Pepsin, a digestive enzyme that works best at low pH is a good example. Pepsin begins the process of protein digestion in the very acidic environment of the stomach. During digestion, The stomach contents pass into the small intestine where the pH of pepsin rises from pH 2 to about 7.5. Pepsin goes above pH 5.5 to denature so that this enzyme will become inactive in the small intestine. From there protein digestion continues with the assistance of trypsin, an enzyme that functions well at the higher pH.