How does pH alter the active site of an enzyme?

pH alters the active site of an enzyme by affecting the ionisation of the amino acids that form the active site.

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are made up of proteins, which are long chains of amino acids. The active site of an enzyme is a specific region where the substrate binds and the chemical reaction takes place. The shape and charge of the active site are crucial for the enzyme's function, and these are determined by the arrangement and ionisation of the amino acids that form the active site.

pH, which measures the acidity or alkalinity of a solution, can affect the ionisation of the amino acids. Each amino acid has an ionisable side chain, and the degree of ionisation depends on the pH of the environment. At a certain pH, known as the isoelectric point, the amino acid is neutral because the positive and negative charges balance each other out. If the pH is above or below this point, the amino acid will carry a net positive or negative charge, respectively.

Changes in pH can therefore alter the charge of the amino acids in the active site, which can in turn affect the binding of the substrate. For example, if the active site contains an amino acid that needs to be negatively charged to bind the substrate, an increase in pH (making the solution more alkaline) could enhance the enzyme's activity by increasing the degree of ionisation of the amino acid. Conversely, a decrease in pH (making the solution more acidic) could inhibit the enzyme's activity by decreasing the degree of ionisation.

In addition, extreme pH values can lead to denaturation of the enzyme, which is a permanent change in the protein structure that renders the enzyme inactive. This is because the hydrogen bonds and other interactions that hold the protein in its specific shape can be disrupted by the high concentration of hydrogen ions (in an acidic environment) or hydroxide ions (in an alkaline environment). The active site is particularly sensitive to these changes, as its shape and charge are crucial for the enzyme's function.