of carbon monoxide in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air. The other major final product of heme degradation is bilirubin. Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage. Iron is removed from heme and salvaged for later use, it is stored as hemosiderin or ferritin in tissues and transported in plasma by beta globulins as transferrins. When the porphyrin ring is broken up, the fragments are normally secreted as a yellow pigment called bilirubin, which is secreted into the intestines as bile. Intestines metabolise bilirubin into urobilinogen. Urobilinogen leaves the body in faeces, in a pigment called stercobilin. Globulin is metabolised into amino acids that are then released into circulation.
-
Oxidation of the heme moiety of Hgb generates biliverdin, which is metabolized into unconjugated bilirubin, and then bound to albumin.
-
There are 3 steps in bilirubin metabolism in the liver:
-
Uptake: The unconjugated bilirubin-albumin complex reaches the hepatocyte; bilirubin dissociates from albumin and then enters the hepatocyte.
-
Conjugation: Unconjugated bilirubin and glucuronic acid combine to make conjugated bilirubin.
-
Excretion: The hepatocyte excretes conjugated bilirubin into the bile.
-
The rate-limiting step of bilirubin metabolism in the liver
-
If excretion is impaired, conjugated bilirubin enters the hepatic sinusoids and then into the bloodstream.
-
-
-
Conjugated bilirubin in the bile is transported through the biliary ducts into the duodenum; it is not reabsorbed by the intestine.
-
Can be excreted unchanged in the stool
-
Can be converted to urobilinogen by colonic bacteria
-
Urobilinogen can be reabsorbed, entering the portal circulation.
-
Some is taken up by the liver and re-excreted into the bile.
-
Some bypasses the liver and is excreted by the kidney, thus appearing in the urine in small amounts.
-
Can be converted in the bowel to stercobilin rendering the stool brown.
-
-
-
Unconjugated bilirubin is not found in the urine because it is bound to albumin and cannot be filtered by the glomeruli.
-
Conjugated bilirubin is filtered and excreted in the urine when there is conjugated hyperbilirubinemia.
Blood cells include: (1) erythrocytes (RBCs), (2) leukocytes (WBCs); and (3) platelets. About 45% of the total blood volume is made up of blood cells.