Primer: catalysts and enzymes

If you pour a glass of hydrogen peroxide (H₂O₂) solution and watch it for a while, not much happens. It sits there, looking very similar to a glass of water. If you get distracted for a while and come back later, you might see a few bubbles on the surface of the glass, but that's about it.

If you know a little bit about chemistry, you know that hydrogen peroxide is not a very stable molecule. Oxygen is one of the biggest electron hogs on the periodic table, and two electron hogs are not going to like being that close together. Oxygen (O₂) and water (H₂O), on the other hand, are very stable molecules. If you know a little more about chemistry, you know that less stable molecules tend to react to form more stable ones. So, where's my water and oxygen?

Molecules don't just react willy-nilly. They need to orient themselves properly for the atoms to interact in new ways. They also need to bring enough energy to form an unstable, temporary, intermediate arrangement of atoms between the reactants and products - a transition state. In our example, the hydrogen peroxide molecules can tumble around in the glass for hours before enough of them meet those conditions for us to actually see oxygen bubbles. But if we drop a little steel wool into the glass, there will be oxygen bubbles everywhere!

The iron in the steel wool is acting as a catalyst. Catalysts increase the probability of molecules reacting by guiding them into the proper orientation, lowering the energy requirements of the transition state, or often both. What catalysts do not do is contribute to the net result of the reaction. Even with the steel wool, our hydrogen peroxide reaction is still only producing oxygen and water. The iron temporarily interacts with the peroxide to form a different, more attainable transition state than the peroxide would form by itself. Since the iron is left behind after the peroxide has reacted, one atom of iron can catalyze any number of hydrogen peroxide reactions.

The chemistry of living things uses catalysts all the time! Life as we know it would be impossible without them. When catalysts appear in a living system, they are called enzymes. Let's look at our hydrogen peroxide example again. Peroxides are damaging to living tissues, so many organisms utilize a protein called catalase, which converts hydrogen peroxide into water and oxygen. Catalase is acting as an enzyme, and similar to our non-biological example, it incorporates iron in its molecular structure.