Any substance that binds specifically and reversibly to a biomacromolecule to form a larger complex and alters its activity or function is called a ligand. In the PDB, drugs, metals, and small molecules are also called ligands.
Here we show examples of ligands bound to protein (figure 19) and DNA (figure 20).
Figure 19 An example of a ligand binding to cytochrome c-553 (in green, yellow, blue) is prosthetic group Haem containing an iron ion.
Figure 20 An example of a ligand binding to DNA: drug, bleomycin a2.