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Current time:0:00Total duration:9:46

ATP hydrolysis: Transfer of a phosphate group

Video transcript

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 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 ATP 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 in 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 poly nucleotide 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 a 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 to ATP molecules make an appearance and specifically they donate one phosphate group each to the phosphate group already on a 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 a DP now there's 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 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 this 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 poly nucleotide 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 involved both the hydroxyl group and 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 and copy and paste this poly nucleotide 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 these 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 a pyro phosphate 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 take away 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 you 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 minds let's also think about this from another perspective let's think about this from the perspective of equilibrium so remember there is a principle in chemistry called lishala a sprin spool and Lashelle EA's principle says that a system such as a chemical reaction will respond to a change in a 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 in our system is that we are degrading our products to remember this phosphate group this pyro phosphate 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 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 energetics 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 zero 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 this topic since our videos 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 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 proceed and so in our example each step of the reaction has a specific enzyme that facilitates or catalyzes each step as a 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