Enzymes

Enzymes are biological catalysts -- almost all of them proteins -- produced by living cells to speed up the specific biochemical reactions that keep the cell functioning. Like any catalyst, an enzyme increases the rate of a reaction by providing a pathway with lower activation energy, without itself being permanently consumed or altered by the overall reaction, so a single enzyme molecule can catalyse the same reaction over and over again.

What sets enzymes apart from simple inorganic catalysts is their remarkable specificity: a given enzyme will typically catalyse the reaction of only one particular substrate, or a small family of closely related substrates, and will not act at all on unrelated molecules. The elementary explanation for this specificity is the lock-and-key model. On the enzyme's folded, three-dimensional surface there exists a small pocket or cleft, called the active site, whose precise shape (and the particular chemical groups lining it) is complementary to the shape of one specific substrate molecule -- much as a key's precise pattern of notches fits only the one lock it was cut for. The substrate binds snugly into this active site, forming a temporary enzyme-substrate complex; the reaction is then catalysed within that complex; and the products, no longer a good fit for the active site, are released, leaving the enzyme free and unchanged to bind another substrate molecule and repeat the cycle. …