Where do enzymes act as catalysts?
Enzymes ARE in fact biological catalysts. So they always act as catalysts whenever they are reacting.
Enzymes are proteins whose main function is to lower the activation energy of any reaction. This means that the reaction would require less energy to proceed and bring about products. So overall, enzymes are catalysts that catalyse biological reactions in all living organisms.
This for example, includes the hydrolysis of hydrogen peroxide ( a toxic substance) by catalase into water and oxygen. This occurs in all cells.