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Main content
Current time:0:00Total duration:8:13
AP.BIO:
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Video transcript

there are all sorts of reactions in biological systems that are energetically favorable but they're still not going to happen quickly or even happen on their own and the phosphorylation of glucose is an example of that and we go into some detail into that on the video on coupled reactions and I think we actually call that the phosphorylation of glucose 6-phosphate but it's super important because by putting the phosphate group on a glucose it's ready to be the input to a whole series of biological mechanisms it allows the glucose to be tagged so it's going to be hard for to escape the cell again and it's a fairly straight it's a fairly straightforward mechanism where you have where you have a lone pair of electrons on this on this hydroxyl group right over here and then it attempts to if it's in the right configuration it could form a bond with the phosphorus in the phosphate group now the reason why it doesn't happen on its own even though it's energetically favourable once you form the bond you have electrons are going to be able to go into a lower energy state so it has a negative Delta G if this is if this is the molecules before the reaction this is how much free energy they have before the reaction after the reaction they have less free energy they have they have been able to release energy so this is something that we would consider to be spontaneous but for the reaction to happen you need a little bit of energy to be put into the system you call this our activation energy you might say well why is that well we have electrons that want to form a bond with this phosphorus but this phosphorous is surrounded by negative charges this oxygen right over here has a negative charge this oxygen right over here has a negative charge and as you can imagine electrons don't like being around other electrons like charges repel each other so in order for this reaction to occur or for it to occur more frequently it has to be catalyzed a catalyst is anything that makes a reaction happen faster or even allows the reaction to happen at all and when we talk about catalysts in biological systems we're typically talking about we're typically talking about enzymes enzymes and the way that an enzyme might catalyze this reaction we actually talk about it it when we talk about coupled coupled reactions as well maybe it can provide some positive charges it can provide some positive charges around these negative charges to pull them further away to create space to create space so that you can actually have the reaction proceed and so what an enzyme would do it would make this curve instead of having this hump on it the curve would look more like this so that the reaction can just proceed but what are these enzymes these things that can you know maybe you could it could play some interesting charge that can allow the cat the reaction to happen a certain way it might bend the molecules in a certain way to to expose some bonds it might have a more acidic or basic environment that might be more more favorable for the reaction what are these seemingly magical things well at a very high level they tend to be these protein complexes plus or minus a few other things so you can view them as proteins and you know maybe sometimes there'll be multiple polypeptide chains put together they might have some other ions associated with them but for the most part they are proteins and the the molecules that are going to react that are going to bind to the proteins we call these the substrates so these in this reaction right here the glucose and the ATP these are going to be the substrates so you can imagine you can imagine the enzyme that does this in the general term for the enzyme that helps that helps phosphorylate a sugar molecule like this we call it a hexo kinase so it might be this crazy looking for we're going to this crazy-looking protein and we're going to take better looks at this in a few moments but the ATP the ATP might bind to it right over there ATP is one of the substrates and then the glucose might bound might bind to it right over there and so this to su subs the two substrates bind in the area where all of this is going on we call that the active site so the active site because that's where all the action is the active site and often when you have the substrates bind they are able to inter act with the protein to make the fit even stronger to make it even more this is more more suitable for the reaction to take place and so the whole the whole protein might bend a little bit to kind of lock these two in place a little bit more and we call that induced fit induced induced fit and so where were these positive charges come from well these would be things that are the side chains of the different amino acids on the actual on the the polypeptide chain on the protein and it could even be other ions that get involved in fact in particular if to facilitate the phosphorylation of glucose a magnesium ion might be involved to help draw some positive charge away but there's other there's other positively charged groups that help draw a charge away so that the reaction is more likely to occur so that's what enzymes are and they tend to be optimally working in certain pH environments or at certain temperatures in general higher temperatures allow more interactions things are bumping around more but if if temperatures get a little bit too high the protein or the enzyme might stop working it might denature it might lose its actual it might lose its actual structure and what I want to now give you an appreciation for is how beautiful and complex these structures are you should appreciate what I'm showing you these are in your cells these are in your area in your and you know look at your hand look at you know everything around you there there's there's a lot of this stuff going on inside of you so hopefully it gives an appreciation for the complexity of you as a biological system but frankly all biological systems so this right over here this is a visualization of a hexokinase one variety of it and just to get a sense of scale this is a glucose molecule and this right over here is an ATP and so they will bind these are the two substrates they will bind at the active site you might have the induced fit where this fits around it it draws some charge away it might bend the molecules in a certain way so that so that they're more likely to interact bring these things close together and so you're going to have the reaction occur and then once the reaction occurs they're not going to want to bind to the substrates anymore I guess you could say the products at that point and then they're going to let go of them and then the enzyme hasn't changed and that's an important property of an enzyme it's not like it just has one use and it goes away it can keep doing this over and over and over again one ends one enzyme will do this many many many many many times in its actual life and so now what I will show you is a little three-dimensional visualization that I got from a website so let me go get that go ahead pause my recording so I could get this this little simulation or this model and this is actually a hexo kinase as well and hexokinase has come in to a bunch of different varieties but this is a pretty neat thing to look at and this has been visualized differently when you look up protein images on the web or anywhere you'll see them sometimes with this ball and stick model sometimes you'll see them in the space-filling models sometimes you'll see them with this kind of where you see the various structures you notice you notice the alpha helixes here that we studied when we talked about protein structures and you can also see some beta sheets but this gives you an appreciation of the binding sites and how these things might interact this right over here that is in that is a molecule of ATP and then right next to it I believe if I'm looking at that right that is a molecule of glucose and notice they have bound either there that they are the two substrates they have bound at the active site and now they can interact with each other the the enzyme the hexokinase in this case can help facilitate the reaction that we care about the phosphorylation of glucose so hopefully images like this and like this give you an appreciation for how complex and how beautiful these things actually are
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