In chemistry, a substrate is the chemical species being observed, which reacts with a reagent. This term is highly context-dependent. In particular, in biochemistry, an enzyme substrate is the material upon which an enzyme acts. Using this same example with enzymes, an enzyme is formed when the enzyme molecule fits with the substrate to create an active site for the enzyme to work. The substrate is what fits into the enzyme to make it function. If the enzyme is denatured, the active site will change (meaning that the enzyme and substrate will not fit in the "lock and key format") and so the substrate and enzyme molecule will not be able to fit meaning the enzyme will not function. When referring to Le Chatelier's Principle, the substrate is the reagent whose concentration is changed.