BMS 531.09 Oxygen Transport and Hemoglobin PDF

Understanding Oxygen: Oxygen Transport and Oxygen Binding Proteins BM S 5 3 1. 09 S P R ING 2 0 2 5 BLO C K 2 L EC T UR E 2  Objectives 1. Evaluate the role of oxygen and oxygen-binding 3. Compare and contrast oxygen and carbon dioxide proteins in normal metabolism...

Understanding Oxygen: Oxygen Transport and Oxygen Binding Proteins BM S 5 3 1. 09 S P R ING 2 0 2 5 BLO C K 2 L EC T UR E 2  Objectives 1. Evaluate the role of oxygen and oxygen-binding 3. Compare and contrast oxygen and carbon dioxide proteins in normal metabolism and disease by: transport within erythrocytes a. Describe the synthesis of heme, structure of heme, the a. Explain the role of bicarbonate in carbon dioxide role of iron in heme, and the importance of heme to regulation myoglobin and hemoglobin b. Summarize the roles of the lungs, kidney, tissues, and b. Compare and contrast the structure, behavior, erythrocytes in CO2 regulation and acid-base balance localization, and uses of myoglobin compared to hemoglobin and determine functional consequences for 4. Explain the importance of the pentose phosphate changes in form or structure c. Explain pulse oximetry and its importance to evaluating pathway to erythrocyte function and oxidative normal and abnormal oxygen transport phosphorylation by d. Explain allosteric regulation of oxygen binding to a. Explain the role of oxidation-reduction reactions in the hemoglobin and identify the consequences to changes in pentose phosphate pathway pH (Bohr Effect), CO2 levels, and 2,3-BPG levels b. Compare the reversible and irreversible steps of the pentose phosphate pathway 2. Evaluate erythrocyte structure, function, and c. Determine the consequence of loss of function or gain of metabolism* function changes in the pentose phosphate pathway a. List the key differences between an erythrocyte and other human cells in terms of cellular structure and contents 5. Compare and contrast the classification, b. Summarize the difference between glucose metabolism in biochemical changes, and clinical consequences for erythrocytes compared to other cell types sickle cell disease and hemaglobinopathies LO1 Oxygen Basics Required for oxidation reactions including ◦ Wide range of metabolic processes ◦ detoxification reactions ◦ ATP generation Main role = electron acceptor Can accept single electrons ◦ Oxygen Radical/Reactive Oxygen Species (more on this next lecture) ◦ Biradical molecule with 2 unpaired electrons in different orbitals ◦ Highly reactive Most of the oxidases, peroxidases, and oxygenases bind O2 and transfer single electrons to it via a metal LO2 Oxygen Transport is via mature Erythrocytes (RBCs) Structurally and metabolically the simplest cell in body Represent 40-45% of blood volume and the majority of formed elements (~90%) in blood Oxygen transport and delivery NOT utilization LO1a Oxygen Binding: A Home for Heme Heme = Iron containing prosthetic group ◦ Porphyrin ring with 4 iron molecules ◦ Iron ions are held in place by nitrogen Oxygen binds to the iron within the heme Heme plays a role in a wide range of processes: ◦ OXYGEN TRANSPORT ◦ Respiration ◦ Sensing of diatomic gases ◦ Detoxification reactions ◦ Signal transduction ◦ Molecular genetics processes including transcription, translation, and processing of microRNAs ◦ Protein stability and import into mitochondria ◦ Differentiation 85% of the heme in human body is located as hemoglobin making erythrocytes a major source of heme production though all cells can produce it (liver = second major source) LO1a Heme Synthesis Heme = porphyrin that is synthesized from glycine and succinyl-coenzyme A forming 5- aminolevulinate (5-ALA) ◦ Enzyme = 5-ALA synthase 4 PBG molecules combine to form linear tetrapyrrole which cyclizes to form ring Decarboxylation and oxidation occur in the mitochondria Addition of iron is the final step for production of heme LO1b Oxygen Binding Proteins: Hemoproteins Myoglobin Hemoglobin ◦ Compact globular protein that binds one oxygen ◦ Tetramer of 4 globin subunits that binds four oxygen ◦ Found in tissues; tissue storage form ◦ Oxygen transport protein for blood without which ◦ Acts as an oxygen storage protein oxygen transport is significantly reduced ◦ High affinity for O2 not dependent on O2 ◦ 3 types: concentration ◦ A: most common; 2 alpha and 2 beta ◦ During exercise, O2 is released and diffuses into ◦ A2: rare; two alpha and two delta mitochondria ◦ F: found in infants; 2 alpha and 2 gamma ◦ Having this tissue-based O2 source is critical to rapid activity during high energy needs of the tissue LO1d Oxygen Transport and Positive Cooperativity Although oxygen binding to hemoglobin follows classic cooperativity binding, the tetramer does NOT have to maintain symmetry over the course of O2 binding and release Ackers and Holt 2006 LO1d Allosteric Regulation of Hemoglobin Oxygen binding affinity is regulated by small molecules Allosteric effectors: H+, CO2, or 2,3- bisphosphoglycerate (2,3-BPG) ◦ Increases in these causes decreased affinity for O2 ◦ Effects of these effectors are additive LO1d Bohr Effect Changes in pH affect oxygen affinity Enables off-loading of O2 in tissues ◦ Hydrogen ions combined with CO2 LO1c Pulse Oximetry Visible and infrared spectra of oxygenated and deoxygenated hemoglobin differ Pulse oximeters evaluate oxygen saturation and enable monitoring of cardiopulmonary status Has predictive clinical value as well as practical applications that result from the potential for at home monitoring (i.e. COVID-19 patient monitoring for hospitalization need) LO3 CO2 and Hydrogen Ion The interconnection between lungs, kidneys, and erythrocytes lies in the acid-base balance Lung = gas exchange Plasma = gas transport = bicarbonate for CO2 Hemoglobin = buffer for H+ from carbonic acid Erythrocyte = O2 and CO2 transport Kidneys = reabsorb filtered bicarbonate and generate new bicarbonate LO3 Carbon Dioxide in RBCs RBCs must carry both O2 (to tissues) and CO2 (away from tissues) While O2 binding to heme is direct, most CO2 must be processed ◦ Formation of bicarbonate To expel the CO2 in the lungs, the process must be reversed LO2 More on Erythrocytes During maturation from reticulocytes, RBCs lose all subcellular organelles ◦ Denucleated thus NO DNA or RNA ◦ No ribosomes or ER so no PROTEIN synthesis or secretion ◦ No mitochondria so no oxidation of fats; Rely exclusively on blood glucose as metabolic fuel ◦ Highest specific rate of glucose use of any cell (10g per kg of tissue per day vs. 2.5 g of glucose per kg of tissue per day for entire remainder of body) ◦ 90% of glucose is metabolized to lactate Metabolism of Glucose is entirely ANAEROBIC LO2, LO4 Anerobic Metabolism in RBCs Glucose is broken down via a series of reactions to generate 2 pyruvate molecules** In RBCs, pyruvate is reduced to lactic acid via anaerobic glycolysis ◦ Generates ATP for use in maintaining electrochemical and ion gradients of the RBC plasma membrane ◦ 10-20% of a glycolysis intermediate (1,3- bisphosphoglycerate) is diverted to generate an allosteric regulator of O2 affinity for hemoglobin (2,3-bisphosphoglycerate) ◦ 10% of glucose metabolism is diverted into the pentose phosphate pathway ◦ Protects from oxidative stress and a source of NADPH **more on this in Glycolysis lecture LO2, LO4 Pentose Phosphate Pathway Irreversible Redox Stage ◦ Generates NADPH and pentose phosphates Reversible Interconversion Stage ◦ Excess pentose phosphates are recycled into glycolytic intermediates LO5 Oxygen Deficiency and Hemoglobinopathies Oxygen deficiency can be linked to mutations in the genes that encode hemoglobin Hemoglobinopathies are classified by the most prominent change in the protein structure, function, or regulation Example: Sickle Cell Disease ◦ Point mutation in gene encoding beta- hemoglobin changing the tertiary structure (valine in place of glutamic acid) ◦ More hydrophobic surface when deoxygenated that forms long polymers that precipitate and distort the shape of erythrocytes (sickled-shape) LO5 Importance of Primary Structure to MBG Callback Function Primary Secondary Quaternary Red Blood Cell and Tertiary Function Structure Structure Shape Structures 1 Val Normal Normal Proteins do not associate; Normal red 2 His β subunit hemoglobin each carries oxygen. blood cells are full of 3 Leu β individual Normal 4 Thr hemoglobin 5 Pro α proteins. 6 Glu 7 Glu 5 µm β α 1 Val Sickle-cell Sickle-cell Hydrophobic interactions Fibers of β subunit hemoglobin between proteins abnormal 2 His lead to aggregation; hemoglobin Sickle-cell 3 Leu oxygen deform red β 4 Thr carrying blood cell 5 Pro α capacity into sickle 6 Val reduced. shape. 7 Glu 5 µm β α © 2016 PEARSON EDUCATION, INC.  Classification Common Mutation Frequency Biochemical changes Clinical consequences LO5 name 6(β) Abnormal HbC Glu →Lys Common Intracellular crystallization of Mild hemolytic anemia; solubility oxygenated protein; increased splenomegaly (enlarged spleen) erythrocyte fragility Hemaglobin- opathies 94(α) Decreased Hb Asp →Asn Very rare Heterodimer interface altered Mild cyanosis (blue-purple skin O 2 affinity Titusville to stabilize T state; decreased coloration from deoxygenated cooperativity blood) 82(β) Increased Hb Helsinki Lys →Met Very rare Reduced binding of 2,3-BPG in Mild polycythemia (increased O 2 affinity T state erythrocyte count) 58(α) Ferric heme HbM His →Tyr Occasional Altered heme pocket Cyanosis of skin and mucous (methemoglob Boston (mutation of distal His) membranes; decreased Bohr in) effect Unstable Hb Gun Hill Δβ91–95 Very rare Misfolding caused by loss of Formation of Heinz bodies protein Leu in heme pocket and (inclusions of denatured Hb); shorter helix jaundice (yellow coloration of integument and sclera); pigmented urine 142(α) Abnormal Hb Tyr →Gln Very rare Loss of termination codon; α-Thalassemia (hemolytic synthesis Constant decreased mRNA stability anemia, splenomegaly and Spring jaundice)