Enzymes are naturally occurring proteins that are found in the bodies of certain living things, including humans and other animals, and that cause chemical changes such as breaking down food in the stomach. Within the human body, enzymes can be found in bodily fluids, such as blood, saliva, the gastric juices or the stomach and fluids in the intestines. In general, enzymes serve as catalysts for biological functions, including natural, involuntary bodily functions, such as blood clotting.
Enzymes have three main characteristics. First, they increase the rate of a natural chemical reaction. Secondly, they typically only react with one specific substrate or reactant, and thirdly, enzyme activity is regulated and controlled within the cell through several different means, including regulation by inhibitors and activators. It is possible to group enzymes into different categories, including oxidases, transferases, hydrolases, lyaes, isomerases and ligases. In naming enzymes, the "-ase" suffix is often appended to the name of the substrate molecule upon which which the enzyme reacts. For example, the enzyme sucrase catalyzes the transformation of the sugar sucrose in to glucose and fructose. In this case, the "sucr-" suffix represents the molecule upon which the sucrase enzyme reacts. Not all enzymes are named according to this convention.