01 Hb Structure.pdf

Field of Medicine Medicine Program Hemoglobin Structure & Function Prof. George N.B. Morcos Date: 30 / 09 / 2024 Objectives At the end of this lecture, the students will be able to: 1. Outline the structure of hemoglobin. 2. Describe the different types of normal hemoglobin. 3. Recognize the fu...

Field of Medicine Medicine Program Hemoglobin Structure & Function Prof. George N.B. Morcos Date: 30 / 09 / 2024 Objectives At the end of this lecture, the students will be able to: 1. Outline the structure of hemoglobin. 2. Describe the different types of normal hemoglobin. 3. Recognize the function of hemoglobin as oxygen transporter. 4. List the different factors affecting hemoglobin function. 5. Recognize the difference between fetal and adult hemoglobin regarding oxygen carrier.  Hemoglobin structure Hemoglobin is a globular protein (hemoprotein) found in the cytoplasm of erythrocytes. Its primary function is the transport of O2 and CO2 between the lungs and the tissues. Normal values: In fetus - just before birth = 16.5 to 18.5 gm/dl. At the end of 1 year = 12.5 gm/dl. Adult male = 14-18 gm/dl. Adult female = 12-15.5 gm/dl.  Hemoglobin structure 1. Hemoglobin has different variants (forms) but they all share the same basic structure. 4 Heme group + 4 globin polypeptide chains (1 heme/ 1 chain) Heme (also called Iron protoporphyrin) is formed of a cyclic tetrapyrrole ring (A, B, C, D) “porphin ring”. D It contains 1 ferrous iron (Fe2+) → bound to the 4 nitrogen of tetrapyrrole ring by 4 coordination bonds. C B Iron in heme can form up to 6 coordination bonds: 4 bonds with the tetrapyrrole ring (A, B, C, D) A 1 with the imidazole ring of histidine amino acid of the polypeptide chain pyrrole ring 1 with the oxygen (it is unoccupied in absence of O2)  Hemoglobin structure 4 Heme groups + 4 globin polypeptide chains The α -chain contains 141 amino acids and β-chain (δ or  chains) contains 146 amino acids. The secondary structure of all globin chain types is formed of 8 α helices identified by the letters A to H. The tertiary folding of each globin chain forms an Globin chain approximate sphere.  Hemoglobin structure Each globin subunit contains a heme-binding pocket. In this pocket, two important histidine residues (His) have a major role in hemoglobin function: The proximal histidine residue in the F helix of the globin chain [His F8] that binds to the 5th coordination site of Fe2+ The distal histidine in the E helix [His E7] that stabilizes oxygen binding.  Hemoglobin structure Role of the globin chain: The globin surrounds the heme. This helps as follows: 1. Globin provides proximal histidine that binds to Fe2+ and the distal histidine that stabilizes oxygen binding. 2. The heme alone interacts with oxygen so that the Fe2+ becomes oxidized to Fe3+ and no longer binds oxygen. The presence of the globin chain helps in reversible oxygen binding to Hb.  Hemoglobin structure The quaternary structure of hemoglobin consists of four subunits tetramer (α2β2 in Hb A). The hemoglobin tetramer is composed of two identical dimers (αβ)1 and (αβ)2. The two chains within each dimer are held together tightly by the ionic bonds and the hydrophobic Hb Globin chains interactions, which prevent their movement relative to each other. However, the two dimers are linked with each other by weaker hydrogen and ionic bonds, so that movement at the interface of these two dimers occurs more freely during oxygenation and deoxygenation. So, 2 forms of Hb can be recognized.  Hemoglobin Function Hb is an important buffer in the erythrocytes and blood. Hb is a carrier of O2 and CO2 Hb binds O2 weakly at low oxygen pressures and tightly at high pressures. The binding of the first O2 to Hb enhances the binding of further O2 molecules. Process of O2 binding to Hb 1st. Hb must be able to bind oxygen in the lungs. 2nd. Hb must be able to release oxygen in the tissues. In these two conditions, Hb exists in 2 different forms: T-form (T = tense) and R-form (R= Relaxed).  Hemoglobin Function Difference between T and R forms of Hb deoxy- & oxyhemoglobin ▪ Following oxygenation, a considerable structural conformational change occurs in Hb. ▪ Oxygenation rotates the 11 dimer in relation to 22 dimer about 15°. ▪ Hb changes from the T → R states. T-state (Taut) R-state (relax) Deoxyhemoglobin Oxyhemoglobin  Hemoglobin Function Differences in the conformation of T and R states help in oxygen transport. The Fe is about 0.6 Å out of the heme plane in the deoxy state. When oxygen binds, it pulls the Fe back into the heme plane. Since the proximal histidine is attached to the Fe2+, this pulls the complete F helix. Oxygenation of one hemoglobin molecule produces rupture of some week noncovalent hydrogen and Oxy Hb DeoOxy Hb ionic bonds and rotation of one dimer ()1 15 degrees relative to the other dimer ()2.  Hemoglobin Function  Hemoglobin Function ✓ Factors affecting Hemoglobin function: Factors affecting the ability of Hb to bind and transport oxygen include: 1) Oxygen 2) 2,3-bisphosphoglycerate (BPG). 3) pH [H+ and CO2] and 4) Carbon monoxide (CO)  Hemoglobin Function ✓ Factors affecting Hemoglobin function: 1) Oxygen The oxygen dissociation curve for hemoglobin is sigmoidal in shape, indicating that the subunits cooperate in binding oxygen. This effect is referred to as heme-heme interaction.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: Although it is more difficult for the first oxygen molecule to bind to hemoglobin, the subsequent binding of oxygen occurs with high affinity, as shown by the steep upward curve in the region near 20 to 30 mmHg. The net effect is that the affinity of hemoglobin for the last oxygen bound is approximately 300 times greater than its affinity for the first oxygen bound. The cooperative binding of oxygen enhances the efficiency of hemoglobin as an oxygen transporter.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: 2) 2,3-bisphosphoglycerate (BPG) is a three-carbon molecule formed during glycolysis [Breakdown of glucose]. - It is formed in human red blood cells. - 2,3 BPG binds with greater affinity to deoxygenated hemoglobin decreasing their affinity for oxygen. This enhances the ability of RBCs to release oxygen near the tissues.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: Hemoglobin in blood is bound to BPG Interaction is electrostatic, between negative charges on BPG and positive side chains of basic amino acid of globin chain (e.g., histidine, Lysine, Arginine) If Hb doesn’t bind to BPG, it remains saturated with O2. BPG binds specifically to the deoxy state and stabilizes it in the T state. In the R state, the central cavity is too narrow for BPG to fit or bind.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: 3) Effect of pH: - High pH= low [H+] or alkaline; this promotes tighter binding of oxygen to hemoglobin. - Low pH= high [H+] or acidic; this permits the easier release of oxygen from hemoglobin. Hb O2 + H + Hb H+ + O 2 The effect of pH on the oxygen binding ability to Hb is called the Bohr effect.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: 3) Effect of pH: The effect of pH on the oxygen binding ability to Hb is called the Bohr effect. The Bohr effect is the reciprocal coupling of protons and O2 binding to Hb. Also, it explains why red blood cells unload oxygen in tissues. More oxygen is released in those tissues that have higher CO2 values. As the pH decreases (H+ increases), the oxygen binding decreases. The opposite effect occurs when the pH increases.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: As oxygen is consumed → CO2 is released (in tissues). CO2 promotes the release of O2 from oxyHb. Carbonic anhydrase catalyzes this reaction in red blood cells. carbonic anhydrase CO 2 + H 2 O H2 CO 3 H2 CO 3 H+ + HCO 3 - The H+ generated from this reaction is taken up by Hb and causes it to release more oxygen. This proton uptake facilitates the transport of CO2 by stimulating bicarbonate formation.  Hemoglobin Function Summary of Bohr Effect: 1. In peripheral tissues, Hb affinity for oxygen is decreased in the presence of carbon dioxide and at lower pH. 2. Carbon dioxide reacts with water to give carbonic acid which dissociate into bicarbonate & free protons via the reaction: CO2 + H2O ---> H2CO3 ---> H+ + HCO3- 3. In addition, hemoglobin transports CO2 from peripheral tissues to the lungs. Hemoglobin carries CO2 as carbamates formed with the amino terminal of the globin chains.  Hemoglobin Function Summary of Bohr Effect: 4. Conversely, in the lung the CO2 levels decrease as it is continuously exhaled, this increases the oxygen affinity of Hb.  Hemoglobin Function ✓ Factors affecting Hemoglobin function: 4) Carbon monoxide CO CO binds tightly to the hemoglobin iron, forming carboxyhemoglobin (HbCO). When CO binds to one or more of the four heme sites, Hb shifts to the relaxed conformation, which causes the remaining heme sites to bind oxygen with high affinity. As a result, the affected hemoglobin is unable to release oxygen to the tissues. Since, the affinity of hemoglobin for CO is about 200 times greater than for oxygen, as a result even minute concentrations of carbon monoxide in the environment can produce toxic concentrations of HbCO in the blood. Carbon monoxide poisoning is treated with 100 percent oxygen therapy, which facilitates the dissociation of CO from hemoglobin.  Hemoglobin Function Tissues Lung 1.High CO2 Low CO2 in lungs 2.Higher H+ Lower H+ 3.Lower pH Higher pH 4.Affinity for O2 decreases Affinity for O2 increases 5.O2 released to tissues O2 binds hemoglobin 6.T state favored R state favored  Fetal Hemoglobin as O2 carrier Fetal hemoglobin has a different subunit (than the β subunit of the adult hemoglobin) called a γ subunit or α2γ2. The γ chain differs from the chain β in that, there is a change in a single amino acid found in the 2,3-BPG 'binding pocket': from Histidine to serine. 2,3 BPG binds less well to HbF than to HbA. This gives HbF a higher affinity for O2 than HbA. In Fetal hemoglobin, BPG does not affect O2 binding and the baby’s blood will get its oxygen from the mother’s hemoglobin. The transfer of oxygen is from the mother (less tightly bonded) to the baby (more tightly bonded).  Hemoglobin Derivatives Oxyhemoglobin (oxyHb) = Hb with O2 Deoxyhemoglobin (deoxyHb) = Hb without O2 Carbaminohemoglobin (HbCO2) - CO2 is non-covalently bound to globin chain of Hb. HbCO2 transports CO2 in blood. Carboxyhemoglobin (HbCO) – carbon monoxide (CO) binds to Fe2+ in heme in case of CO poisoning or smoking. CO has more than 200x higher affinity to Fe2+ than O2.  SUMMARY -Hemoglobin Structure. -Hemoglobin Structure. -Different factors affecting hemoglobin function. -Fetal hemoglobin (Hb F).  References – Lippincott's Illustrated Reviews: Biochemistry, 5th Edition. Chapter 3. pp 25-35 – https://www.youtube.com/watch?v=jVUwn4wWTXI – https://www.youtube.com/watch?v=Qv-KExGKAYw&t=183s