Final answer:
The protein-ligand binding equilibrium is described by the equilibrium constant Kd, which represents the ligand concentration at which the protein binding site is half occupied. Kd is measured in molar units and is used to quantify the affinity between a protein and a ligand in a reversible binding interaction.
Step-by-step explanation:
The mathematical expression that describes protein-ligand binding equilibria is the equilibrium constant (Kd), which is the dissociation constant for the binding of a ligand to a protein. The equilibrium equation is given by:
- Protein (P) + Ligand (L) ↔ Protein-Ligand complex (PL)
- Kd = [P][L] / [PL]
The term Kd represents the ligand concentration at which the binding site on a protein is half occupied, which can be seen on a saturation curve. It is measured in molar units (M). The Kd is used to understand and compare the affinity of different proteins for the same ligand or the same protein for different ligands. The equation essentially shows that when a protein and ligand are at equilibrium, the rate of their association is equal to the rate of their dissociation.