pH: All enzymes work at their maximum rate at a narrow range of pH, called the optimum pH. A slight change (increase or decrease) in this pH causes retardation in enzyme activity or blocks it completely. Every enzyme has its specific optimum pH value.

For example, Pepsin (working in the stomach) is active in an acidic medium (low pH) while trypsin (working in the small intestine) shows its activity in an alkaline medium (high pH). A change in pH can affect the ionization of the amino acids at the active site.