Receptor Affinity
Definition
Receptor affinity describes how strongly a ligand (such as a peptide) binds to its target receptor. It is quantified by the dissociation constant (Kd) — a lower Kd indicates tighter binding and higher affinity. Receptor affinity is one of the most important parameters in peptide pharmacology, as it determines potency and selectivity.
How Receptor Affinity Is Measured
Affinity is typically determined through competitive binding assays, surface plasmon resonance (SPR), or isothermal titration calorimetry (ITC). In competitive binding studies, a labelled reference ligand is displaced by increasing concentrations of the test peptide, generating an IC50 value that can be converted to Ki (inhibition constant).
Related but distinct concepts include:
- Selectivity — Preferential binding to one receptor subtype over others (e.g., MC4R over MC3R)
- Efficacy — The magnitude of biological response once bound (a high-affinity ligand can be a full agonist, partial agonist, or antagonist)
- Potency (EC50) — The concentration required to produce 50% of the maximal response
Why Affinity Matters in Peptide Design
Peptide researchers modify sequences to optimise receptor affinity while maintaining selectivity. Substituting amino acids, introducing unnatural residues, or constraining peptide conformation through cyclisation can all alter binding characteristics. For example, BPC-157 fragments and melanocortin peptide analogues are studied with specific modifications designed to enhance affinity for their respective targets.
Related Peptides
Peptide profiles that reference “Receptor Affinity” in their research content.