Lock and key enzyme action. Computer artwork of the lock and key mechanism of enzyme action. Enzymes are proteins that act as biological catalysts. That is they regulate the rates of reactions in the body. In this image, the enzymes (red, pink, green and blue) have a depressed region on their surface representing an active site" which allows them to bind with the yellow substrate molecule. This is known as the "lock and key" mechanism. Once bound, the enzyme chemically changes the substrate, for example, to break it into smaller parts during the digestion of food molecules. The active site of each enzyme is usually specific for it's substrate."