Hemoglobin and Myoglobin Binding 3

Overview

This lecture continues the discussion on hemoglobin and myoglobin, focusing on their oxygen binding properties, binding curves, and the concepts of allostericity and cooperativity.

Oxygen Binding Curves

• Myoglobin displays a hyperbolic oxygen binding curve, reflecting non-allosteric (non-cooperative) binding.
• Oxygen concentration is measured as partial pressure (kilopascals) for gases.
• Theta (θ) represents the fraction of occupied binding sites: θ = pO₂ / (pO₂ + P₅₀).
• P₅₀ (for gases) is the partial pressure at which half the binding sites are occupied, analogous to Kd.
• Hemoglobin shows a sigmoidal (S-shaped) binding curve due to allosteric behavior.

Hemoglobin: States and Function

• Hemoglobin exists in two forms: T (tense, low affinity, deoxyhemoglobin) and R (relaxed, high affinity, oxyhemoglobin).
• In the lungs (high pO₂), hemoglobin shifts to R state and binds oxygen; in tissues (low pO₂), it shifts to T state and releases oxygen.
• The allosteric transition between T and R states enables efficient oxygen uptake and release.

Allostericity and Cooperativity

• Allosteric proteins have ligand binding at one site affecting binding at another site.
• Cooperativity: the binding of one ligand affects the likelihood of further ligand binding (can be positive or negative).
• Positive cooperativity: binding of the first ligand increases affinity for subsequent ligands (e.g., hemoglobin).
• Negative cooperativity: binding of one ligand decreases affinity for others.

Hill Coefficient and Analysis

• Archibald Hill introduced the Hill coefficient (n) to quantify cooperativity.
• Hill equation: generalizes the fraction of occupied sites to include n, the Hill coefficient.
• n = 1: no cooperativity (myoglobin).
• n > 1: positive cooperativity (hemoglobin, n ≈ 3).
• n < 1: negative cooperativity.
• Hill plot: log[θ/(1−θ)] vs. log(pO₂); the slope gives the Hill coefficient, indicating type and degree of cooperativity.

Key Terms & Definitions

• Partial Pressure (pO₂) — The pressure exerted by oxygen in a mixture of gases.
• P₅₀ — The partial pressure of oxygen where 50% of binding sites are occupied.
• Allosteric Protein — A protein whose function at one site is affected by binding at another site.
• Cooperativity — Interaction where ligand binding at one site affects binding at another site, positively or negatively.
• Hill Coefficient (n) — A measure of cooperativity; slope from a Hill plot.
• T State — Tense, low-affinity conformation of hemoglobin (deoxy form).
• R State — Relaxed, high-affinity conformation of hemoglobin (oxy form).
• Hill Plot — Plot to analyze cooperativity from binding data.

Action Items / Next Steps

• Review structural differences between R and T states of hemoglobin.
• Prepare for continued discussion on cooperativity models in the next lecture.