Enzymes are biological catalysts. A catalyst speeds up a chemical reaction without being used up. Enzymes are proteins, and they make the reactions of life happen fast enough at body temperature.
Each enzyme has a region called the active site, which has a very particular shape. The substance the enzyme acts on is the substrate. The substrate fits into the active site like a key fits into a lock, and the two form an enzyme-substrate complex. The reaction happens, the products leave, and the enzyme is not used up, so it can work again.
Enzymes are specific. The shape of an active site is complementary to only one kind of substrate, so each enzyme catalyses only one reaction. Amylase breaks down starch, but its active site does not fit protein molecules, so it cannot break them down.
