An enzyme is a protein produced by a living organism that functions as a catalyst.
Enzymes increase the speed and likelihood of chemical reactions within an organism's body by lowering the activation energy necessary for a reaction to occur. This happens because the enzyme selects the particular configuration of the substrate and bends the bonds into the energetically-favorable orientation, with a consequent decrease in entropy. They are essential to all cellular functions, and most life could not exist without them, although viruses lack enzymes, but use them from their host cell. The active site, a three-dimensional structure, is the only region of the enzyme that binds to the substrate. It permits only a particular substrate to bind to the site.