Effect of pH on Enzyme-Catalyzed Reactions

pH effect on active site ionization: The concentration of protons ([H+]) affects reaction velocity in several ways. First, the catalytic process usually requires that the enzyme and substrate have specific chemical groups in either an ionized or unionized state in order to interact. For example, catalytic activity may require that an amino group of the enzyme be in the protonated form (−NH3+). Because this group is deprotonated at alkaline pH, the rate of the reaction declines.
pH effect on enzyme denaturation: Extremes of pH can also lead to denaturation of the enzyme, because the structure of the catalytically active protein molecule depends on the ionic character of the amino acid side chains.
Variable pH optimum: The pH at which maximal enzyme activity is achieved is different for different enzymes and often reflects the [H+] at which the enzyme functions in the body. For example, pepsin, a digestive enzyme in the stomach, is maximally active at pH 2, whereas other enzymes, designed to work at neutral pH, are denatured by such an acidic environment (Fig. 1).

Figure 1 Effect of pH on enzyme-catalyzed reactions.