There are some human immune system cells that actually use H 2 O 2 to kill foreign invaders. Hydrogen peroxide is rendered harmless in cellular organelles called peroxisomes. During reduction of molecular oxygen to water, hydrogen peroxide is generated. It can damage DNA, protein and lipid membranes and may even be a causative factor in cancer. It is important to realize that each type of organism has its own catalase, which probably differs at least slightly in other species of organism.
Enzymes work best within an optimum range of physical and chemical conditions.
It is important to remember that enzymes act by binding substrates, forming a complex. The complex stresses chemical bonds forming a transition state. This makes the substrate more reactive. Energy is needed to form this state and it is provided by the enzyme. The enzyme's site of attachment and the parts that stress the substrate's bonds is known as the active site.