Protein-ligand interaction refers to the specific binding between a protein and a ligand, which is a molecule that can attach to the protein. This interaction is crucial for many biological processes, including enzyme activity, signal transduction, and cellular communication. The ligand can be a small molecule, ion, or even another protein, and it typically binds to a specific site on the protein known as the active site. These interactions are essential in drug design, where ligands are developed to target specific proteins involved in diseases. Understanding the nature of protein-ligand interactions helps researchers create more effective medications and therapeutics by enhancing the binding affinity and specificity of the ligands to their target proteins.