Glossary
Enzyme
What is an Enzyme?
An enzyme is a biological catalyst: it speeds up a biochemical reaction without being consumed by it. Most enzymes are proteins, though some RNA molecules also act as catalysts.
How enzymes work
A reaction needs an initial input of energy to get going, called activation energy. Enzymes lower that barrier. Reactant molecules, called substrates, bind at an enzyme's active site, where the reaction converts them into products. The enzyme can then take part in another reaction.
The active site's shape and chemical properties help determine which substrates bind. That selectivity is useful, but it is not perfect: some enzymes work on multiple substrates or catalyze side reactions.
How cells regulate them
Cells control metabolic activity partly by controlling enzymes. Inhibitors and activators can change enzyme activity, as can covalent modifications and allosteric regulation—binding at a site that changes how the enzyme works elsewhere.
The distinction is between the speed of a reaction and its energy balance. Enzymes change the rate; they do not change the free-energy difference between reactants and products.