Each enzyme has optimal activity at a characteristic pH, called the pH optimum for that enzyme. Deviations from the pH optimum will decrease the reaction rate because the pH affects the shape of the enzyme and charges within the active site. The pH optima of different enzymes can be quite different, pepsin has a pH optimum of 2, for example, while trypsin is most active at a pH of 9. Many enzymes require metal ions in order to be active. These ions are therefore said to be cofactors for the enzymes.
Many enzymes require smaller organic molecules for activity.
These smaller organic molecules are called coenzymes. Many coenzymes are derived from water-soluble vitamins. Coenzymes transport hydrogen atoms and small substrate molecules from one enzyme to another. The rate of enzymatic reactions increases when either the substrate concentration or the enzyme concentration is increased. If the enzyme concentration remains constant, the rate of the reaction increases as the substrate concentration is raised, up to a maximum rate.