A ligand is any molecule that binds a target biomolecule — typically a receptor, enzyme or transport protein — with defined specificity and affinity, forming a non-covalent complex. Ligands may be endogenous (hormones, neurotransmitters, growth factors) or exogenous (synthetic agonists, antagonists, tool compounds).
A ligand's pharmacological properties are described by two independent axes: affinity, which quantifies the strength of binding (Kd, Kᵢ), and efficacy, which describes how well the bound state activates the receptor (full agonist, partial agonist, antagonist, inverse agonist).
Every peptide in this catalogue is, functionally, a ligand for one or more molecular targets; ligand is the parent concept underneath agonism, antagonism and affinity.