Myoglobin & Hemoglobin - 2023-2024 PDF

THE UNIVERSITY OF ZAMBIA SCHOOL OF MEDICINE DEPARTMENT OF PHYSIOLOGICAL SCIENCES PGY 3419 (MEDICAL BIOCHEMISTRY & GENETICS) 2023/2024 Hemoglobin and Myoglobin Dr. Lubinda Mukololo Email: [email protected] Objectives 1. Protein structure 2. Protein func...

THE UNIVERSITY OF ZAMBIA SCHOOL OF MEDICINE DEPARTMENT OF PHYSIOLOGICAL SCIENCES PGY 3419 (MEDICAL BIOCHEMISTRY & GENETICS) 2023/2024 Hemoglobin and Myoglobin Dr. Lubinda Mukololo Email: [email protected] Objectives 1. Protein structure 2. Protein function 3. Genetic defects/ diseases 2 Medical/ Biomedical Importance Hemoglobin & Supply O2 Myoglobin Transports CO2 and H+ from tissues Hemoglobin to lungs Hemoglobin and Vulnerable to CO and Cyanide Cytochrome Oxidase poisoning, respectively. Molecular basis of genetic diseases Hemoglobin such as sickle cell and Thalasemias Provides valuable information for Hemoglobin management of diabetes mellitus. 3  Oxygen is required for Cellular Respiration But its poor solubility in aqueous solutions presents a challenge. Hemoglobin (Hb)- red blood cells; Myoglobin (Mb)- muscle cells. 4  Oxygen is bound to heme Hb & Mb contain heme, a complex of ferrous iron (Fe2+) and protoporphyrin IX 5  Structure of heme Porphyrins are cyclic compounds formed by the linkage of four pyrrole rings through -HC= methenyl bridges A characteristic property of the porphyrins is the formation of complexes with metal ions bound to the nitrogen atoms of the pyrrole rings. Porphyrin Heme 6 Structure of heme cont. 7  Structure and function of Myoglobin Globular protein made up of single polypeptide chain of 153 a.a. residues. Folded into 8 𝛼-helices. Helical segments named A to H. Amino acid residues designated either by position in a.a sequence or location within sequence of particular 𝛼-helical segments. Functions as a reservoir of oxygen in muscles. 8  Structure and function of Myoglobin cont. His residue coordinated to the heme in myoglobin is His93 (also called F8). Bends in structure are designated AB, CD, EF, FG, etc. 𝛼-helical regions are terminated by presence of Proline or by β-bends and loops. 9  Structure and function of Myoglobin cont. Heme sits in crevice lined with nonpolar a.a, except His residues. Proximal His (F8) binds Fe2+ Distal His (E7) stabilizes binding of O2 to Fe2+ 10  Structure and function of Hemoglobin Composed of four polypeptide chains- two 𝛼 chains and two β chains, held together by non covalent interactions. Each subunit has stretches of 𝛼-helical structures and a heme-binding pocket similar to that of Myoglobin. Besides O2, hemoglobin binds H+ and CO2 O2-binding properties are regulated by interactions with allosteric effectors 11  Structure and function of Hemoglobin cont. i. T form: Deoxy form is T (tense). In T form two 𝛼β dimers interact via ionic and H-bonds; movement of polypeptide chains constrained. Low affinity for O2 12  Structure and function of Hemoglobin cont. ii. R form: ionic and H-bonds between 𝛼β dimers ruptured upon O2 binding Relaxes (R) the structure Polypeptide chains have freedom of movement High O2-affinity 13  Binding of O2 to Myoglobin & Hemoglobin Myoglobin can bind only one molecule of O2; hyperbolic; Hemoglobin binds four molecules; Sigmoidal. 14 Affinity for O2 by Myoglobin & Hemoglobin Myoglobin has higher affinity for O2 than does Hemoglobin 15  Affinity for O2 by Myoglobin & Hemoglobin Apoproteins Provide a Hindered Environment for Heme Iron 16 Effect of CO on Hemoglobin Effect of carbon monoxide on the oxygen affinity of hemoglobin. CO-Hb = carbon monoxyhemoglobin. 17  Binding of O2 to Hemoglobin Hemoglobin can bind four molecules of O2; Sigmoidal Subunits cooperate in binding O2 Binding of an oxygen molecule at one heme group increases the oxygen affinity of the remaining heme groups in the same haemoglobin molecule. 18  Allosteric effects Ability of hemoglobin to reversibly bind O2 is affected by allosteric effectors (pO2, pH, pCO2, and 2,3-Bisphosphoglycerate). Binding of allosteric effectors on one site of hemoglobin affects binding of O2 to heme groups at other locations on the molecule. Binding of O2 to myoglobin is not influenced by allosteric effectors. 19  Allosteric effects cont. Heme-heme interactions: The cooperative binding of O2 allows hemoglobin to deliver more O2 to the tissues in response to relatively small changes in pO2. 20  Allosteric effects cont. pH 21  Allosteric effects cont. pCO2 About 15% of CO2 is carried as carbamate bound to the N-terminal amino groups of hemoglobin; forming carbaminohemoglobin. The binding of CO2 stabilizes the T state, resulting in a decrease in its affinity for oxygen. In the lungs, CO2 dissociates from the hemoglobin, and is released in the breath. The rest of the CO2 produced in metabolism is hydrated and transported as bicarbonate ions. 22  Allosteric effects cont. The Bohr effect Deoxyhemoglobin binds one proton for every two O2 molecules released. Lower pH of peripheral tissues, stabilizes the T state and thus enhances the delivery of O2. In the lungs, the process reverses. As O2 binds to deoxyhemoglobin, protons are released and combine with bicarbonate to form carbonic acid. Dehydration of H2CO3, catalyzed by carbonic anhydrase, forms CO2, which is exhaled. This reciprocal coupling of proton and O2 binding is termed the Bohr effect. The Bohr effect is dependent upon cooperative interactions between the hemes of the hemoglobin tetramer. 23  Allosteric effects cont. Effect of 2,3-Bisphosphoglycerate (2,3-BPG) on hemoglobin affinity for oxygen 2,3-BPG is the most abundant organic phosphate in RBC. Binds deoxyhemoglobin but not oxyhemoglobin. This stabilizes the T conformation and decreases oxygen affinity of hemoglobin. 24 Adaptation to high altitude, hypoxia, or anaemia In the above conditions, 2,3- BPG concentration in RBC increases. This lowers oxygen affinity of hemoglobin but promotes oxygen unloading in tissues. 25 Developmental changes in hemoglobin Normal adult human hemoglobins 26  Minor Hemoglobins Fetal hemoglobin (Hb F) Two 𝛄 chains replace the β-chains. 𝛄 chains lack some positively charged a.a found in β- chains, hence only weakly binds 2,3-BPG Therefore Hb F has high affinity for O2 than does Hb A. This facilitates transfer of O2 from maternal circulation across placenta to fetal RBCs. 27  Minor Hemoglobins Hemoglobin A (Hb A ) 2 2 Hb A2 is a minor component of normal adult hemoglobin. Appears shortly before birth; constitutes about 2% of the total hemoglobin. Composed of two α-globin chains and two δ-globin chains (α2δ2) Has no physiological importance. Used as a tool to diagnose the Beta-Thalassemia trait. 28  Minor Hemoglobins Hemoglobin A1c (HbA1c) Under physiological conditions, Hb A gets glycosylated, nonenzymatically. The extent of glycosylation depends on the plasma concentration of a particular hexose. The abundant form of glycosylated hemoglobin is Hb A1c, with glucose residues attached predominantly to the NH2 groups of the N-terminal valines of the β-globin chains. Increased amounts of Hb A1c are found in RBCs of patients with diabetes mellitus. 29  Hemoglobinopathies Sickle cell anaemia symptoms include: episodes of pain, hyperbilirubinemia, renal dysfunction, increased susceptibility to infections etc. 30  Hemoglobinopathies Sickle cell anaemia How does Sickle Cell trait protect against malaria? 31 β-Thalassemia symptoms include: severe anaemia; regular transfusion of blood is required Precipitated 𝛼-chains cannot form stable tetramers, causing premature death of cells destined to become RBCs. 32 𝛼-Thalassemia If all four 𝛼-globin genes are defective, fetal death results. 𝛼- globin chains are required for synthesis of Hb F. 33 Summary 34  Sample question Which statement below describes the effect of increased pCO2 on the stability of ionic bonds that occur between the (𝛼β)1 and (𝛼β)2 dimers of hemoglobin and the overall fidelity of the hemoglobin molecule? a) Increased pCO2 will destabilize the ionic bonds between the dimers and increase the affinity of hemoglobin for oxygen b) Increased pCO2 will stabilize the ionic bonds between the dimers and reduce the affinity of hemoglobin for oxygen c) Increased pCO2 will disrupt the ionic bonds between the dimers and relax the hemoglobin structure, thereby increasing its affinity for oxygen d) Increased pCO2 will stabilize the ionic bonds between the dimers, contribute to the T-conformation of hemoglobin, and increase its affinity for oxygen 35 36