BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 7. ENZYMOLOGY

7.5. Fundamentals of Enzymatic Reaction Kinetics

7.5.3. Effect of Medium pH on the Rate of Enzymatic Reactions

Enzyme activity depends on the pH of the solution in which the enzymatic reaction takes place. For every enzyme, there is an optimal pH value at which its maximum activity is observed. Deviations from this optimal pH lead to a decrease in enzymatic activity.

The Effect of pH on enzyme activity is associated with The ionization of functional groups in The amino acid residues of the protein, which maintain the optimal conformation of the Active Site. When the pH deviates from optimal values, the ionization state of the protein's functional groups changes. For instance, acidification of the medium leads to the protonation of free amino groups (NH3), whereas alkalinization leads to deprotonation of carboxyl groups (COO^-). This alters the conformation of both the enzyme molecule and its active site, consequently disrupting the binding of the substrate, Cofactors, and Coenzymes to the active site. Furthermore, the medium pH can affect the ionization state or spatial arrangement of the substrate, thereby influencing its affinity for the active site. Significant deviations from the optimal pH can also induce Denaturation of the protein molecule, resulting in a complete loss of enzymatic activity.

The pH optimum varies among different Enzymes (Fig. 7.18). Enzymes operating in acidic environments (such as Pepsin in The Stomach or lysosomal enzymes) have evolved a conformation adapted to function efficiently at acidic pH values. However, the majority of human body enzymes exhibit a pH optimum close to neutrality, which corresponds to the physiological pH (Table 7.1).

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Fig. 7.18. Dependence of the enzymatic reaction rate (V) on the pH of the medium

Table 7.1

Optimal pH values for selected enzymes