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Current time:0:00Total duration:6:59

Reaction coupling to create glucose-6-phosphate

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Video transcript

it's super valuable in biological systems to be able to take a glucose molecule and to phosphorylate it so let's start with a glucose molecule and phosphorylated and the reason why is once you have this phosphate group let make sure I put that charge right over there once you have this phosphate group or once you have this negative charge on this glucose 6-phosphate it becomes much harder for it to leave the cell the cell wants to hog as many glucose molecules as it can when the glucose isn't charged it's able to pass through the cellular membrane but then once it becomes phosphorylated it's going to stay in the cell and glucose 6-phosphate right over here this is a very important input to a whole series of processes inside of cells now unfortunately this this this make this reaction of taking glucose and phosphorylating it it requires energy it's endergonic it's not going to happen spontaneously it has a positive Delta G it is ender and ER it is endergonic and so you can imagine what we're going to need to make it happen we're going to have to use the energy currency of the cell our good friend ATP and the way that we're going to make this reaction happen is we're in a couple what's essentially you could view it as a hydrolysis of ATP although we won't have exactly a water molecule in the mechanism but what's what's functionally the hydrolysis of ATP into ADP and a phosphate group which is very energetically favorable it is exergonic it would happen spontaneously under the right conditions it won't just always happen in inside of an aqueous solution it needs a little bit of activation energy or an enzyme to lower the activation energy but the net reaction it is exergonic so we can do is we can couple these two reactions and so when we couple the two reactions when we couple the two reactions we have ATP ATP plus glucose plus glucose reacting and there we use an enzyme that the general term for it is hexokinase to facilitate this reaction to lower the activation energy it's going to yield it's going to yield glucose 6-phosphate glucose glucose six phosphate phosphate and ADP and and ADP a d B now what's the Delta G for this reaction going to be well it's a coupled reaction you can view it as a combination of these two reactions and so roughly speaking you can say well let's just add the Delta G's so if you add the Delta G's here you're going to get if you add this negative Delta G's exergonic and this positive Delta G you're going to get negative thirty point five plus thirteen point eight that's going to be negative sixteen point seven kilojoules kilojoules per mole and so this coupled reaction is going to be exergonic not quite as extra Ghatak as a hydrolysis because now you're going to be using some of that energy but this can happen spontaneously especially if you can lower the activation energy enough for it to happen and so let's now look at the mechanism of how it happens now without an enzyme without an enzyme the way that this reaction needs to occur is that you have an electron you have an electron pair right over here on this hydroxyl group that it needs to do what's called a nucleophilic attack on this on this phosphorus right over here but without an enzyme it's going to be very hard for it to do it's going to have a high activation energy because it's going to be impaired by all of this negative charge from these oxygens right over here you can imagine electrons don't like going through a lot of negative charge they're repulsed by negative charge so we're going to need an enzyme to help facilitate this reaction to help lower the to help lower the energy to actually start and essentially and get these electrons out of the way and the enzyme or the general term for the enzymes that do this it's called hexokinase and hexokinase let me write this down heck so kinase and the way it does is it provides ions to one way to think about is to keep these electrons over here busy and in particular it has a magnesium ion a magnesium ion right over here and that's this is bound to the rest of the hexokinase remember this is all happening in three mentioned so the hexokinase is kind of wrapping around it so these can these can keep these electrons busy there's other ions on the hexokinase that can keep these electrons busy other positive ions keep these electrons busy and so these electrons can sneak in and do the nucleophilic attack and remember when we talk about enzymes these are these protein these protein let me do the same color that I wrote the hexokinase in these are these complex protein structures right over here just like this and so you might have the magnesium ion let me do that in that purple color just right over there and then maybe the glucose molecule the glucose molecule gets bound right over here and then maybe you have your ATP gets bound right right over here and I'm obviously I'm just kind of giving you an example this isn't exactly what's happening but by getting by essentially wrapping it with this positive charge it's all able to pull the electrons away to help facilitate this nucleophilic attack that needs to happen for the reaction to proceed and so this bond right over here between this oxygen and this and this phosphorus that is going to be that is going to be this bond right over here and as this happens then then this character then these these two electrons can be taken by this character and so that this oxygen is this oxygen right over here and now has a negative charge and so what we've just resulted with is glucose 6-phosphate and ADP and ADP and it's go it's energetically favorable it's exergonic it's going to happen assuming that you have the enzyme there to to help distract these these electrons lowering the activation energy and I know what you're thinking we had this hydrogen right over here so this hydrogen should be right over here still and then another water molecule could come and nab the proton the the hydrogen proton and so you're left once again with just the glucose 6-phosphate so this hopefully this gives you a sense of how reaction coupling occurs and also a sense of how ATP is actually useful when I first learned about it to be I'm like okay fine it's you know that really wants to let go of this phosphate group it's energetically favorable but how is that actually used to to to to drive things to to actually do a to do things in the system that might not be energetically favorable and hopefully this gives you a sense of how it's done and also the importance the importance of an enzyme in in facilitating it
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