Enzymes are proteins produced by all living organisms. They selectively accelerate chemical reactions as part of the essential process of life such as digestion, breathing, metabolism, and maintenance of tissues. Enzymes can increase the speed of a reaction by a factor of up to 10^17 times more than the uncatalyzed reaction. An enzyme combines with its <em>substrate </em>to form an enzyme-substrate complex as an essential step in enzymatic catalysis.
Substrate binding occurs only in a specific and small region of the enzyme. This region to which the substrate binds is called the ACTIVE CENTER or Active Site of the enzyme. A molecule to be accepted as a substrate must have the appropriate spatial shape to fit into the active center and chemical groups capable of establishing precise bonds with the active center radicals.
