Effect of pH on enzyme activity
pH is a measure of the concentration of hydrogen ions in a solution. The higher the hydrogen ion concentration, the lower the pH. Most enzymes function efficiently over a narrow pH range. A change in pH above or below this range reduces the rate of enzyme reaction considerably.
Changes in pH lead to the breaking of the ionic bonds that hold the tertiary structure of the enzyme in place. The enzyme begins to lose its functional shape, particularly the shape of the active site, such that the substrate will no longer fit into it, the enzyme is said to be denatured. Also changes in pH affect the charges on the amino acids within the active site such that the enzyme will not be able to form an enzyme-substrate complex.
The pH at which an enzyme catalyses a reaction at the maximum rate is called the optimum pH. This can vary considerably from pH 2 for pepsin to pH 9 for pancreatic lipase.