Enzymes are a type of protein and act as catalysts in biochemical reactions, meaning that they speed the reactions up. Each enzyme recognizes one or more substrates, the molecules that serve as starting material for the reaction it catalyzes. Different enzymes participate in different types of reactions and may break down, link up, or rearrange their substrates.
One example of an enzyme found in your body is salivary amylase, which breaks amylose (a kind of starch) down into smaller sugars. The amylose doesn’t taste very sweet, but the smaller sugars do. This is why starchy foods often taste sweeter if you chew them for longer: you’re giving salivary amylase time to get to work.