Glycolysis 4 - Online Tutor, Practice Problems & Exam Prep

In glycolysis, step 9 involves enolase converting 2-phosphoglycerate to phosphoenolpyruvate (PEP), releasing H₂O. Step 10, catalyzed by pyruvate kinase, converts PEP to pyruvate, generating ATP through substrate-level phosphorylation. Key irreversible steps (1, 3, and 10) drive the pathway forward. Various substrates, including lactose and glycerol, can enter glycolysis, but glycerol generates excess NADH, limiting fermentation. Glycogen is broken down to glucose-1-phosphate, which is converted to glucose-6-phosphate to enter glycolysis, illustrating the interconnectedness of metabolic pathways.

Glycolysis 4

Step 9 is carried out by enolase and again, this has a ΔG close to 0, meaning we've got our forward and reverse arrows present. Additionally, let's put in our carbon numbers because again this is important. Your professor likes to test it. So we have carbons 3 or 4, 2 or 5, and 1 or 6. This reaction is going to take 2-phosphoglycerate and turn it into phosphoenolpyruvate (PEP), and I just love calling it PEP. Actually, in the process, we will be releasing an H2O. So, we're oxidizing this molecule right now and let's put our carbon numbers on the product here. So 2, 5, and 1 or 6. Let's go ahead and take a look at step 10. Step 10 is carried out by pyruvate kinase. This is another enzyme, a really important one that you should know. And you can see that this has a very negative ΔG, meaning it's a super favorable reaction. We're going to talk more about that later. 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 the keto form but you know, it actually does immediately come out in the enol form. Anyhow, just to be super thorough, let's number our carbons again: 3, 4, 2, 5, and 1 or 6.

You may have noticed that a lot of the glycolytic reactions are readily reversible. This is because, remember that reaction 7, it had a super negative ΔG 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 what I call the commitment steps, and 10, this last step that converts PEP by pyruvate kinase, also has a very negative ΔG. These three reactions ensure that the whole pathway moves forward in that one direction because they have a negative ΔG and are very favorable reactions.

It's important to note that other molecules can also feed into glycolysis, but it's always going to cost 2 ATP. So, there's no cheating the system really. You know, you're going to 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, needs a few reactions before it's actually ready to enter the glycolytic pathway. Mannose gets converted to mannose-6-phosphate, which gets converted into fructose-6-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

Here’s what students ask on this topic:

What is the role of enolase in glycolysis?

Enolase catalyzes the ninth step of glycolysis, where it converts 2-phosphoglycerate (2-PG) into phosphoenolpyruvate (PEP) while releasing a molecule of water (H₂O). This reaction is crucial because it prepares the substrate for the final step of glycolysis, where ATP is generated. The reaction has a ΔG close to 0, indicating it is readily reversible under cellular conditions. Enolase's activity is essential for maintaining the flow of metabolites through the glycolytic pathway.

Why are steps 1, 3, and 10 of glycolysis considered irreversible?

Steps 1, 3, and 10 of glycolysis are considered irreversible because they have highly negative ΔG values, making them energetically favorable and driving the pathway forward. Step 1 involves hexokinase converting glucose to glucose-6-phosphate. Step 3 involves phosphofructokinase converting fructose-6-phosphate to fructose-1,6-bisphosphate. Step 10, catalyzed by pyruvate kinase, converts phosphoenolpyruvate (PEP) to pyruvate, generating ATP. These steps are crucial for regulating glycolysis and ensuring the pathway proceeds in one direction.

How does glycerol enter the glycolytic pathway?

Glycerol enters the glycolytic pathway by first being converted into glycerol-3-phosphate, which then gets oxidized to dihydroxyacetone phosphate (DHAP). DHAP is an intermediate in glycolysis and can be readily converted to glyceraldehyde-3-phosphate (G3P). However, glycerol's entry into glycolysis is less efficient under anaerobic conditions because it generates excess NADH, which can disrupt the balance needed for fermentation. Therefore, glycerol is primarily utilized in aerobic conditions where NADH can be reoxidized in the mitochondria.

What happens to pyruvate after glycolysis?

After glycolysis, pyruvate can undergo several fates depending on the cellular conditions. Under aerobic conditions, pyruvate is transported into the mitochondria and converted into acetyl-CoA by the pyruvate dehydrogenase complex, entering the citric acid cycle for further energy production. Under anaerobic conditions, pyruvate can be reduced to lactate in animals (lactic acid fermentation) or to ethanol and CO₂ in yeast (alcoholic fermentation). These pathways regenerate NAD⁺, allowing glycolysis to continue in the absence of oxygen.

How does glycogen enter the glycolytic pathway?

Glycogen enters the glycolytic pathway through its breakdown into glucose-1-phosphate by the enzyme glycogen phosphorylase. Glucose-1-phosphate is then converted to glucose-6-phosphate by the enzyme phosphoglucomutase. Glucose-6-phosphate is a key intermediate in glycolysis and can directly enter the pathway at the second step, bypassing the initial ATP-consuming step of converting glucose to glucose-6-phosphate. This allows for a more efficient use of stored glycogen for energy production.