Glycolysis 4 - Video Tutorials & Practice Problems
On a tight schedule?
Get a 10 bullets summary of the topic
concept
Glycolysis 4
Video duration:
10m
Play a video:
Video transcript
Step 9 is carried out by enolase and again, this has a delta G close to 0. Meaning, we've got our forward and reverse arrows present. And additionally, let's put in our carbon numbers because again this is important. Your professor likes to test it. So we have 3 or 4, 2 or 5, and 1 or 6. And this reaction is going to take 2 phosphoglycerate and turn it into phosphoenolpyruvate or PEP and I just love calling it PEP. Actually, in the process, we will be releasing an H2O. So we're we're oxidizing this molecule right now and let's put in our carbon numbers on the product here. So 2, 5, and 1 or 6. And let's go ahead and, take a look at step 10. And step 10 carried out by pyruvate kinase. This is another enzyme, really important one that you should know. And you can see that this has a very negative delta g, meaning it's a super favorable reaction. We're going to talk more about that later. And, it takes PEP and ADP and converts PEP into pyruvate and generates an ATP through substrate level phosphorylation. And actually, pyruvate comes out of this reaction in its enol form and is quickly converted, by itself. It converts to the keto form. So, here we're showing, we're showing that keto form but you know, it it actually does immediately come out in the enol form. Anyhow, just to be super thorough, let's number our carbons again. 3425, 16. And then finally, here, 3, 4, 2, 5, and 16. Alright. So you may have noticed that a lot of the glycolytic reactions are readily reversible. Conditions conditions. Because remember that reaction 7, it had a super negative delta g but in cellular conditions, it's actually much closer to 0. That's why it has the forward and reverse arrows. But reactions 1, 3, and 10 are not readily reversible. Those are, reactions 1 and 3 are like what I call the commitment steps. And 10, this last step that converts PEP into pyruvate kinase is also, has a very negative delta g. And these three reactions one are ensuring the whole pathway moves forward in that one direction and that's because again, they have a negative delta G and are very favorable reactions. Now, it's important to note that other molecules actually can feed into glycolysis but it's always going to cost 2 ATP. So, there's no cheating the system really. You know, you're you're gonna have to pay the piper no matter what you do, but you don't necessarily have to start with glucose which is a good thing because your body is not always going to have glucose available to it. So, obviously, if you do start with glucose, you turn it into glucose 6 phosphate and that's what feeds into glycolysis, right? That is the substrate for step 2 of glycolysis. If you have lactose, lactose is a disaccharide and it gets broken up into glucose and galactose. Well, we know what's going to happen to glucose, right? Glucose is going to get converted to glucose 6 phosphate and enter, glycolysis that way. Galactose on the other hand reactions before it's actually ready to enter the glycolytic pathway. Manose or manose gets converted to mannose-six phosphate which gets converted into fructose-six phosphate and that's what enters the glycolytic pathway and fructose gets converted into fructose 1 phosphate and then that is actually broken into glyceraldehyde and DHAP, both of which will get converted into G3P. So we can see this here is fructose. This molecule right here. I'll even write it in. Fructose. Here, we have fructose 1 phosphate. This is fructose 1 phosphate. And let me just jump out of the image here. Right next to that, we have DHAP and G3P. So, here's our DHAP. This is our G3P And the molecule glyceraldehyde, well, that's basically just G3P without that phosphate group, right? G3P is glyceraldehyde 3 phosphate. So basically, what's going to happen is you're just going to add a phosphate on to glyceraldehyde and then you have glyceraldehyde 3 phosphate. And DHAP, well, we already looked at how you turn that into G3P. So the main point is, fructose is going to enter as G3P. Now, glycerol can also enter glycolysis but, and this is going to become more apparent in a moment when we talk about fermentation. Glycerol is not the best choice for entering glycolysis and the reason for that is glycerol gets converted into glycerol 3 phosphate which gets converted into, G3P which gets converted into glyceraldehyde 3 phosphate. And, this process uses 1 ATP and generates 2 ATP. Now remember, we're saying before it always costs 2 ATP. Well, it costs 2 ATP per 6 carbons. This glycerol is only contributing 3 carbons so it only costs 1 ATP. That's where that discrepancy comes in. And it generates 2 ATP. So that's normal, right? You pay 1 ATP per 3 carbons. You get 2 ATP back per 3 carbons. So if we're talking about 6 carbons, that would be paying 2 ATP and generating 4 ATP for the 6 carbons. The problem is that normally for a 6 carbon molecule like glucose, right, we generate 2 NADH, right? So, the problem with glycerol is that for 3 carbons, we generate 2 NADH. So we're generating twice as much NADH as we want to be. And, we'll see why momentarily why this is a problem, why generating this excess NADH means that glycolysis won't be sustained with fermentation. So the end game of all this is that glycerol is really only used in aerobic conditions because it's going to generate, more NADH than, the other feeder molecules. And because of that, we can't rely on fermentation to keep glycolysis going and and this is going to again become apparent when we turn our attention to glycolysis. The last thing I want to point out is glycogen phosphorylate. So, before when we were talking about bio signaling, we talked about how the cell will store its glucose as glycogen. Well, when glycogen gets broken down, it actually doesn't get broken down back into glucose. It gets broken down into glucose 1 phosphate. So glycogen phosphorylase actually breaks down, breaks down glycogen into glucose and phosphorylates it. And that glucose 1 phosphate has to be turned into glucose 6 phosphate to enter glycolysis. This is a simple rearrangement phosphate coming off and it has to be converted into glucose 6 phosphate before it can actually enter glycolysis. Alright. Let's flip the page.