Hemoglobin Chemistry - SRM Medical College Hospital PDF

Hemoglobin chemistry DEPARTMENT OF BIOCHEMISTRY SRM MEDICAL COLLEGE HOSPITAL & RC COMPETENCY BC 5.9 & 6.12 Describe the major types of Hemoglobin and its types, derivatives & variants found in the body and their physiological / pathological relevance.  SPECIFIC LEARNING OBJECTIVE A...

Hemoglobin chemistry DEPARTMENT OF BIOCHEMISTRY SRM MEDICAL COLLEGE HOSPITAL & RC COMPETENCY BC 5.9 & 6.12 Describe the major types of Hemoglobin and its types, derivatives & variants found in the body and their physiological / pathological relevance.  SPECIFIC LEARNING OBJECTIVE At the end of the session the I MBBS students should be able to correctly describe: Structure of Hemoglobin and structure function relationship.  Transport of oxygen and carbon dioxide  Oxygen binding, Oxygen dissociation curve, bohr effect Chloride shift  CONTENT Deoxy Hb & oxy Hb Cooperative binding of O2 to Hb, Bohr effect Chloride shift  CASE SCENARIO During winter early morning, an elderly couple was taken to the emergency department. They were in confused state. Previous night theelderly couple kept the coal furnace in the bedroom throughout the night. Next day morning had complaints of severe headache, confusion, fatigue, nausea. Physical examination: lips appeared red. What is the most likely cause?  Hemoglobin structure Hemoprotein ( complex protein globin + prosthetic group) Quaternary structure : 4 subunits Prosthetic group – Heme 4 polypeptide chains 4 molecules of heme 4 ferrous (Fe2+) ions  Erythrocytes Normal Hb level – 14-16 g/dl - Males 13-15 g/dl – Females The adult hemoglobin (HbA) has 2 α chains & 2 β chains Mol.wt of HbA is 67,000 daltons Function i)Transport oxygen from lungs to tissues and carbon dioxide from tissues to the lungs ii)Intracellular buffer involved in acid base balance  Structure of Heme Iron containing compound Protoporphyrin Four pyrrole rings linked by methene (= CH) bridge Tetrapyrrole ring (porphyrin) Methyl – 4 Vinyl – 2 Propionate – 2  Structure of Heme Fe2+ ion has 6 valencies: 4 linked with N of pyrrole ring, 5th – imidazole N of proximal histidine Oxy Hb – 6th binds to O2 In deoxy Hb – water molecule is present between Fe2+ and distal histidine.  Structure of Globin Four polypeptide chains -Two alpha (α) chains – 141 AA -Two beta β/ γ/ δ/ ε – 146 AA Eight right-handed helices (A to H) One heme molecule for one polypeptide chain  Structure of Globin Hydrophilic amino acids: outer surface – solubility Hydrophobic amino acids: inner surface (heme binds)  Structure of Globin α 1 – β1 & α 2 – β2 interact by hydrophobic, hydrogen bonds and salt bridges. α 1 - α 2 & β1 – β2 are only polar interactions  Structural organization  Primary structure – α globin chain -141 aa, β,γ,δ -146 aa  Secondary structure – α – 7 helices, β,γ,δ – 8 helices  Tertiary structure – globin chain is looped about itself to form a pocket for heme binding.  Quaternary structure – Hb is tetramer, 2α 2 β –subunits with 4 heme  Binding sites for O2, CO2, H+ O2 is bound to ferrous atoms of the heme to form oxyHb. H+ is bound to R group of histidine residues in alpha & beta chains. CO2 is bound to N-terminal end of each polypeptide chains of Hb to form carbamino Hb.  Transport of O2 Hb is a soluble protein Transport large quantities of oxygen Can take up and release oxygen at appropriate partial pressures. Powerful buffer.  Binding of O2 Formation of oxyHb: O2 binds to α chain because heme pockets are more readily accessible. β chains are blocked by valine residue. Conformational changes in tertiary and quaternary structure of Hb. 1.Widening of heme pockets 2. Movement of subunits 3. Movement of iron atom T state  R state Binding of O2 to heme changes the structure of Hb  H bon d Salt bon d Protective hydrophob ic pocket  Movement of Fe atom Deoxy Hb : Fe atom is 0.06 nm beyond the plane of the heme ring Oxy Hb: Fe atom moves into the plane of the Co-operative binding of O2 to Hb Co-operative binding of O2 to Hb Binding of O2 to heme will increase binding of O2 to the other heme. Last O2 binds with affinity 100 times greater than the 1st O2. Positive operativity – Homotropic interaction. Lungs: O2 bins to Hb Tissues: O2 is release to tissues.  O2 Dissociation curve The binding ability of Hb with O2 at different partial pressures of oxygen (pO2) can be measured by a graphic representation known as ODC. Ability of Hb to unload and load O2 at physiological Po2. Hb is oxygenated. Graphical presentation: X axis – plotting partial pressure of O2; Y axis – Hb saturation with O2.  Sigmoid shaped curve The steep slope of the oxygen-dissociation curve over the range of oxygen concentrations that occur between the lungs and the tissues permits hemoglobin to carry and deliver oxygen efficiently from sites of high to sites of low pO2. LUNGS – High PO2 – loading of O2 TISSUES – Low PO2 – unloading of O2  Sigmoid curve explains how Hb transports O2 from the lungs to tissues  In pulmonary alveoli – Hb is 97% saturated with O2 – High O2 partial pressure in lungs.  In tissue capillaries – pO2 is 40 mmHg, the Hb is about 60% saturated. Physiologically 40% of O2 is released. TISSUES – O2 is liberated from Hb LUNGS – O2 is taken by Hb.  Factors affecting Oxygen Dissociation curve C - CARBONDIOXIDE A - ACIDITY (Increased H+ concentration) B – 2,3 Bisphosphoglycerate E – EXERCISE T – TEMPERATURE ODC shifts to RIGHT SALT BRIDGES Salt bridges ↓ Restrict movement of subunits ↓ Heme pocket not accessible for O2 ↓ Decreased affinity for O2 2,3-BPG formation in Glycolysis  Effect of 2,3 BPG Binds to Hb and stabilizes it by formation of salt bridges (negative cooperativity) ↓ Oxygen released to tissues (2,3 BPG reduces the affinity of Hb for O2) ↓ Reduced affinity allows O2 efficiently at partial pressure found in tissues by shifting ODC curve to right  2,3 BPG Significance: Level of RBC 2,3 – BPG are related to tissue demands of O2 supply. ↑ 2,3 BPG - Chronic hypoxia – High altitude, COPD ↑ 2,3 BPG – Severe anemia  Blood transfusion – blood stored in acid citrate-dextrose – decreases 2,3 BPG in RBC  Fetal Hb – binding ability of BPG to HbF is low. Facilitates trans placental O2 transfer.  Bohr’s effect Influence of pH & pCO2 to facilitate oxygenation of Hb in lungs and deoxygenation at the tissues is known as Bohr effect. Bohr effect – changes in H+ and CO2 conc promotes release of O2 in tissues. EFFECT OF CO2 & pH  Effect of H+/ pCO2 Metabolically active tissues ↓ Increased CO2 production Increased H+ concentration (increased carbonic anhydrase activity) ↓ stabilizes deoxy Hb (low affinity / T form) by formation of salt bridges O2 delivered to metabolically active tissues Chloride Shift/ Hamburger effect Venous side of the RBCs are bulged ? Transport of CO2  DISSOLVED FORM 5-10 % CARBAMINO Hb 15% ISOHYDRIC TRANSPORT 75-80 %  Summary Hb – non-protein: Heme – proporphyrin –Fe2+ Protein: 4 globin chains – α –141 aa β – 146aa O2 binding – T and R forms of Hb. widening of heme pockets movement of subunits movement of Fe atoms. 2,3 – BPG Bohr’s effect, chloride shift Met Hb – fe 3+  ODC: binding ability of Hb at different partical pressures of pO2 can be measured by graphic representation. Ability of Hb to load or unload O2 at physiological pO2. Bohr effect:influence of pH and pCO2 to facilitate oxygenation of Hb in lungs and unloading of O2 at the tissues.  Short notes 1. Illustrate and explain the structure of hemoglobin and mention its functions. 2. Illustrate and explain the Bohr Effect and Chloride shift. 3. Explain the structure and function of hemoglobin with a neat diagram. 4. Write briefly about the following: a) Cooperative binding of O2 with Hb and formation of oxyHb (4 marks) b) Role of 2,3 BPG (3 marks) c) Effect of pH and CO2 on binding of oxygen to Hb. (3 marks)  POST TEST 1. The four globin chains of HbA1 is? (A) a. 2 alpha 2 beta b. 2 alpha 2 gamma c. 2 alpha 2 epsilon d. 2 alpha 2 delta 2. An increased affinity of hemoglobin for O2 may result from which of the following? (E) a. Acidosis b. Higher 2,3-bisphosphoglycerate (BPG) levels within erythrocytes c. High CO2 levels d. More ferric ion e. Initial binding of O2 to one of the four sites available in each deoxy hemoglobin molecule  MCQ 3. 2,3-BPG binds Hb by salt bonds by cross linking between? [A] a.β1 β2 b.α1α2 c.α1 β1 d.Α2β2 4. Hemoglobin is a: [D] a.Monomeric protein b.Dimeric protein c.Trimeric protein d.Tetrameric protein 5. ODC shift is right occurs in all, except: [B] a.Increased CO2 b.Decreased H+ c.Anemia d.Exercise THANK U