Iron Absorption and Metabolism

Iron is an essential element for almost all forms of life, but most important as an oxygen transporter. When iron is in its ferrous state, oxygen binds to it within the hemoglobin molecule allowing erythrocytes to circulate and deliver oxygen to all the human bodies cells and tissues. The human body also requires iron in order to obtain ATP from cellular respiration (Oxidative phosphorylation). Although iron is an essential element to the body, like anything in nature, too much of it can be toxic. Its ability to donate and accept electrons readily means that it can spontaneously catalyze the conversion of hydrogen peroxide into free radicals. Free radicals cause a wide array of damage to cellular structures and tissues. To minimize the chances of toxicity, almost every iron atom is bound to protein structures, an example being hemoglobin. The iron is bound to the globin protein. To learn more about the structure of hemoglobin, review the previous article written. There is tight regulation of iron metabolism that allows the body to remain in homeostasis. Understanding iron metabolism is important for understanding multiple diseases of iron overload, and iron deficiency.

Iron Absorption

Most of the bodies iron comes from dietary uptake. There is continuous iron recycling occurring within the body from the sequela of hemoglobin metabolism by the spleen. The macrophages of the reticuloendothelial system store iron from the process of breaking down engulfed red blood cells. Its stored as hemosiderin. Hemosiderin is just a defined deposit of protein and iron that occurs as a result of iron overload, either systemically or locally. The metabolic functions of iron depend on the ability to change its valence state from reduced ferrous state (Fe2+) to the oxidized ferric state (Fe3+). Ferrous iron in the lumen of the duodenum is transported across the luminal side of the enterocyte by a protein called divalent metal transporter-1 (DMT1). Once iron has been absorbed across the cell membrane of the enterocyte, it can either be stored by binding to apoferritin or the cell can release the iron through the help of another transporter called ferroportin. Ferroportin is the only know protein that exports iron across cell membranes. One of the ways that the human body manages iron homeostasis is by the production of hepcidin. When iron stores are adequate, the liver will produce hepcidin, which competitively binds to ferroportin and inactivates it. When iron stores begin to drop, suppression of synthesis of hepcidin allows ferroportin to transport iron again. Before iron is taken by ferroportin across the membrane, it must be converted to its ferric form. Hephaestin, another protein on the enterocyte cell membrane oxidizes iron as it exits to its ferric form (Fe3+). Once oxidized and in its ferric state, the iron binds to apotransferrin (ApoTf). This iron:apotransferrin complex is known as transferrin (Tf). Its important to note that two molecules of ferric iron can bind to one molecule of apotransferrin.

Mechanism-of-intestinal-iron-absorption-at-low-and-high-serum-iron-levels

Iron Uptake into Cells

Individual cells regulate the amount of iron they absorb to avoid adverse toxicity. Cells possess a receptor for transferrin (Tf), called transferrin receptor-1 (TfR1). The physiological pH of the plasma and extracellular fluid allow for a strong affinity to transferrin for TfR1. Through receptor mediated endocytosis transferrin saturates the TfR1 and once a critical mass has accumulated, endocytosis begins. The iron is passed into the cell into an endosome vesicle. Hydrogen ions are then pumped into the endosome and as a result the pH drops causing dissociation of the iron from the transferrin. Almost simultaneously the affinity for TfR1 to apotransferrin increases so it remains bound to the receptor while the iron remains free. The iron is then exported from the endosome vesicle into the cytoplasm by divalent metal transporter 1 (DMT1). The molecules of iron are then either stored, or transported into the mitochondria where they are incorporated into cytochromes or heme for the production of hemoglobin. While the iron is transported in the cytoplasm, the endosome fuses again with the cell membrane and in the extracellular space pH, TfR1 has a low affinity for apotransferrin so it dissociates and begins circulating again in the plasma for free transferrin. Again transferrin being a diiron:apotransferrin complex. Cells are able to store iron so they have a reserve if needed. Ferric iron (Fe3+) is stored in a protein called apoferritin. When iron binds to it it known as ferritin. Ferritin can be used at anytime during iron depletion by lysosomal degradation of the protein.

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Regulation

Just like hepcidin, there are other ways that the body maintains iron homeostasis. Transcription of TfR1 on the surface of the cells can either decrease or increase depending on iron stores within the cell. When iron stores are sufficient, production of TFR1 decreases, and vice versa. This is also useful in diagnosis of iron deficiency. Turns out there is a truncated form of TfR1 that circulates in serum as soluble transferrin receptors (sTfR). These sTfRs reflect the amount of tFR1 in the body. So in iron depletion there will be more circulating sTfRs indicating more production of TfR1 on the cells surface. A useful tool in the diagnosis of iron deficiency anemia.

Iron Recycling

When cells die, they are sequestered by the spleen and liver in which mechanisms salvage iron. These mechanisms are often referred to as the haptoglobin-hemopexin-methemalbumin system. Free hemoglobin in the plasma is quickly complexed with haptoglobin. By binding haptoglobin, the hemoglobin, and consequently, the iron avoid filtration by the glomerulus in the kidneys. This complex is taken up by macrophages, primarily those in the liver, spleen, bone marrow and even in the lungs. These macrophages express CD163, which is the haptoglobin scavenger receptor. The entire complex is internalized into the macrophage within a lysosome. Inside this lysosome, the iron is salvaged, the globin is catabolized as any protein would be, and the protoporphyrin is converted to unconjugated bilirubin. To learn more about the process of bilirubin metabolism, review the previous article. The haptoglobin is also degraded by the lysosome. The iron in free hemoglobin becomes oxidized to its ferric state (Fe3+), and as a result, forms methemoglobin. The heme (metheme) molecule of the free hemoglobin binds to hemopexin, preventing oxidative injury to the cells and tissues, as well as prevents loss of iron through glomerulus filtration. Albumin acts as a carrier for many proteins, including metheme. So albumin acts as a carrier for metheme to find hemopexin, which has a much higher affinity for the metheme itself. This allows for more rapid degradation of the toxic metheme.

There was a lot to learn in this article. Read carefully and go back and refer. I will try to highlight certain areas that I think are more important to the bigger picture. The next step is what happens in certain physiological disease states that leads to either iron overload or iron deficiency.

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Thalassemias

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The major hemoglobin that is present in adults is hemoglobin A (HbA). This is a heterotetramer that consists of one pair of alpha-globin chains and one pair of beta-globin chains. Alpha-globin chains are encoded by two copies of the alpha gene present on chromosome 16. Beta-globin chains are driven by one gene on chromosome 11. Normally there is tight regulation of the production of alpha and beta-globin chains and the ratio of production, but sometimes that regulation can be interrupted. This offsets the balance of globin chains being produced. These types of hematological disorders are coined thalassemias. They are a quantitative defect characterized by reduced or absent production of one and rarely two of the globin chains.

Alpha thalassemia is largely due to the inadequate production of alpha globin chains, which leads to an excessive production of either gamma-globin chains or beta-globin chains. In the fetus alpha thalassemia leads to excess gamma chains and in adults it largely leads to excess beta chains. In neonates the absence of alpha-globin chains is incompatible with life, leading to hydrops fetalis or hemoglobin Barts and absolute death after delivery. Hb Barts cannot deliver the oxygen to the tissues because its affinity to oxygen is too high. The hydronic state is reflected in the fetus by heart failure and massive total body edema. Excess beta-globin chains are capable of forming homotetramers and precipitate that leads to a variety of clinical manifestations.

Beta thalassemia is an inherited hemoglobinopathy in which production of beta-globin chains is impaired. There are different classifications corresponding to the degree of reduction in the beta chains. Beta thalassemia major is due to mutations that completely stop all production of beta-globin chains. These are individuals who are homozygous for the disease. They lose the ability to make HbA and because of this will experience severe manifestations and are transfusion-dependent for the rest of life. Symptoms typically begin during late infancy (6-12 months), but some newborns are asymptomatic because the major hemoglobin in newborns is HbF (4A:4G) which is constructed by gamma-globin chains and not beta-globin chains. Beta-thalassemia major presents with pallor, jaundice, and bilirubin in the urine which indicates hemolysis. Hepatosplenomegaly is present as well as heart failure. Failure to thrive and recurrent infections are also other signs. There is so much hemolysis because of the faulty hemoglobin present in the red cells that the bone marrow can’t keep up with production so extra medullary hematopoiesis occurs that results in skeletal abnormalities in the face and long bones. Iron overload is often a symptom of late untreated disease which can affect almost every organ in the body. Mortality is upwards of 85% by age five if untreated. If treated the survival rate is only 60 years of age if lucky.

Beta thalassemia major is also called transfusion-dependent beta thalassemia. There is also a subtype called non-transfusion-dependent beta thalassemia otherwise known as beta thalassemia intermedia. These individuals present with a less severe phenotype of the disease. There is significant variability with the clinical findings in individuals with beta thalassemia intermedia; from osteoporosis to thrombosis to diabetes mellitus. Some individuals will develop hepatosplenomegaly and extramedullary hematopoiesis and some won’t. Also some individuals will have to become transfusion-dependent, but that is typically in the late decades of life.

Anemia is a severe clinical manifestation of both alpha and beta thalassemia. The pathophysiology of beta thalassemia causes excess alpha-globin chains to precipitate in the developing erythrocytes in the bone marrow. This causes inclusion bodies. The inclusion bodies create oxidative stress and damages the cellular membranes. Apoptosis gets activated downstream and the red cell precursors are subsequently phagocytized and destroyed in the bone marrow by activated macrophages. This is also called ineffective erythropoiesis. The bone marrow in an effort to compensate releases these red cell precursors into the peripheral blood riddled with these inclusion bodies. These cells are subsequently sequestered by extravascular hemolysis by the RES which  further contributes to the anemia. The red cells that survive are microcytic and hypochromic and have a significantly shortened life span. Severe tissue hypoxia is seen due to the increased HbF as a compensatory mechanism. HbF has an increased affinity for oxygen and causes a shift to the left on the oxygen dissociation curve.

Typical laboratory findings for an individual with beta thalassemia is a slightly decreased red cell count and a marked decrease in hemoglobin of usually about 2-3 g/dL (12.5-16.5 g/dL). There will be marked anisocytosis (microcytosis) and poikilocytosis, target cells, basophilic stippling, slight increase in reticulocytes and nucleated red cells.

The pathophysiology for anemia associated with alpha thalassemia is associated with precipitation of HbH. Remember HbH is formed when there is decreased production of the alpha-globin chains so there is an excess of beta-globin chains. The precipitation of HbH creates inclusion bodies, typically called Heinz Bodies. These inclusions are recognized by the RES and remove the red cells via extravascular hemolysis.

Laboratory findings for an individual with alpha thalassemia is very similar to that of an individual with beta thalassemia. Decreased hemoglobin, marked anisocytosis (microcytosis) and poikilocytosis, target cells, basophilic stippling, and reticulocytes and NRBCs. The HbH inclusions can be see seen using a cresyl blue stain.

Thalassemias are a quantitative hemoglobinopathy meaning that there is a deficiency or an excess of production of globin chains leading to clinical manifestations. They are inherited and some subtypes can significantly elevate mortality. It is important to diagnose early and to treat early.

 

 

 

Acid-Base Balance

An acid is any compound that can donate H+ when dissolved in water. A base is any compound that can donate OH- ions. A buffer system is a combination of a weak acid or base and its salt or conjugate that resists changes in pH. The human body has incredible mechanisms to maintain an acid-base balance. Changes in pH put the body in different physiological states that can cause an array of problems. Acidosis is when the pH falls below the reference range of 7.34. Alkalosis is when the pH increases above the reference range of 7.44.

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The most important buffer system in the body is the bicarbonate (HCO3)/carbonic acid (H2CO3) system. Carbonic acid works to allow the human body to rid of toxic CO2 via respiration to maintain a normal pH of 7.4. There normally is a 20:1 ratio of bicarbonate to carbonic acid.

The red cells pick up CO2 from tissues and throughout its travel through the blood vessel its converted to carbonic acid. That carbonic acid is then broken down into bicarbonate and hydrogen. The excess hydrogen ions are buffered by hemoglobin. Bicarbonate leaves the red cell and goes into circulation. Bicarbonate enters the plasma through an exchange mechanism with chloride to maintain a state of electroneutrality in the cell. When the red cells reach the lung the hemoglobin will release the excess hydrogen ions by the binding of oxygen to hemoglobin. The excess hydrogen ions bind to bicarbonate to form carbonic acid. Carbonic acid then dissociates into H20 and CO2 which is expelled.

As mentioned above, an individual can be in a state of acidosis or alkalosis. This can be caused by ventilation and is called respiratory acidosis or respiratory alkalosis or it can either be caused by HCO3-. This is called metabolic acidosis or alkalosis.

Respiratory acidosis is an increase in PCO2. Conversely respiratory alkalosis is a decrease in PCO2. Metabolic acidosis is a loss of HCO3- or an addition of H+. Metabolic alkalosis is a loss of H+ or an increase of HCO3-. The body will naturally compensate for the pH changes. Some of the compensatory mechanisms are increasing respiration in metabolic acidosis. Hyperventilation increases the amount of CO2 that is expelled and raising the pH. In respiratory acidosis the kidney will increase its reabsorption of HCO3-.

Metabolic acidosis can be caused by multiple different disease states. Excessive loss of HCO3- by diarrhea can cause metabolic acidosis. Diabetic ketoacidosis can cause it. Other causes are ingestion of acids or renal tubular failure where there is no renal reabsorption of HCO3-.

Metabolic alkalosis is caused by excess or an overdose of HCO3-. Excessive vomiting causes a loss of hydrochloric acid with the stomach contents. Vomiting also results in hypokalemia and hyponatremia which are both positively charged ions (acids) leading to an increase in the pH. Excessive diuretic use can sometimes initially cause an increase in chloride, but most commonly results in hyponatremia and causing a contractile alkalosis.

Respiratory acidosis is most commonly caused by CO2 retention usually due to ventilation failure. Decreased cardiac output and hypotension also cause acidosis. Less blood is pumped to the heart so less CO2 is getting transported to the lungs to be expelled. Chronic lung conditions such as COPD result in an inability to ventilate properly and to expel CO2. Certain drugs cause depression of the respiratory center in the brain and can cause respiratory acidosis. Some of these drugs are barbiturates, opiates and ethanol (alcohol).

Respiratory alkalosis is primarily caused by hyperventilation (increased alveolar ventilation). This results in a decreased arterial PCO2. Any condition which decreases pulmonary compliance causes a sensation of dyspnea. Dyspnea is not a single sensation and there are at least three distinct sensations including air hunger, work/effort, and chest tightness. These sensations cause a state of hypoxia which is caused by the hyperventilation.

-Caleb

Disseminated Intravascular Coagulation

Disseminated intravascular coagulation (DIC) is a generalized activation of homeostasis secondary to a systemic disease. There are multiple different diseases that can activate different homeostatic factors that can contribute to DIC. Conditions such as physical trauma and endothelial cell damage exposing tissue factor that finds its way into circulation; or being exposed to tissue factor through vasodilation from hypovolemic shock, malignant hypertension or even heat stroke. There is a long list of secondary conditions that can set off an array of events that can lead to DIC. DIC involves all aspects of homeostasis; the vascular intima, platelets, leukocytes, coagulation, coagulation regulation, and fibrinolysis. DIC is often called a consumptive coagulopathy as it is consuming platelets at a rapid rate that form fibrin microthrombin that partially occlude small vessels. These thrombi that form are small and ineffective so systemic hemorrhage occurs which is often the first sign of DIC or one of the first signs prevalent.

To understand the pathophysiology of DIC its important to have a handle on normal primary and secondary homeostasis in the body. Normal physiological coagulation initiation begins on tissue-bearing cells such as fibroblasts and the subendothelial cells. This is called the extrinsic tenase complex which is composed of tissue factor, factor VIIa, and calcium. When tissue factor that is released from damaged subendothelial cells comes into contact with coagulation factor VII it activates it and that complex produces factors Xa, and IX and miniscual amounts of thrombin. There is also a minute amount of factor VIIa that is circulating in the blood that is resistant to breakdown from tissue factor pathway inhibitor (TFPI) and can bind to tissue factor to start coagulation. The initiation phase and the small amount of thrombin produced starts the initial fibrin formation by splitting the fibrinogen peptides A and B from fibrinogen and activates platelets through cleavage of protease activated receptors PAR-1 and Par-4, cofactors, factor Va released from the platelet alpha granules, factor VIIIa to be released by vWF, and procoagulants such as factor IX to be used further in propagation.

Coagulation Cascade

Propagation is where more than 95% of the thrombin is generated and occurs on the surface of the platelets. Initiation attracts a copious amount of platelets to adhere to the site of the injury from both the low-level thrombin released and exposed collagen. These initial platelets are sometimes called COAT-platelets or platelets partially activated by collagen and thrombin. The COAT-platelets have a higher level of procoagulant activity than platelets activated by collagen alone. These platelets also provide a surface for the intrinsic and prothrombinase tenases to form. Factors Va and VIIIa that were activated by thrombin in initiation bind to platelet surfaces and become receptors for factors IXa and Xa. Factor IXa binds to VIIIa and forms in the intrinsic complex. The intrinsic complex then activates factor Xa, which binds to factor Va that forms the prothrombinase complex. The prothrombinase complex activates prothrombin which generates thrombin. Thrombin activates factor XIII to stabilize the fibrin clot by covalently cross-linking the fibrin polymers initiated by the extrinsic tenase, binds to its cofactor thrombomodulin to activate the protein C pathway and also activates thrombin activatable fibrinolysis inhibitor (TAFI) to inhibit fibrinolysis to protect the formation of the fibrin clot.

Platelets as mentioned above have an important role in homeostasis. Platelet activation occurs once the platelet binds to collagen or the vWF that is present on the surface of the damaged endothelial cells. Adhesion occurs through the integrin GP IX V platelet receptor. Upon binding they secrete their primary granules that secretes molecules such as ADP, epinephrine, serotonin, and calcium. Calcium and other molecules like ADP activate phospholipase A2 through GCPRs otherwise known as 7 transmembrane receptors. Thomboxane A2 (TXA2) is then synthesized by thromboxane synthase in multitude of events. TXA2 generates secondary messengers DAG and IP3. DAG helps mediate actin contraction for shape conformational changes and IP3 binds to the IP3 receptors in the dense tubular system that opens calcium channels to allow release of more calcium. The activation of DAG and IP3 induces a conformational change that activates the fibrinogen receptor GP IIb/IIIa which allows adjacent platelets to aggregate and form the initial platelet plug. Platelets are also important in that they allow a surface for propagation of coagulation to occur.

With a basic background of primary and secondary homeostasis it will now be easier to understand what actually occurs during DIC. Triggering events may activate coagulation at any point in its pathway. Circulating thrombin that is released activates platelets, activates coagulation proteins that have positive feedback loops within the coagulation cascade and catalyzes fibrin formation. The fibrinolytic system enzymes such as plasminogen and TPA may become active subsequent to fibrin clot formation. Monocytes may also be induced to released tissue factor caused by inflammation in DIC. Normally thrombin cleaves fibrinogen creating fibrin monomers which spontaneously polymerize to from this insoluble gel which is strengthened through factor XIII. In DIC, a high percentage of the fibrin monomers fail to polymerize and just circulate in plasma as soluble monomers. These circulating monomers coat platelets and coagulation proteins which doesn’t allow any binding creating an anticoagulant effect. Plasmin, which is the activated form of plasminogen is a part of the fibrinolytic system. In normal homeostasis plasmin only cleaves the solid fibrin clot formed. Although in DIC, plasmin circulates in the plasma and degrades all forms of fibrin. It is because of this that fibrin degradation products, otherwise known as D-dimers become detectable in the plasma in concentrations commonly exceeding 20,000 ng/mL. The normal range for the D-dimer is 0-240 ng/mL. At the same time coagulation pathway control is lost as protein C, protein S, and anti-thrombin are consumed by the plasmin. Plasmin also digests factors V, VIII, IX, and XI. The platelets become enmeshed within the fibrin monomers and become exposed to thrombin which triggers platelet further platelet activation and consumption. Plasmin can also trigger complement which causes hemolysis and the kinin system which triggers inflammation and hypotension and as an end result shock.

It is important to diagnose DIC early and as a physician be aware of the early signs. A lot of times the symptoms of DIC are masked by the underlying disease and may be chronic or acute. The initial laboratory testing includes a platelet count, blood film examination, PT, aPTT, D-dimer and fibrinogen assay. The PT time is usually >14 seconds (Ref. 11-14) aPTT is usually >35 seconds (Ref. 25-35). These time intervals clue in that there is a coagulation issue as the PT tests the function of the extrinsic pathway and the aPTT tests the function of the intrinsic pathway. The platelet count is lower than 150,000 uL (Ref. 150,000-450,000 uL). The D-dimer as noted previously is significantly elevated. Although a D-dimer alone can’t diagnose DIC because a D-dimer is elevated in other conditions such as inflammation, pulmonary embolism and deep vein thrombosis. The fibrinogen levels may drop below 220 mg/dL (Ref. 220-498), but that provides little diagnostic information because in a lot of cases the fibrinogen level may not rise or may become elevated because of the level of inflammation occurring while the patient is in DIC. A peripheral blood smear confirms thrombocytopenia as well as the presence of schistocytes. Schistocytes are broken red blood cells because the microvessel walls are occluded they get shredded while passing through the blood vessels. Although not one test result can rule in DIC or rule it out, a panel of specialized tests can help in the diagnosis. It’s important to get the whole picture.

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Treatment of chronic DIC is to diagnose and treat the underlying condition. This may include surgery, anti-inflammatory agents, or antibiotics to stabilize homeostasis. Supportive therapy to maintain fluid and electrolyte balance is important in the treatment of chronic DIC. In acute DIC where there is multi organ failure from microthrombi and hemorrhagic bleeding therapies are targeted at slowing the clotting process and to replace the consumed coagulation factors and proteins. Unfractionated heparin is commonly used for its anti-thrombotic properties. Normally the aPTT is used to monitor heparin therapy, but in the case of DIC other assays must be used so it’s important to pay close attention to the patient when administering heparin as it can aggravate bleeding tendencies. Physicians may also order fresh frozen plasma (FFP), platelets, and red cell transfusions as needed. FFP will replace the coagulation factors and proteins. The platelets will correct for the thrombocytopenia and the red cells are transfused because of the resulting anemia. Cryoprecipitate can also be administered to replace the low levels of fibrinogen. A physician may use an INR to figure out the best way to treat the DIC as well as monitor the therapy. The INR or international normalized ratio is a way of standardizing the PT results, regardless of test methods and where the testing occurred. A normal INR should be between 2-3. An INR too low puts the patient as risk for blood clots, on the contrary and INR too high puts the patient as too high of a risk for bleeding. As the underlying condition begins to stabilize the DIC will begin to subside and patient will slowly recover.

There will be more covered about DIC and how it relates to different leukemias and solid tumor cancers. This article provides an overview and how it affects normal homeostasis.

-Caleb

Hemoglobin, What is it?

Function

Hemoglobins main function is to carry oxygen to the tissues and cells of the body. Hemoglobin can bind and transport four molecules of oxygen. Hemoglobin is made up of 4 polypeptide globin chains, two alpha and two beta that forms a tetramer heme group with iron located within the tetramer. Globin is the protein part of hemoglobin. The different globin chains are typically 141-146 amino acids in length and each chain is designated by a greek letter. Each chain is subdivided into eight helices designated an alphabetical letter with each helice divided by seven non-helical segments. The globin chains loop to form a cleft pocket for heme which is suspended between the E and the F helices of the polypeptide chain. The ferrous iron in each heme molecule reversibly binds to one oxygen molecule.

Heme is a four ring consisting of carbon, hydrogen and nitrogen atoms called the protoporphyrin IX. The single carbon atoms act as connecting bridges. There are alternating double bonds where the electron resonation absorbs light which is the reason why heme is colored. There are two propionic acid side chains on end rendering it polar, with the rest of the heme molecule being non-polar and hydrophobic.

Globin Synthesis

Globin is a protein so naturally translation and synthesis occurs in the ribosomes while transcription occurs in the nucleus. Transcription of the alpha globin gene which occurs on chromosome 16 produces more mRNA than the beta globin genes which is transcribed on chromosome 11. There are four alpha genes, and only two beta genes. To make up for that discrepancy, translation of the alpha globin is less efficient than that of the beta globin so there are equal amounts of both produced in Hgb A.

Heme Synthesis

Heme is synthesized in the mitochondria and cytoplasm of the bone marrow erythrocyte precursor cells. Biosynthesis begins in the mitochondria with the condensation of glycine and succinyl CoA catalyzed by aminolevulinate synthase (ALA synthase) to form ALA in the cytoplasm. Porphobilinogen synthase converts ALA to porphobilinogen (PBG). Porphobilinogen synthase is the enzyme that is inhibited by lead. PBG is then converted to hydroxymethylbilane which is further converted to uroporphyrinogen III. Uroporphyrinogen III is converted to coproporphyrinogen III. Synthesis then continues back in the mitochondria by the conversion of coproporphyrinogen III to protoporphyrinogen IX. Protoporphyrinogen IX is then converted to protoporphyrin IX by protoporphyrinogen oxidase. From there protoporphyrin IX is converted to heme in the presence of ferrous iron and ferrochelatase. Heme has a negative feedback mechanism on ALA by inhibiting the transcription of the ALA synthase enzyme.

Hemoglobin Synthesis

With the synthesis of globin and heme covered the next step is the assembly of the hemoglobin molecule as one. After the globin is released by the ribosomes, each polypeptide chain binds to a single heme molecule. An alpha globin:heme complex and a beta globin:heme polypeptide then combine to form a heterodimer. Now remember that step is repeated as there are four polypeptide chains in a hemoglobin molecule. Two heterodimers then combine to form a tetramer to complete the assembly of the hemoglobin molecule.

hhemo_rib1

Oxygen Binding

When in the deoxygenated state, the iron within the heme molecule is pulled out of plane of the heme ring. When oxygen binds it pulls the iron back into the plane of the heme ring and also causes a shape change in the polypeptide chains which causes a ripple effect among all four polypeptide chains. This phenomenon is allosteric regulation. The allosteric effect is the conformational change in the entire hemoglobin molecule caused by the binding of one oxygen molecule to one ferrous iron molecule within the heme cleft. In the deoxygenated state ionic bridges form creating a stable and rigid configuration. A single molecule of 2,3-DPG binds adjacent polypeptides to further stabilize the hemoglobin molecule. In the oxygenated state there is that allosteric effect which alters the shape of the hemoglobin molecule enough so that ionic bridges are broken which causes the globin molecules to relax and heme cleft enlarges. This allows the remaining three oxygen molecules to bind readily to the ferrous iron.

Oxygen/Hemoglobin Dissociation Curve Effects

The oxyhemoglobin is formed when oxygen binds during physiological respiration within the pulmonary capillaries. Various factors such as pH, CO2 concentration, and 2,3-DPG concentration affect the way that oxygen binds. When talking in terms of the oxygen/hemoglobin dissociation curve it is generally said that curve is sigmoidal; meaning that there is low hemoglobin affinity for oxygen at low oxygen tension and high affinity for oxygen at high oxygen tension. There are values such as the P50 which is defined in terms of the amount of oxygen needed to saturate 50% of the hemoglobin molecule. The PO2 of the lungs are ~ 100 mm/Hg so that means that hemoglobin is 100% saturated when the RBC is in the lungs. Normally a P02 of 27 mm/Hg results in 50% hemoglobin saturation. You can also have shifts to the right or left of the curve. These shifts come from the various factors that were mentioned above. Hemoglobin exists as two forms, a taut (tense) phase and a (R) form or relaxed form. Low pH, high CO2 (such as in the tissues), and high 2,3-DPG initiate a shift to the right, meaning that there is less affinity for oxygen and the oxygen is released into the tissues resulting in the taut form of hemoglobin. Conversely a high pH, low CO2 (such as in the lung capillaries), and low 2,3-DPG results in the relaxed form and a shift to the left creating a higher affinity to oxygen. The partial pressure of the system also affects the affinity to oxygen. At high PO2 levels, such as those present in the lungs, a high affinity relaxed state is favored. In a low PO2 state, such as in the tissues, the low affinity, taut form is favored. Hemoglobin needs to be able to bind oxygen and release it. There is no point in the molecule binding if there is no chance at releasing it. The sigmoidal curve causes it to be efficient at taking up oxygen in the lungs and efficient at releasing it in the tissues

oxygen-dissociation curve

Hemoglobin is an essential aspect of homeostasis for the cells. It needs to functioning and be assembled correctly to efficiently do its job. This is just a brief overview of what it does and the structure of it. More will come on the different hemoglobinopathies (qualitative) and thalassemias (quanitative).

-Caleb