All enzymes have a certain pH range at which they work best. An enzyme is a protein composed of molecules called amino acids, and these amino acids have regions that are sensitive to pH. The pH scale defines how acidic or basic a solution is, with low pH being acidic and high pH being basic. The human stomach has a pH of 2, and enzymes that work in the stomach are adapted to function at that pH level.

When we ingest food and drink, bacteria come along with them. Our bodies are able to protect themselves from infection by killing bacteria in the stomach. At a pH of 2, the gastric juices of the stomach are acidic enough to kill the bacteria we ingest. The cells that line the stomach -- called parietal cells -- secrete hydrochloric acid or HCl, and this acid gives gastric juices their low pH. HCl does not digest food, but it kills bacteria, helps break down the connective tissue in meat, and activates pepsin, the stomach's digestive enzyme.

Chief cells, which also line the stomach, produce a pro-enzyme called pepsinogen. When pepsinogen contacts the acidic environment of the stomach, it catalyzes a reaction to activate itself and becomes the active enzyme called pepsin. Pepsin is a protease, or an enzyme that breaks chemical bonds in protein. Pepsin uses the carboxylic acid group on one of its amino acids to break the chemical bond between nitrogen and oxygen in the proteins found in food.

The reason pepsin functions best at pH 2 is because the carboxylic acid group on the amino acid in the enzyme's active site must be in its protonated state, meaning bound to a hydrogen atom. At low pH the carboxylic acid group is protonated, which allows it to catalyze the chemical reaction of breaking chemical bonds. At pH values higher than 2, the carboxylic acid becomes deprotonated and thus unable to participate in chemical reactions. Pepsin is most active at pH 2, with its activity decreasing at higher pH and dropping off completely at pH 6.5 or above. In general, enzyme activity is sensitive to pH because the catalytic group of an enzyme -- in pepsin's case, the carboxylic acid group -- will be either protonated or deprotonated, and this state determines whether or not it can participate in a chemical reaction.

Following digestion in the stomach, food exits through the pyloric sphincter into the duodenum of the small intestine, where the pH is much higher. Pepsin becomes inactive in this environment because the concentration of hydrogen atoms is lower. The hydrogen on pepsin's carboxylic acid in the enzyme active site is then removed, and the enzyme becomes inactive. The chemical reaction catalyzed by pepsin depends on the presence of a protonated carboxylic acid, so the enzyme's activity is highly dependent on the pH of the solution it's in. Low pH leads to high activity and high pH gives little or no activity.