An enzyme is a type of protein that is used by cells to speed up chemical reactions. It acts as a catalyst, which means it participates in and affects a reaction but is not itself altered by that reaction.
Enzymes are extremely powerful in increasing the rates of chemical reactions. Most reactions in cells happen about a million times faster with an enzyme present than they would without one. Enzymes tend to be very specific, with each enzyme only affecting one type of chemical reaction. Cells have the ability to regulate enzymes, turning them on and off depending on their needs.