An enzyme acts on a substrate to form a product. The substrate can be a small molecule like ATP or a large molecule like DNA or another protein. Enzymes are catalysts Enzymes do not get used up in the reaction. Enzymes do not alter the equilibrium ratio of substrate and product. Enzymes speed up reaction rates.
Enzymes can catalyze the reverse reaction as well.
Any molecule that binds to a protein is called a ligand. If ligand binding involves H-bonding, water provides competition. The reaction that an enzyme catalyzes occurs in the enzyme active site. The 3-dimensional arrangement of amino acids in the active site defines the active site. These other sites are called allosteric sites.