An enzyme is a protein functioning as a biological catalyst. Enzymes accelerate (often by several orders of magnitude) chemical reactions in the cell that would proceed imperceptibly or not at all in their absence. The enzyme is not permanently modified by its participation.

Most enzymes demonstrate great specificity, reacting with only one or small group of closely related chemical compounds, thus, sometimes several enzymes are required for efficient catalytic function. 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. X-ray crystallography is used to analyze the structure of enzymes.

Over 1000 different enzymes have been identified, and the exact sequence of Amino Acids (subunits of protein) has been determined for many proteins since 1967, when the first such determination was made.

It is believed that enzymes function by attaching the substrate molecule to a specific molecular site, so that electrostatic forces of nearby atoms sharply reduce the energy needed to cleave and re-form the appropriate chemical bonds.