Enzyme

An enzyme is a molecule that speeds up a chemical reaction in a living thing without being consumed, so each enzyme molecule can act again and again. Almost all enzymes are proteins, although some RNA molecules, called ribozymes, also work as catalysts. Each enzyme is specific: it binds particular reactant molecules, its substrates, at a pocket on its surface called the active site, and a cell contains thousands of different enzymes for its many reactions.

Enzymes work by lowering the activation energy, the energy barrier a reaction must get over before it can proceed. They hold substrates in the right orientation, strain their bonds or provide a favorable chemical setting, but they do not change how much energy the reaction releases overall. Their activity depends on temperature and pH, and outside its preferred range an enzyme can lose its shape and stop working. Many need helpers: inorganic cofactors such as zinc or magnesium ions, or organic coenzymes, often made from vitamins.

Cells tune enzyme activity with inhibitors, activators and feedback from the end products of their pathways. Enzyme names usually end in -ase, as in amylase, lactase and DNA polymerase.