Overview
This lecture covers the structural and functional properties of adult human hemoglobin and myoglobin, focusing on their roles in oxygen transport and storage, molecular structure, and the mechanism of oxygen binding.
Hemoglobin Structure and Function
- Hemoglobin transports oxygen throughout the body, allowing cells to acquire oxygen for aerobic respiration.
- It is present in the cytoplasm of red blood cells.
- Hemoglobin consists of four subunits: two alpha and two beta, each with an iron-containing heme group.
- Cooperative binding occurs as oxygen binding to one subunit increases the affinity of others through protein conformational changes.
- Hemoglobin is a globular protein with hydrophilic amino acids on its exterior, making it soluble in the cytoplasm.
Myoglobin Structure and Function
- Myoglobin stores oxygen in muscle cells and is structurally similar to hemoglobin's beta subunit.
- It contains one polypeptide chain and a single iron-bound heme group.
- Myoglobin is a monomeric, globular protein with hydrophobic residues inside and hydrophilic residues outside, increasing its cytoplasmic solubility.
Heme Group and Oxygen Binding
- Both hemoglobin and myoglobin have a heme group consisting of an iron ion in a protoporphyrin ring.
- The iron is in the 2+ oxidation state and can switch between high-spin (5-coordinate, deoxygenated) and low-spin (6-coordinate, oxygenated) states.
- Oxygen binds reversibly to the iron in the heme group, affecting the geometry and color of blood (octahedral for oxygenated, square pyramidal for deoxygenated).
- The iron’s interaction with histidine and movement into the plane of the ring upon oxygen binding is crucial for function.
Cooperative Binding in Hemoglobin
- The movement of the iron and histidine leads to a conformational change, increasing hemoglobin's affinity for additional oxygen molecules.
- This change enables efficient oxygen uptake and delivery by hemoglobin.
Key Terms & Definitions
- Hemoglobin — Oxygen-transport protein in red blood cells, containing four subunits.
- Myoglobin — Oxygen-storage protein in muscle cells, consisting of a single polypeptide.
- Heme Group — Iron-containing prosthetic group enabling oxygen binding in hemoglobin and myoglobin.
- Cooperative Binding — Increased affinity of a protein’s subunits for a ligand (oxygen) as other subunits bind.
- Globular Protein — Protein with a compact, spherical shape that is soluble in water.
Action Items / Next Steps
- Review diagrams of hemoglobin and myoglobin structures.
- Study the mechanism of cooperative binding and oxygen affinity changes in hemoglobin.