enzyme active site

Enzyme active sites are like locks on a door. All enzymes have a special shape made up of amino acids (the building blocks of proteins). The parts of the enzyme that fit into another molecule are the "active sites". Those molecules are like different-shaped keys that can fit into the locks. When they do, they start a chemical reaction that changes one molecule into another. Every enzyme has it's own special and unique active site, which means only certain molecules can fit and start the reaction.