Overview
This lecture reviews Chapter 3 of Lippincott's Biochemistry, focusing on the structure, function, and clinical significance of globular proteins, especially heme proteins such as myoglobin and hemoglobin.
Heme Proteins
- Heme proteins are a type of globular protein that contain a heme group (an Fe²⁺ ion complexed with protoporphyrin IX).
- The iron (Fe²⁺) in the heme group forms six bonds:
- Four with nitrogen atoms in the porphyrin ring.
- One with a histidine residue (an amino acid in the protein).
- One available for binding oxygen (O₂) or other ligands.
- The role of the heme group varies depending on the protein and its environment.
Myoglobin
- Myoglobin is a simple, single-chain (monomeric) globular protein found in heart and skeletal muscle.
- Functions mainly as an oxygen reservoir and, to a lesser extent, as an oxygen carrier.
- Structure:
- Composed of a single polypeptide with alpha-helical content.
- Contains a hydrophobic crevice lined with nonpolar amino acids, which holds the heme group in place.
- The outer surface is lined with polar amino acids, making myoglobin soluble in water.
- Oxygen affinity:
- Myoglobin has a very high affinity for oxygen.
- It becomes saturated at low partial pressures of oxygen and only releases oxygen when levels are extremely low.
- This makes it ideal for oxygen storage rather than transport.
Hemoglobin Structure & Function
- Hemoglobin is a more complex globular protein found in red blood cells.
- Structure:
- Tetramer: composed of four polypeptide chains (2 alpha and 2 beta chains).
- Arranged as two identical dimers (αβ₁ and αβ₂).
- Each subunit contains a heme group, so each hemoglobin molecule can bind up to four oxygen molecules.
- Functional states:
- T (taut) state: deoxygenated form, low affinity for oxygen.
- R (relaxed) state: oxygenated form, high affinity for oxygen.
- Hemoglobin shifts between these states depending on oxygen binding.
- Additional functions:
- Hemoglobin can also transport hydrogen ions (H⁺) and carbon dioxide (CO₂).
Oxygen Dissociation Curves
- Myoglobin:
- Exhibits a hyperbolic oxygen dissociation curve.
- Becomes saturated with oxygen at low partial pressures and releases oxygen only at very low levels.
- Hemoglobin:
- Shows a sigmoidal (S-shaped) oxygen dissociation curve.
- This shape reflects cooperative binding: as each oxygen molecule binds, hemoglobin’s affinity for oxygen increases.
- At high partial pressures (e.g., in the lungs), hemoglobin binds oxygen tightly; at lower pressures (e.g., in tissues), it releases oxygen more readily.
- Key point: The sigmoidal curve allows hemoglobin to efficiently pick up oxygen in the lungs and release it in tissues.
Allosteric Effectors & the Bohr Effect
- Allosteric effectors influence hemoglobin’s oxygen affinity:
- Hydrogen ions (H⁺): Increased acidity (lower pH) decreases O₂ affinity.
- Carbon dioxide (CO₂): Increased CO₂ decreases O₂ affinity.
- 2,3-Bisphosphoglycerate (2,3-BPG): A byproduct of glycolysis in red blood cells; stabilizes the T state, lowering O₂ affinity.
- Bohr effect:
- Describes how increased H⁺ and CO₂ shift the oxygen dissociation curve to the right (decreased O₂ affinity), promoting oxygen release in metabolically active tissues.
- 2,3-BPG:
- Levels increase in conditions like hypoxia or anemia, enhancing oxygen release.
- Stored blood loses 2,3-BPG over time, causing transfused red blood cells to hold onto oxygen more tightly; this can be corrected with a rejuvenation solution.
- CO₂ transport:
- Most CO₂ is carried as bicarbonate (HCO₃⁻) and H⁺ in the blood.
- Some CO₂ binds directly to hemoglobin as carbaminohemoglobin, further stabilizing the T state and promoting O₂ release.
Special Conditions & Minor Hemoglobins
- Carbon monoxide (CO) poisoning:
- CO binds to heme with much higher affinity than O₂, occupying binding sites and reducing O₂ carrying capacity.
- CO binding also increases the affinity of remaining sites for O₂, making it harder for hemoglobin to release oxygen to tissues.
- Types of hemoglobin:
- Hemoglobin A (HbA, α₂β₂): Main adult form (~90%).
- Hemoglobin A₂ (HbA₂, α₂δ₂): Minor adult form.
- Hemoglobin F (HbF, α₂γ₂): Fetal hemoglobin; higher O₂ affinity than adult hemoglobin, allowing the fetus to extract O₂ from maternal blood. HbF is replaced by HbA after birth.
- Hemoglobin A1c (HbA1c): Glycated form of HbA; levels increase in chronic high blood sugar (e.g., diabetes).
Hemoglobin Genetics & Disorders
- Genetics:
- Alpha globin genes: Located on chromosome 16; two genes per chromosome (total of four).
- Beta globin gene: Located on chromosome 11; one gene per chromosome (total of two).
- Genes are transcribed to mRNA, spliced to remove introns, and translated into polypeptides.
- Sickle cell anemia (HbS):
- Caused by a single nucleotide substitution in the β-globin gene (Glu → Val).
- Results in hemoglobin that forms sickle-shaped red blood cells.
- Symptoms: pain, anemia, increased risk of infection, and reduced red blood cell lifespan (<20 days).
- Heterozygotes (carriers) are protected against malaria; homozygotes have full disease.
- Hemoglobin C disease:
- Caused by Glu → Lys substitution in β-globin.
- Usually results in mild, chronic hemolytic anemia; no severe crises.
- Hemoglobin SC disease:
- One sickle cell gene and one hemoglobin C gene; variable clinical severity, less frequent crises than sickle cell anemia.
- Methemoglobinemia:
- Fe²⁺ in heme is oxidized to Fe³⁺ (ferric state), which cannot bind oxygen.
- Causes cyanosis (brownish blood); can be congenital or drug-induced.
- Treatment: methylene blue (converts Fe³⁺ back to Fe²⁺).
- Thalassemias:
- Beta thalassemia: Reduced β-globin production; increased α-globin, more HbA₂ (α₂δ₂) and HbF (α₂γ₂) produced. Symptoms appear after birth; regular transfusions may be needed.
- Alpha thalassemia: Reduced α-globin production; more severe because α-chains are needed for all hemoglobins. Severity depends on how many of the four α-globin genes are affected:
- 1 gene: silent carrier
- 2 genes: trait carrier
- 3 genes: variable severity
- 4 genes: fatal (no functional hemoglobin)
Key Terms & Definitions
- Heme: Iron-containing prosthetic group in myoglobin and hemoglobin.
- Myoglobin: Muscle oxygen storage protein; single polypeptide chain.
- Hemoglobin: Blood oxygen transport protein; tetramer (α₂β₂).
- T state (taut): Deoxyhemoglobin; low oxygen affinity.
- R state (relaxed): Oxyhemoglobin; high oxygen affinity.
- 2,3-BPG: Glycolysis byproduct that decreases hemoglobin’s oxygen affinity.
- Bohr effect: Decreased oxygen affinity due to increased CO₂ and H⁺.
- Methemoglobinemia: Oxidation of Fe²⁺ to Fe³⁺ in heme, preventing oxygen binding.
- Thalassemia: Inherited defect in hemoglobin chain synthesis.
Action Items / Next Steps
- Review and memorize: Oxygen dissociation curves for myoglobin and hemoglobin.
- Understand: Effects of 2,3-BPG, CO₂, and pH on hemoglobin’s oxygen affinity.
- Study: Hemoglobin genetic disorders (sickle cell, thalassemias, etc.) and their clinical consequences.
- Practice: Answering chapter review questions to reinforce understanding.
- Highlight: Key differences between myoglobin and hemoglobin, and the clinical significance of their properties.