What is a ligand in the context of protein-ligand interactions?

• A. A molecule that catalyzes a reaction by converting substrates into products.
• B. A protein that binds to DNA to regulate gene expression.
• C. A large biomolecule that binds to a smaller molecule.
• D. A small substance that can bind and form a complex with a larger biomolecule, typically a protein.

Answer: (D)