ATP hydrolysis: Transfer of a phosphate group

Oftentimes in biochemistry, we write energy-requiring processes such as in the conversion of a monomer to a larger polymer as one-step reactions that are ultimately fueled by a separate reaction involving the breakdown, or hydrolysis-- remember, hydrolysis just means reaction with water-- of ATP to produce ADP and a free phosphate group. Now, when we write it this way, it implies that the breakdown of ATP is entirely separate from the biosynthesis reaction that it's fueling. When in fact, in the body, coupled reactions such as these often occur simultaneously. In fact, almost always, ADP is directly involved in the chemical reaction that it's fueling by directly donating or transferring one of its phosphate groups to the reaction it's involved in. And this case, in our example, it donates a phosphate group directly to the monomer. To understand this mechanism even better, let's go ahead and take a look at a more tangible example in which we take a monomer and convert it into a polymer. So let's actually go ahead and take a look at building up nucleotides into a polynucleotide chain such as DNA. So for the sake of simplicity, I'm going to go ahead and just draw a symbol for our nucleotide instead of writing out the entire chemical formula. But let's go ahead and review what the components of a nucleotide are. So remember that a nucleotide is composed of a sugar and nitrogenous base along with a phosphate group. And I'm going to highlight this phosphate group, because it will be particularly important in the covalent bonds that form between nucleotides to form DNA. In addition, the sugars have a hydroxyl group that are also involved in these bonds. I'm going to highlight that here as well. In the very first step of the reaction, two ATP molecules make an appearance. And specifically, they donate one phosphate group each to the phosphate group already on the nucleotide. Let's go ahead and draw out what our products look like. So we have our nucleotide and our untouched hydroxyl group. But now, instead of one phosphate group, we have a total of three phosphate groups that are covalently bonded to one another. And because ATP gave up its phosphate group, we are left with two molecules of ADP. Now, there is one point of confusion when we're talking about the breakdown of ATP into ADP. Many of us fall into this habit of saying that it's the breakdown of ATP into ADP that provides the energy for a reaction. But it turns out that in this step the reaction in which we're essentially stacking up all these negatively charged phosphate groups that don't want to be next to each other has a positive delta G value and cannot drive the reaction forward. But what does drive the overall reaction forward is the second step of the reaction. So let's talk about what happens in the second step. In the second step of the reaction, in comes a growing polynucleotide chain. So I'm going to go ahead and just draw two of these nucleotides. And remember that on one end of the growing polynucleotide chain is a hydroxyl group, which I'm highlighting in green. And on the other end is a phosphate group, which I'm highlighting in blue. Bonds between the nucleotides involve both the hydroxyl group in green and the phosphate group in blue. In this step of the reaction, the hydroxyl group of the growing polynucleotide chain essentially displaces the phosphate groups that were added in the previous step of the reaction. Now, recall that this is possible because having three phosphate groups all crowded together like this makes the electrons in the bonds between the phosphate groups very energetically unstable. Think about all that negative charge that's close together that doesn't want to be. So let's go ahead and draw out what our final product looks like. And I'm going to go ahead copy and paste this polynucleotide chain so we don't have to redraw all of it. So here it is. Go ahead and erase the parts that we don't need here. And now at the hydroxyl end, which is this green end right here, we have covalently bonded our new nucleotide. And so I'm just going to color code this by noting that the nucleotide with the stripes is the one that we're adding. And this covalent bond is between the hydroxyl group as well as the phosphate group, which I'm indicating here in blue. And the other hydroxyl group remains on the other end of the nucleotide ready for another nucleotide to attach. Now, let's not forget the paired phosphate group that was displaced in forming this covalent bond. This paired phosphate group, which has a name that you don't need to remember-- it's called the pyrophosphate group-- undergoes a hydrolysis reaction, that is, reaction with water, to produce two independent phosphate groups. Now, the particular reaction mechanism isn't that relevant. But just so you know, the water essentially inserts itself asymmetrically in between these two phosphate groups and basically donates a hydroxyl group to one and a hydrogen to another. And because the split was asymmetric, these two phosphate groups still remain identical to one another. Now, the key point to takeaway here is that this reaction with water, this hydrolysis reaction, is very energetically favorable. Specifically, we say that it has a fairly large negative delta G value. And so it is this step of the reaction that ultimately drives this entire reaction forward. Now, if that doesn't fully click in your mind, let's also think about this from another perspective. Let's think about this in the perspective of equilibrium. So remember, there's a principle in chemistry called Le Chatelier's principle. And Le Chatelier's principle says that a system, such as a chemical reaction, will respond to a change in the system, such as a change in temperature or change in concentrations of the reactants or products, by shifting in the direction that counters that change. So in this case, what is the change in our system? Well, the change our system is that we are degrading our products. So remember, this phosphate group, this pyrophosphate group, is being broken down into individual phosphate groups. And so essentially what we're doing is we are removing the products of our reaction. And if we do this, the reaction will shift in the forward direction to produce more products. And the shift in the forward direction pushes this reaction forward. So ultimately, the reason I wanted to go through this mechanism with you was to really show you that ATP provides energy to a reaction by donating one of its phosphate groups or transferring one of its phosphate groups, and not through a simple hydrolysis reaction, even though that's often how it's written for convenience. Now, one question that I had about ATP hydrolysis when I was learning it, was that if this overall reaction here, this coupled reaction, is energetically favorable, so this overall reaction has a negative delta G value, why doesn't ATP just donate its phosphate groups to any substrate in the body? Because ATP is floating around in an aqueous solution, so potentially it could donate its phosphate group to water and expend a lot of wasteful energy. The answer to this question involves reminding ourselves of the two factors that control whether or not a reaction will occur. So the first one is energetic. So is this reaction energetically favorable? And we usually look at the value of delta G. In this case, our reaction is energetically favorable because delta G is less than 0. But the other factor that controls whether or not a reaction will occur is kinetics. And we usually talk about a parameter called the activation energy, which I'm going to abbreviate here as E sub A. And it turns out, in this example, ATP has a very high activation energy. And I'm just going to skim the surface of the topic since our video's devoted just to this topic alone. But essentially, we say that this reaction is kinetically stable because it has a very high activation energy. And the way that our body deals with this is that our body has enzymes that can lower the activation energy and allow the reaction to proceed when it should precede. And so in our example, each step of the reaction has a specific enzyme that facilitates or catalyzes each step of the reaction. And this is actually very beneficial for the body, because by having an energetically favorable but kinetically stable reaction, the body can selectively control whether or not energy will be used by controlling the enzymes that are available for the reaction to proceed. And so it's a very elegant system. And I hope that ultimately, this entire video has given you a better picture of how ATP fuels energy requiring processes on a molecular level.