Cooperative Binding — Hemoglobin

Cooperative binding in hemoglobin: binding of one O₂ molecule changes the protein's conformation (T→R state), increasing affinity for subsequent oxygen molecules. This is captured by the Hill equation: θ = pO₂ⁿ/(P50ⁿ + pO₂ⁿ). When n=1, binding is hyperbolic (myoglobin). When n>1, the curve is sigmoidal — more efficient O₂ loading in the lungs (high pO₂) and unloading in tissues (low pO₂). The Bohr effect: low pH (acidosis, e.g. in exercising muscle) right-shifts the curve, promoting O₂ release exactly where it's needed. The Hill plot (log[θ/(1-θ)] vs log pO₂) linearizes the sigmoid; slope = Hill coefficient n.