Enzymes have a pH range where they work most effectively. This range is called their optimum pH. In most cases it matches the pH of the environment where they function. For example, pepsin is a digestive enzyme found in the stomach. It works best at a pH of 2 which is about the pH of stomach digestive fluids. Mild changes in pH can change the ionic state of polar groups in the enzyme causing them not to give up H+ ions that an enzymatic reaction needs. The pH optimum occurs when the active ionic state of an enzyme has one part that is donating H+ and one part that is donating an electron pair.