In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity.  The binding partner of the macromolecule is often referred to as a ligand. Ligands may include other proteins (resulting in a protein–protein interaction), enzyme substrates, second messengers, hormones, or allosteric modulators.  The binding event is often, but not always, accompanied by a conformational change that alters the protein's function.  Binding to protein binding sites is most often reversible (transient and non-covalent), but can also be covalent reversible or irreversible.