This is because increasing the temperature increases the energy in the total population of reactant molecules, thus increasing the proportion of reactants that have the activation energy. At a few degrees above body temperature, however, most enzymes start to denature, which decreases the rate of the reactions which they catalyze. 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.