Hemoglobin and Oxygen Binding

Introduction to Hemoglobin and Cooperative Binding

• Michaelis-Menten Enzymes: Characterized by a single active site and a hyperbolic reaction rate graph.
• Allosteric Enzymes: Possess multiple active sites, exhibit a sigmoidal (s-shaped) reaction curve, and demonstrate cooperative binding.
  • Cooperative Binding: The binding of a substrate to one active site increases the affinity of other active sites for subsequent substrate molecules, making their binding easier.
• Hemoglobin: A protein, not an enzyme (does not catalyze reactions), but serves as a crucial model system to study allosteric effects and cooperative binding.
  • It is easily obtainable and found abundantly in red blood cells.
  • Hemoglobin is the primary oxygen-carrying molecule in the blood.
  • It binds four oxygen molecules; the binding of the first oxygen facilitates the binding of the subsequent three.
• Reversible Binding: Oxygen binding to hemoglobin is a reversible process.
• R vs. T States: The concepts of Relaxed and Tense states are applied:
  • Relaxed (R) State: The active form, capable of binding substrate (e.g., oxygen).
  • Tense (T) State: The inactive form, less able to bind substrate.

Myoglobin: A Simpler Model

• Structure: A tertiary protein structure, essentially a single subunit. It has one active site.
  • Can be envisioned as one of the four subunits that make up hemoglobin.
• Location: Found in muscle cells (skeletal and cardiac muscle), not in the blood. Highly concentrated in deep-diving mammals like the sperm whale, enabling prolonged dives.
• Binding Behavior: Behaves like a Michaelis-Menten enzyme, exhibiting a hyperbolic binding curve.
  • No Cooperative Binding: Due to having only one active site, myoglobin does not exhibit cooperative binding.

The Heme Group: Oxygen Binding Site

• Prosthetic Group: The heme group is a non-protein component (a