Glycolysis 2 - Video Tutorials & Practice Problems
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Glycolysis 2
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As we move through the glycolytic reactions, the things that I want you to pay attention to are the name of the enzyme at each step which is going to be the first thing you see. The delta G of that reaction which is going to tell you whether or not this is a readily reversible reaction and the more negative the reaction, the less able to be reversed it is or to put it in a less confusing way. If you have a delta G that's close to 0, you have a readily reversible reaction. But the reactions you have, for your negative value, those are going to be what I'm going to call commitment steps because we're going to expend energy and commit to the pathway. So, you also lastly, you want to pay attention to your reactants or substrates I should say, your substrates and your products. So, looking here, the first step of glycolysis is performed by hexokinase. That's our enzyme. We have a pretty negative value for delta g, meaning this is not going to be readily reversible. That's why we have a one way arrow here. And we're going to take glucose or hexokinase is going to take glucose I should say and it's going to turn it into glucose 6 phosphate. It's going to add a phosphate group onto glucose and this actually happens as soon as glucose enters the cell. As soon as glucose enters the cell, hexokinase performs this regardless of whether or not glucose is actually going to, be broken down through And you can see that we have to expend an ATP. And that ATP expenditure is going to allow us to attach that phosphate group onto glucose and we're going to, end up with ADP and a proton because we are breaking is going to leave ADP behind. Alright, moving on to the second reaction, the glycolytic pathway, we have phosphohexoseisomerase and you can see here that the delta g of this reaction is pretty close to 0 and that's why we have these, both ways arrows, right? This means it's readily reversible reaction. And that makes sense because this is an isomerase, right? We're not really doing a whole lot here. We're just rearranging the molecule a little bit. We're taking glucose 6 phosphate and we're turning it into fructose 6 phosphate. And you can see basically we're just changing the ring structure a little bit so that carbon 1 is no longer part of the ring. Now the, ring starts with carbon 2, right? We've moved from a a 6 membered ring to a 5 membered ring. Now, step number 3 carried out by phosphofructokinase. That's an enzyme name that you want to remember because it's going to come up many, many times even beyond this course probably. And phosphofructokinase, you can see has a pretty negative delta G. And we have a one way arrow here, right? This is going to be our second commitment step. And, we have to burn another ATP or expend another ATP and we're going to do that and we're going to phosphorylate fructose 6 phosphate. So we're going to take, fructose 6 phosphate from previous reaction and we're going to add another phosphate group on to it. You can see it right there making it a bisphosphate, 2 phosphates. So, this is our second commitment step. This is going to be the last step of directly investing energy but we're not quite at the energy payoff phase yet. So, remember, in terms of looking at glycolysis as a whole, we expend 2 ATP per glucose. So this is the second ATP we're expending. Now, 4th step of the reaction is performed by aldolase. Let me just hop out of the image here so you can see the whole thing. So, So, notice notice here, aldolase has a pretty high delta g, right? So this is probably making you wonder how this reaction occurs at all. We'll notice that this is the delta g prime naught there, right? Meaning this is a delta g at, you know, standard biochemical, conditions. But actually in in the cell at cellular conditions, the delta G of this reaction is actually somewhere between negative 60. That's why this is going to be a readily reversible reaction. So don't be fooled by this delta G of 23.8 because it's cellular conditions. This is actually going to be more like -six or 0. And what we're going to be doing, we're going to be taking that fructose 16 bisphosphate. I've abbreviated it there to fit it all in one line, Great. So, that's this molecule right here. And we're going to actually we're going to split it up, right, aldolase. We're going to break up that aldol and we're going to break it up into 2 molecules. One of which is glyceraldehyde 3 Phosphate which I'm going to abbreviate G3P. You may have seen this molecule before. Some other, it it's sometimes called PGAL which stands for phosphoglyceraldehyde. I personally prefer G3P, glyceraldehyde 3 phosphate. Or rather preferred G3P as my abbreviation because I think it's closer to the name glyceraldehyde 3 phosphate than PGAL. Doesn't really matter. We're going to call call it G3P. End of story. Now, the other molecule that it gets broken up at into is dihydroxyacetone phosphate or DHAE. So here we have G3P and this is DHAP. Now, G3P is ready to basically continue on in glycolysis. That's going to be the substrate of the next step. DHAP can't just jump into the next step though. DHAP actually has to be converted into G3P which I'm sure you can tell by looking at the structure. Not going to be the hardest chemical reaction to pull off as they're fairly similar molecules. So let's turn the page and take a look at what happens to DHAP.