Why does the enzyme catalase break down hydrogen peroxide?
In: Biology
There is a concept called Activation Energy, which is the energy you need to put in to a system in order for a reaction to start. Think of this like the amount of energy you need to push on a domino in order for it to tip over, or the heat that the friction of striking a match generates.
There’s another concept called a Maxwell-Boltzmann distribution, which basically says that the molecules in a substance is not all the same temperature. Some of them are hotter than the average, some of them colder. This means that even if the solution of Hydrogen Peroxide isn’t hot enough for it all to decompose, a couple of molecules will be above the Activation Energy, a couple of them will be. So at room temperature, Hydrogen Peroxide will slowly decompose on it’s own.
What catalysts do is they lower the Activation Energy so that more of the molecules in a substance have enough energy. It’s thought that the shape of the active site on catalase is structured in such a way that when Hydrogen Peroxide attaches to it, it can decompose through an alternate route made up of two reactions which take less energy to start (lower Activation Energy). At the end, the water and O2 are released and the catalase is free to act upon the next H2O2 it encounters.
It takes hydrogen peroxide and combines together to make oxygen and water! That’s why your not supposed to but hydrogen peroxide on your cuts because the bacteria on your cut are already exposed to the air and use it to live and divide. If you put hydrogen peroxide on them, since they are aerobes, they effectively dilute it! That’s why you see bubbles when you poor it on the skin, it’s the oxygen! Also hydrogen peroxide acts as a mechanical cleaner of the skin so it can actually harm our skin by killing our fibroblast cells. Source: college medical microbiology
The other answers are good to answer how catalase works, but in case your question is why we have this enzyme in the first place instead of letting hydrogen peroxide just stick around, here goes. H2O2 is something that acts as a free radical, giving off OH- ions. These are bad for your cells and can wreak all sorts of havoc. So, your cells have an enzyme that rapidly converts things to water and oxygen, which are much better for your cells.
TL;DR: catalase *’catalyses’* the breakdown of peroxide by forming a temporary intermediate. The formation of this intermediate has a lower **activation energy** than the normal break down of peroxide. Now, what does that mean?
To understand activation energy, imagine there’s a frog in a slippery bowl, hopping around randomly. Its hops are random in height, but they’re within a range. Most of the time, it’ll land on the side and slide back into the bowl. Sometimes, it’ll hop really strongly, or maybe hop twice back to back, and it’ll make it out of the bowl. The amount of energy the frog has to use to hop out the bowl is the activation energy of it jumping out.
Peroxide naturally breaks down into oxygen and water. But it has a high activation energy. Peroxide molecules are all bumping into each other randomly, and transferring energy as the do so. Every now and then, one molecule will get enough energy, and break down – it’ll hop out of its bowl. If you heat up the peroxide, that’s like kicking the bowl as the frog jumps. It makes it easier for it to hop out.
Catalysis is like adding a step into the bowl. It’s easy for the frog to jump on the step, so most frogs do that. It’s also easy for a frog on the step to jump out of the bowl. But it’s difficult for the frog to jump back *in* to the bowl, step or no step. The result is most of your frogs end up outside their bowls.
So what does the ‘step’ actually look like? Peroxide is H-O-O-H. The O-O bond is not that stable, because oxygen is large and greedy for electrons. This means it needs more than a single bond to safely hold two oxygen molecules together. Also, since oxygen is greedy for electrons, and electrons are negatively charged (like a magnet), there’s a strong negative charge focused right around the O-O bond.
Catalase has a little slot that’s perfectly cosy for a peroxide molecule to fit in. And right in the middle, it has a little nub that’s naturally positively charged. This attracts peroxide and makes it stick around. When the peroxide is in this position, it also pulls on the catalase like a magnet.. This makes catalase fold around it, and shoves one of the H atoms in front of a nub with a negative charge. The result is that H holds onto its electron less strongly, so the adjacent O easily steals it. H-O-O-H becomes H+ (sorta-bonded to one part of the catalase) and O-O-H- (sorta-bonded to another part).
This makes the exposed O happier to lose another electron to its sibling O. So the O-O bond breaks as well. But that negative charge on the oxygen is what was keeping the peroxide in place. This means the now severed H+ and OH- ions are thrown out. They’ll quickly hook up and make water. In the mean time, the O- ion that’s left now has a REALLY focused, strong negative charge.
Which means it’ll gladly attack any nearby peroxide molecules, and break it down by basically ripping an O atom away. This again leaves H+ and OH-, which immediately turn into water. The new O2 molecule is very satisfied with its electron state, and much less electrocally charged, and so is no longer that attracted to the catalase, so it floats off. The result is two peroxide molecules have been turned into one O2 and two water molecules. The catalase comes out of the ordeal same as it started.
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