What is a general term for a molecule that binds to a specific site on a protein?

A ligand is a general term for any molecule that binds specifically to a site on a protein, often resulting in a change in the function or activity of that protein. Ligands can include a wide variety of substances, such as ions, small organic molecules, or larger macromolecules, depending on the nature of the protein they interact with. When a ligand binds to its specific site on a protein, it can facilitate processes like signal transduction or enzyme activation, illustrating the important role ligands play in biological systems.

Enzymes, on the other hand, are specific types of proteins that catalyze biochemical reactions, and they often require substrates to bind to their active sites in order to function. Cofactors are non-protein molecules (which may or may not be ligands) that assist enzymes in their activity, while substrates refer specifically to the reactants that enzymes act upon. Therefore, while these other options relate to proteins and their interactions, they are not general terms for molecules that bind to proteins in the same way that a ligand is.