Enzymes are proteins that regulate chemical reactions but are themselves unchanged by the reaction. Because they're often required to start or speed up a reaction, enzymes also are called catalysts. Without enzymes, many biochemical reactions would be energetically inefficient.
Enzymes form temporary bonds with one or more of the reactants, or substances that are transformed during the reaction, to make the products of the reaction. These bonds reduce the amount of energy needed to begin the reaction, which speeds up the process.
Names for enzymes usually end in the suffix "-ase," making enzymes easy to distinguish. For example, an enzyme responsible for removing a phosphate group from a molecule is called a phosphatase, and an enzyme responsible for breaking down proteins is called an protease.
The International Union of Biochemistry recognizes about 300 different types of enzymes. Specific enzymes are involved in making chemical energy for cells, breaking down proteins or nucleic acids or catalyzing oxidation-reduction reactions.
Without enzymes, the energy stored in molecules might be all but inaccessible to cells. Just as wood doesn't spontaneously catch fire, breaking chemical bonds to release energy requires an investment of energy.
Just as applying heat starts the burning process, enzymes allow chemical reactions to proceed by lowering the amount of energy required to run the reaction so the cell can function efficiently.