Catalase is an enzyme which is present in every living organism that is exposed to oxygen. Catalase's activity is affected by temperature, pH and the concentration of substrate. In catalase's case, the substrate is the hydrogen peroxide. If any of these factors change, the effectiveness of the catalase to catalyze the decomposition of hydrogen peroxide will deteriorate. If the temperature and pH change, the shape of the enzyme will change as well. This denaturation of the enzyme will obstruct the hydrogen peroxide molecules to fit in the catalase's active site.