Enzymes

Enzymes are biological catalysts: proteins that speed up reactions without being used up. Each enzyme has an active site with a specific shape. Only a substrate with a matching shape fits, like a key in a lock, forming an enzyme–substrate complex. The substrate is broken down or joined, and the products leave.

Enzyme activity rises with temperature up to an optimum, often about 37 °C in humans, because molecules collide more often. Above the optimum, the active site changes shape and the enzyme is denatured: it no longer works. Each enzyme also has an optimum pH; for example, the protease pepsin in the stomach works best at about pH 2, while amylase in the mouth works best near pH 7.