An enzyme is a specialized protein that acts as a biological catalyst. A catalyst is something that speeds up a chemical reaction. Therefore, enzymes are biological molecules which increase the rates of chemical reactions.
Enzymes are highly specific and are the most efficient catalysts known.
The enzyme is not permanently modified by its participation. Some enzymes depend on the presence of coenzymes for their function. For the enzyme to continue to be effective, its three-dimensional molecular structure must be maintained. It is believed that an enzyme functions by attaching the molecule it acts on to a specific molecular site, so that the electrostatic forces of nearby atoms sharply reduce the energy needed to cleave and re-form the appropriate chemical bonds.