About this video
ATP hydrolysis releases energy because ADP plus phosphate has lower free energy, giving a negative Delta G of about minus 30.5 kJ/mol. Although the reaction is spontaneous, it needs an activation energy hump, which enzymes called ATPases lower by positioning ATP and nearby ions. The released energy is then coupled to useful work, such as phosphorylating a molecule or moving molecules against a gradient.
You'll be able to
- explain why ATP hydrolysis is exergonic with a negative Delta G.
- explain the role of activation energy and how enzymes lower it.
- describe how the energy from ATP hydrolysis is used for cell work.
Key words
- Hydrolysis
- A reaction that breaks a bond by adding water, here removing a phosphate group from ATP.
- Delta G
- The change in free energy; a negative value means a reaction is spontaneous.
- Activation energy
- The energy hump a reaction must get over before it can proceed.
- ATPase
- An enzyme that catalyzes the hydrolysis of ATP and often couples the released energy to other work.
- Nucleophilic attack
- When an electron-rich atom, such as the oxygen in water, bonds to an electron-poor atom, such as phosphorus.
Transcript
- We've talked a lot about ATP being the energy currency of cells, but I want to dig a little bit deeper into that in this video. And as we'll see when we go from ATP to ADP, ADP + a Phosphate group, we have a release of free energy. If we look at just the system, ATP's free energy is over here, but once hydrolysis has taken place and now it's ADP + a Phosphate group, the free energy has dropped by roughly 30.5 KJ/mol our Delta G is -30.5 Kilojoules per mole and if you watch the videos on Gibbs Free Energy this tells us that this is a spontaneous reaction.
Delta, Delta G is less than zero, which tells us that this is going to be spontaneous. Now when I first learned this I was like, well if it's going to be spontaneous why wouldn't all of the ATP just spontaneously and all of the water turn into ADP and just release it's energy as heat or whatever else?
And the key is, it has to get over this hump. You have this activation energy, it has to go uphill a little bit if there's no enzyme to catalyze it. And the reason why we have this uphill hump is the way that ATP gets broken up is that you have to, if we're talking about a water molecule doing the hydrolysis which is typically what people think about hydrolysis, although it can sometimes be done by a different molecule, if you think about the water molecule what needs to happen is, you need this lone pair of electrons on this oxygen to be able to do what we call a nucleophilic attack on this Phosphorous in this phosphate group and if that happens, it forms this bond and then these electrons can be taken back by this oxygen
which gives its negative charge right over there. Now you might say, "Okay, this makes a lot of sense", but you have to rememeber this, electrons are negative and they're surrounded by these negative charges. So they have to overcome getting close to these things so as they're approaching these negative charges, they want to repel each other, so you have to overcome that.
And the way that it's overcome is a class of enzymes called ATPases, ATPase. And what they do is, remember these enzymes are these big protein complexes and the ATP molecule combined in the right place. And they essentially try and surround the ATP molecule with some positive ions.
So let's say there's a positive ion here. So it can keep these electrons busy while the water, or whatever is doing the nucleophilic attack, can not have to worry about these electrons over here. So it might have a positive ion here. And remember, if we think in three dimensions, this thing is all wrapped around in different ways around the ATP molecule, so this is the ATPase.
And so by having the enzyme over here, you're lowering the activation energy. And so it might end up looking something more like this. And so the reaction can actually occur. So the reason why you don't just see this happening all the time without an enzyme is why you have to overcome this hump.
But once you have the enzymes they can actually allow the reaction to occur. So this attacks this, forms this bond, and then you say well, "You have another hydrogen right over here, you have another hydrogen right over here." But this could get nabbed up by another water molecule becoming a hydronium ion, which is actually what's going to happen.
And then, this character takes away this pair of electrons, becomes a negative charge, and we're left with that Phosphate group has been broken off and then we have our ADP and we have our release of energy. Now, typically you don't want this to release energy for no good reason.
Most ATPases are going to leverage that energy that release this reaction to phosphorylate a molecule. And in this case you can kind of think that this hydroxide has been phosphorylated, but it might phosphorylate something else. Or it might change the confirmation of this ATPase so it can do some other type of work, transfer molecules against their concentration gradient or chemoelectric gradient, whatever it might be.
You don't want to just release the energy. Sometimes it might happen if you just want to generate some heat, but usually you're going to do some useful work for the actual cells. Hopefully this gives you a little more appreciation for ATP and the reaction by which the hydrolysis can occur.
This is called hydrolysis because, at least in this example we're using a water molecule to break it up or to take off a phosphate group.
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