Enzymes /ˈɛnzaɪmz/ are macromolecular biological catalysts. They are responsible for thousands of metabolic processes that sustain life. The study of enzymes is called enzymology. Enzymes are highly selective catalysts, greatly accelerating both the rate and the specificity of metabolic chemical reactions: from the digestion of food to the synthesis of DNA. Most enzymes are proteins, although a few are catalytic RNA molecules, such as the ribosome. Enzymes fold into a specific three-dimensional structure, and may use organic (e.g. biotin) and inorganic (e.g. magnesium ion) cofactors to aid catalysis.