3. Hemoglobin
Page 2: What is Hemoglobin?
Overview
Definition: Hemoglobin is the red, oxygen-carrying pigment found in red blood cells (RBCs) of vertebrates.
Composition:
It comprises about 95% of the cytoplasmic content of RBCs.
Hemoglobin is a globular molecule consisting of four subunits.
Page 3: Function of Hemoglobin
Key Functions
Oxygen Transport: Transports oxygen from the lungs to tissues.
Carbon Dioxide Transport: Transports carbon dioxide from tissues back to lungs for exhalation.
Acid-Base Balance: Helps maintain acid-base balance by binding and releasing hydrogen ions.
Nitric Oxide Transport: Carries nitric oxide, which regulates vascular tone.
Page 4: Hemoglobin Structure
Structural Components
Subunits: Each subunit contains a heme moiety that is conjugated to a polypeptide.
Heme Structure: Heme is an iron-containing porphyrin derivative.
Page 5: Polypeptides of Hemoglobin
Globin Portion
Polypeptides: Collectively referred to as the globin portion of hemoglobin.
Helical Structure:
Each globin chain is organized into eight helices separated by seven nonhelical segments.
The helices are labeled A to H and contain amino acid sequence subgroup numberings.
Page 6: Heme Group and Iron Behavior
Iron Coordination
Each chain has a heme group situated between the E and F helices of the polypeptide chain.
Iron Positioning: The iron atom is bound by two histidine radicals, forming:
Proximal Histidine Bond within F8.
Associated with Distal Histidine Residue in E7.
Iron States: Remains in its divalent ferrous form (Fe2+) whether oxygenated or deoxygenated.
Page 7: Hemoglobin Types
Polypeptide Composition
Hemoglobin A (HbA): Consists of two alpha (α) chains and two beta (β) chains (designated as α2β2).
Other Forms: About 2.5% of hemoglobin comprises hemoglobin A2, which has δ chains (α2δ2).
Page 8: Glycated Hemoglobins
Clinical Relevance
Glycated Hemoglobins: Hemoglobin A derivatives include hemoglobin A1c (HbA1c).
Clinical Marker: Increased in patients with poorly controlled diabetes mellitus.
Mechanism: Glucose attaches to the terminal valine in each β chain.
Page 9: Binding Reactions of Hemoglobin
Oxygen Binding
Oxygen Reaction: O2 binds to Fe2+ in heme, forming oxyhemoglobin.
Reaction Notation: Expressed as Hb + O2 ↔ HbO2.
The iron stays in the ferrous state, so
that the reaction is oxygenation (not
oxidation).
Page 10: Oxyhemoglobin Formation
Reaction Characteristics
Representation: Hemoglobin can be depicted as Hb4 and reacts with four O2 molecules to form Hb4O8.
Reaction Time: This oxygenation reaction is very rapid, taking less than 0.01 seconds.
Page 11: Hemoglobin Quaternary Structure
Structural Changes
Oxygen Affinity: Affected by the quaternary structure.
In deoxyhemoglobin, globin units are in a tense (T) configuration, reducing O2 affinity.
Binding of O2 shifts to a relaxed (R) configuration, exposing more binding sites.
Page 12: Factors Influencing Affinity
Affinity Modifiers
Influencing Factors: Affinity of hemoglobin for O2 is altered by:
pH, Temperature, 2,3-bisphosphoglycerate (2,3-BPG).
Competition: 2,3-BPG and H+ compete with O2 for deoxygenated hemoglobin's binding, reducing oxygen affinity.
Page 13: Carbon Dioxide Interaction
Physiologic Ligands
Carbaminohemoglobin: Formation upon CO2 binding to hemoglobin.
Binding Site: CO2 combines with globin portion, not heme, aiding to transport CO2 from tissues to lungs.
Page 14: Methemoglobin Formation
Oxidative Stress
Methemoglobin Formation: Generated by conversion of Fe2+ to Fe3+ via drugs/oxidative agents.
Consequences: Methemoglobin cannot carry oxygen because ferric iron cannot bind it, leading to decreased oxygen delivery and skin discoloration resembling cyanosis.
Page 15: Regulation of Methemoglobin Levels
Enzymatic Conversion
NADH-Methemoglobin Reductase: Enzyme system that converts methemoglobin back to hemoglobin in red cells.
Congenital Absence: Hereditary methemoglobinemia can occur due to the absence of this system.
Page 16: Sulfhemoglobin Characteristics
Formation and Impact
Sulfhemoglobin: Formed from irreversible oxidation of hemoglobin by specific drugs or sulfur chemicals.
Characteristics: Has a greenish pigment (cyanosis) and is ineffective for oxygen transport; persists in cells for their lifespan.
Treatment: by avoidance of offending agent
Page 17: Carbon Monoxide Effects
Toxic Interaction
Carboxyhemoglobin: Formed when carbon monoxide binds to hemoglobin.
Affinity Comparison: Hemoglobin's affinity for carbon monoxide drastically reduces its oxygen-carrying capacity.
Page 18: Fetal Hemoglobin
Structure and Functionality
Fetal Hemoglobin (HbF): Similar to hemoglobin A, but β chains are replaced by γ chains (designated α2γ2).
Replacement: Fetal hemoglobin is gradually replaced by adult hemoglobin after birth.
Page 19: Persistence of Fetal Hemoglobin
Clinical Aspects
Clinical Significance: In some individuals, HbF persists throughout life, enabling better oxygen content at given PO2 due to lower affinity for 2,3-BPG.
Oxygen Movement: Critical for O2 transfer from maternal to fetal circulation, especially during increased oxygen demand stages.
Page 20: Formation of Hemoglobin
Synthesis Process
Origin: Begins in polychromatophil erythroblasts and continues through reticulocytes.
Hemoglobin Formation: Reticulocytes continue to synthesize hemoglobin for a short period post bone marrow exit.
Page 21: Initial Steps in Hemoglobin Synthesis
Building Blocks
Succinyl-CoA and Glycine: Combine to form pyrrole, a precursor in hemoglobin synthesis.
Protoporphyrin IX: Composed of four pyrrole molecules, combines with iron to form heme.
Page 22: Hemoglobin Chain Formation
Final Assembly
Combination: Each heme molecule binds to a polypeptide chain (globin), forming hemoglobin chains.
Molecular Weight: Each chain weighs about 16,000; four chains assemble to create a complete hemoglobin molecule.
Page 23: Variability in Hemoglobin Chains
Types of Chains
Types: Hemoglobin chains vary based on amino acid sequences (α, β, γ, δ chains).
Common Form: Hemoglobin A, with a molecular weight of 64,458, consists of two α and two β chains, allowing it to bind four oxygen molecules.
Page 24: Abnormal Hemoglobin Characteristics
Changes in Affinity
Binding Affinity: Altered by variations in hemoglobin chains; abnormalities can affect physical properties.
Sickle Cell Anemia: Specific mutations (valine for glutamic acid) lead to abnormal crystal formations under low oxygen conditions, causing cell fragility.
Page 25: Functionality of Hemoglobin
Oxygen Binding Dynamics
Reversible Binding: Major function involves reversible binding with oxygen, facilitating O2 uptake in lungs and release in tissues, where the gaseous tension of oxygen is much lower than in the lungs.
Page 26: Nature of Oxygen Binding
Coordination with Iron
Binding Mechanics: Oxygen binds loosely via coordination bonds rather than ionic bonds, ensuring reversible release in tissues as molecular oxygen.
Page 27: Importance of Iron
Iron in Metabolism
Role in Body: Vital for hemoglobin, myoglobin, cytochromes, and more; total body iron averages about 4 to 5 grams with 65% as hemoglobin.
Distribution:
4% in myoglobin,
1% in other heme compounds,
0.1% in transferrin,
15-30% stored mainly as ferritin.
Page 28: Iron Transport and Storage
Iron Absorption
Transferrin Formation: Upon intestinal absorption, iron binds to apotransferrin in plasma to transport in the body.
Iron Release: Iron is released to tissues where necessary; excess is stored mainly in liver and reticuloendothelial cells.
Page 29: Iron Compartments in the Body
Iron Content Breakdown
Iron Distribution:
Functional Iron (Hemoglobin): 68% (2.4g)
Myoglobin Iron: 10% (0.36g)
Enzymatic Iron (peroxidase, etc.): 3% (0.12g)
Storage Iron (ferritin/hemosiderin): 18% (0.67g)
Transport Iron (transferrin): <1% (0.001g)
Page 30: Ferritin Storage
Ferritin and Hemosiderin
Ferritin: Main storage form of iron, can vary in iron content depending on body levels.
Hemosiderin: Forms when iron levels exceed storage capacity, can be seen microscopically as large accumulations.
Page 31: Differences Between Ferritin and Hemosiderin
Storage Forms
Ferritin: Small, dispersed particles; typical visible only under an electron microscope.
Hemosiderin: Larger, insoluble clusters collected in cells when excess iron exists.
Page 32: Iron Regulation and Transport
Cytoplasmic Iron Utilization
Iron Mobilization: When plasma iron decreases, iron from ferritin is released and transported via transferrin to erythroblasts for hemoglobin synthesis.
Consequences: Insufficient transferrin can lead to hypochromic anemia (low hemoglobin content in RBCs).
Page 34: RBC Lifespan and Iron Recycling
Hemoglobin Breakdown
Lifecycle of RBCs: Approximately 120 days; macrophages digest released hemoglobin to reclaim iron for new RBCs or storage.
Bilirubin Formation: The hemoglobin's heme is converted to bilirubin for excretion.
Page 35: Daily Iron Loss
Iron Excretion
Average Loss:
Adult men: approximately 0.6 mg daily
Adult women: around 1.3 mg/day (due to menstruation).
Page 36: Intestinal Iron Absorption
Absorption Pathway
Apotransferrin: Combines with free iron in the duodenum to create transferrin for iron absorption through intestinal epithelial cells.
Page 37: Absorption Efficiency
Rate of Absorption
Iron Absorption: Typically slow at a maximum of a few mg/day due to the low absorption rate, regardless of dietary iron quantity.
Page 38: Regulation of Iron Absorption
Iron Homeostasis
Saturation and Regulation: Increased iron exposure reduces absorption rates, while depleted stores can accelerate absorption dramatically.
Page 39: Hepcidin's Role
Hormonal Regulation
Hepcidin: Polypeptide secreted by the liver that regulates iron absorption by altering ferroportin channels.
Feedback Mechanism: Production negative feedback loop adjusts based on body's iron needs.
Page 40: Iron Homeostasis Pathway
Detailed Regulation Diagram
Role of Hepcidin: Demonstrates modulation of iron transport processes in response to body iron status.
Page 41: Food Iron Types
Iron Sources
Heme Iron: Found in animal products.
Nonheme Iron: Comes from plant sources.
Factors Affecting Absorption: The body's state of iron saturation impacts uptake mechanisms.
Page 42: Hemoglobin Destruction Overview
Hemoglobin Breakdown Phases
Phagocytosis: Released hemoglobin from ruptured RBCs is quickly ingested by macrophages.
Iron Recycling: Iron enters plasma through transferrin for new RBC production or storage.
Page 43: Intravascular Hemolysis
Destruction in Circulation
Causes of Rupture: Aging of RBCs leading to membrane rigidity; defects can lead to hemolysis.
Page 44: Hb-Haptoglobin Complex
Transferring Breakdown Products
Haptoglobin's Role: Binds released hemoglobin in plasma for transport to the liver for degradation.
Page 45: Heme Decomposition
Product Conversion
Conversion Process: Heme is cleaved by heme oxygenase to release carbon monoxide, iron, and form biliverdin.
Bilirubin Formation: Biliverdin is further processed to bilirubin for excretion.
Page 46: Extravascular Hemolysis
Macrophage Function
Ingestion of Old RBCs: Macrophages phagocytize aged RBCs; two major changes affect susceptibility to destruction.
Page 47: Decreased Deformability
Mechanical Fragility
Increased Rigidity: Conditions like spherocytosis or Hb-C disease lead to fragile RBCs more prone to lysis.
Page 48: Surface Property Alteration
Immunologic Impacts
Cell Membrane Changes: Binding of antibodies leads to oxidative changes on RBC membranes, making them targets for macrophage ingestion.
Page 49: Bilirubin Excretion
Conversion in Gut
Bilirubin Processing: In the intestine, bilirubin turns into urobilinogen. Some may be reabsorbed and eventually excreted as stercobilin in feces, while a fraction appears in urine.
Page 50: Erythrocyte Recycling Overview
Lifespan and Biochemical Fate
Cycle Duration: Erythrocytes remain in circulation for approximately 120 days.
Component Recycling: Successful phagocytosis leads to recycling or waste excretion of hemoglobin components.
Page 51: Understanding Anemia
Definition and Causes
Anemia: Defined as a deficiency in hemoglobin levels which can arise from inadequate RBC count or insufficient hemoglobin within cells.
Page 52: Blood Loss Anemia
Mechanisms
Post-Hemorrhage: Fluid replacement occurs rapidly, but RBC concentration takes weeks to normalize. Chronic blood loss leads to microcytic hypochromic anemia due to iron absorption deficits.
Page 53: Aplastic Anemia
Bone Marrow Dysfunction
Definition: Aplasia of bone marrow leads to insufficient erythropoiesis. Can result from radiation, toxic chemicals, or autoimmune disorders.
Treatment: Supportive care includes transfusions and bone marrow transplant.
Page 54: Megaloblastic Anemia
Nutritional Deficiencies
Causes: Results from deficiencies of vitamin B12, folic acid, or intrinsic factor impacting erythoblast reproduction, leading to oversized RBCs (megaloblasts).
Page 55: Additional Insights on Megaloblastic Anemia
Proliferation Issues
Intestinal Absorption: Conditions like intestinal sprue lead to poor nutrient absorption, resulting in weakness and fragility of megaloblasts.
Page 56: Hemolytic Anemia
RBC Fragility
Cell Dynamics: Hereditary and acquired defects create fragile RBCs prone to destruction, causing increased destruction rates that surpass erythropoiesis.
Page 57: Sickle Cell Anemia
Genetic Mutation Effects
Hemoglobin S Presence: Results in abnormal long crystalline formations under hypoxic conditions, causing RBC sickling and fragility.
Page 58: Sickle Cell Crisis
Pathophysiological Cycle
Crisis Mechanism: Low oxygen levels initiate sickling, leading to rapid RBC destruction and further hypoxia, creating a vicious cycle that can be fatal.
Page 59: Erythroblastosis Fetalis
Hemolytic Consequence
Maternal Antibody Impact: Rh-negative mothers produce antibodies against Rh-positive fetal RBCs, leading to anemia requiring rapid erythroblast production.
Page 60: Secondary Polycythemia
Oxygen-Related Adaptation
Condition Overview: Occurs with chronic hypoxia (e.g., high altitudes); compensatory increase in RBC production results in elevated RBC counts.
Page 61: Polycythemia Vera
Pathological Condition
RBC Overproduction: Genetic aberrations cause continuous excess production of RBCs leading to elevated RBC counts and hematocrit.
Page 62: Vascular Effects of Polycythemia Vera
Hemodynamic Changes
Blood Volume and Viscosity: Polycythemia vera can double blood volume and drastically increase blood viscosity, causing vascular complications.
Page 63: Erythropoietin Feedback Mechanism
Hormonal Regulation of Production
Oxygen Sensor Role: Changes in erythropoietin secretion adjust RBC production according to oxygen delivery needs; elevated erythrocyte mass reduces erythropoietin production.