Protein-ligand interactions refer to the binding of a ligand, which can be a small molecule, ion, or another protein, to a protein. These interactions are crucial for many biological processes, including enzyme activity, signal transduction, and cellular communication. The binding typically occurs at a specific site on the protein, known as the active site or binding site, where the ligand can influence the protein's function. The strength and specificity of these interactions are determined by various factors, including hydrogen bonds, ionic interactions, and hydrophobic effects. Understanding protein-ligand interactions is essential in fields like drug design, where researchers aim to develop compounds that can effectively target specific proteins to treat diseases.