Explain an optimum, not just a peak

Explanation

Enzymes act only in solution, where enzyme and substrate molecules can move and collide. An effective collision is one in which the substrate binds to the active site and an enzyme-substrate complex forms. As temperature rises towards the optimum, enzyme and substrate molecules gain kinetic energy. The frequency of effective collisions increases, so enzyme-substrate complexes form more often and the rate rises.

Above the optimum, the extra kinetic energy makes the enzyme vibrate enough to break the hydrogen bonds and ionic bonds that hold its 3D shape. The 3D shape of the active site changes, so the substrate is no longer complementary to it. The enzyme is denatured, fewer enzyme-substrate complexes form and the rate falls, even though collisions are more frequent. The measured optimum depends on the enzyme, assay conditions and duration; a sparse set of tested temperatures does not locate it exactly.

pH affects ionisation of groups in the enzyme and sometimes the substrate. Changed charge can break ionic bonds and hydrogen bonds that hold the 3D shape of the active site, or change the charge of groups that bind the substrate, so fewer enzyme-substrate complexes form. Extreme pH may denature the protein. Different enzymes have different suitable pH ranges.

Compare results at a fixed temperature when testing pH and at fixed pH when testing temperature. A low rate alone does not identify the cause: too little enzyme, depleted substrate or an unsuitable assay could also produce it.