Overview
This lecture covers the structure and function of hemoglobin, the oxyhemoglobin dissociation curve, and how different organisms adapt their hemoglobin for oxygen transport.
Hemoglobin Structure and Function
- Red blood cells carry oxygen using a protein called hemoglobin.
- Hemoglobin consists of four subunits, giving it a quaternary structure.
- Each subunit contains a heme group with an iron ion (Fe²⁺) that binds to oxygen.
- When oxygen binds to hemoglobin, it forms oxyhemoglobin in a reversible reaction (loading and unloading).
- Hemoglobin’s affinity for oxygen refers to its tendency to bind with oxygen.
Oxygen Loading and Unloading
- In the lungs (high oxygen concentration), hemoglobin loads oxygen due to high affinity.
- In tissues (low oxygen concentration), hemoglobin unloads oxygen for cellular respiration due to lower affinity.
Oxyhemoglobin Dissociation Curve
- The curve plots partial pressure of oxygen (x-axis) against percentage saturation of hemoglobin (y-axis).
- The curve is S-shaped (sigmoidal) due to cooperative binding.
- At low oxygen, saturation rises slowly; after first oxygen binds, affinity increases, causing a steep rise.
- As saturation nears maximum, it plateaus since fewer binding sites remain.
Bohr Effect and Curve Shifts
- Increased carbon dioxide shifts the curve to the right (Bohr effect), lowering hemoglobin’s affinity for oxygen.
- Lower affinity promotes greater unloading of oxygen to tissues during high activity or exercise.
- To interpret curves, compare percentage saturation of hemoglobin at tissue oxygen levels between different curves.
Hemoglobin Adaptations in Organisms
- Hemoglobin structure can differ between organisms due to variations in amino acid sequences, altering affinity.
- Left-shifted curve: hemoglobin has higher affinity, loading more oxygen at lower concentrations (useful in low oxygen environments, e.g., fetal hemoglobin).
- Right-shifted curve: hemoglobin has lower affinity, unloading more oxygen to tissues (useful for organisms with high metabolic rates).
Key Terms & Definitions
- Hemoglobin — protein in red blood cells that carries oxygen.
- Affinity — tendency of hemoglobin to bind with oxygen.
- Oxyhemoglobin — compound formed when oxygen binds to hemoglobin.
- Partial pressure of oxygen — measure of oxygen concentration.
- Bohr effect — decrease in hemoglobin’s oxygen affinity due to increased CO₂.
- Quaternary structure — protein structure with multiple polypeptide chains.
Action Items / Next Steps
- Review and interpret example oxyhemoglobin dissociation curves.
- Study differences in hemoglobin adaptation for various organisms.
- Prepare for questions on curve interpretation and exam scenarios involving hemoglobin affinity.