Cambridge O Level Biology · Syllabus 5090 · Enzymes
Optimum pH
What is Optimum pH?
The pH at which an enzyme has its greatest activity. Moving away from it alters the interactions that hold the enzyme's folded shape, so the active site becomes a less good fit for the substrate, fewer enzyme-substrate complexes form and activity falls; at extreme pH the enzyme is denatured. Different enzymes have different optimum pH values.
This definition is part of the Enzymes chapter in Cambridge O Level Biology.
Optimum pH in context
An enzyme is a protein that works as a biological catalyst: it increases the rate of a metabolic reaction and is not changed by that reaction. Each enzyme has an active site with a particular shape. A substrate whose shape is complementary to that active site fits into it and an enzyme–substrate complex forms; the reaction happens, the products no longer fit in the same way and leave, and the unchanged enzyme is free to work again. Raising the temperature towards the optimum gives molecules more kinetic energy, so collisions are more frequent and more of them are effective, and the rate rises. Above the optimum, increased vibration disrupts the bonds holding the enzyme's three‑dimensional shape, the active site changes shape, the substrate no longer fits, and the rate falls rapidly — the enzyme has been denatured. pH behaves the same way around an optimum pH that differs from enzyme to enzyme.