Enzyme

An enzyme is a biological catalyst: it increases the rate at which a reaction proceeds without being consumed as a net reactant. Most enzymes are proteins whose folded structures create a catalytic environment. Some RNA molecules, called ribozymes, also catalyse reactions. Enzymes can work inside cells or, after extraction, in laboratory, medical and industrial processes.

Reactant molecules acted on by an enzyme are called substrates. A substrate binds at or near a region of the enzyme known as its active site, where chemical interactions help stabilize the reaction's transition state and lower the activation energy. The enzyme then releases product molecules and can catalyse another reaction. Enzymes are often selective for particular substrates or reaction types, although their selectivity varies. They change reaction rates, not the overall equilibrium position or thermodynamics of a reaction.

An enzyme's three-dimensional structure and chemical environment help determine its activity. Some enzymes require helper components such as metal ions or organic cofactors. Temperature and pH can affect the shape and reaction rate; conditions far from an enzyme's usual range may disrupt its structure. Inhibitors reduce activity by binding to an enzyme or otherwise interfering with catalysis, while other molecules can regulate activity by binding at sites distinct from the active site.

The International Union of Biochemistry and Molecular Biology's Enzyme Commission system classifies enzymes according to the reactions they catalyse and assigns EC numbers. Enzymes are essential to cellular metabolism and are used in applications such as food processing, detergents, diagnostics and biotechnology. Their usefulness in a particular process depends on such factors as substrate, temperature, pH, stability and cost.