Gluconeogenesis 2: Videos & Practice Problems

Gluconeogenesis reverses key glycolytic reactions, specifically those catalyzed by pyruvate kinase, phosphofructokinase, and hexokinase. The process begins with pyruvate carboxylase converting pyruvate to oxaloacetate using ATP, followed by PEP carboxykinase transforming oxaloacetate to phosphoenolpyruvate (PEP) with GTP. Fructose 1,6-bisphosphatase then converts fructose 1,6-bisphosphate to fructose 6-phosphate, and glucose 6-phosphatase finalizes the pathway by converting glucose 6-phosphate to glucose, a reaction exclusive to liver cells, crucial for maintaining blood sugar levels.

Gluconeogenesis 2

Let's cover the reactions specific to gluconeogenesis in the order that they appear in the gluconeogenic pathway. That means that the first reactions we're going to talk about are actually undoing the reaction from the very end of glycolysis. So, these first two reactions we're going to talk about are reversing the action of pyruvate kinase or reaction number 10 of glycolysis. The reason that this actually has to be done in 2 steps rather than just 1 is because the action of pyruvate kinase is so favorable, right? It cannot be simply undone in just one step. Pyruvate kinase takes PEP, turns it into pyruvate, generates an ATP in the process. These two steps that undo the action of pyruvate kinase and convert pyruvate back into PEP both require energy input in the form of a nucleoside triphosphate. Pyruvate Kinase generates an ATP. These two reactions are going to burn 2 nucleotide triphosphates to get back to that point, of having PEP. So, the first reaction is carried out by pyruvate carboxylase. This is like our first enzyme of the gluconeogenic pathway and it's going to take pyruvate and turn it into oxaloacetate and it's going to use an ATP to do that. And so we're going to be left with an ADP. And this reaction adds a CO₂ to pyruvate. Here, we have our pyruvate and we're actually going to add CO₂ onto our pyruvate. The next reaction is carried out by PEP carboxykinase and this is going to take that oxaloacetate and that's a name to remember because it's going to become a very important molecule, later in the next unit when we talk about the citric acid cycle. So, PEP carboxykinase is going to take oxaloacetate and turn it into PEP and it's going to use a GTP in the process. I always use the phrase "burn" because I like to think of it as using up the energy from the molecule, but I really just mean break that phosphate bond to harness the energy released. So this reaction actually removes the CO₂ that was just put on in the previous reaction. So, we add the CO₂ and then we get rid of it. But in the process, we add on a phosphate group to our oxaloacetate. And that leaves us with PEP. So we are now back at reaction, well, sort of the end of reaction 9 of glycolysis, but this is actually going to be the beginning of reaction 3 of gluconeogenesis. Moving on to what is reaction 3 of glycolysis which is carried out by phosphofructokinase, or PFK1 as I'm abbreviating it here. So phosphofructokinase, you might recall, is going to take fructose 6-phosphate and add another phosphate group onto it. It's going to be phosphorylated. So, the enzyme that's going to reverse the action of PFK 1 is fructose-1,6-bisphosphatase, right? Phosphofructokinase using a phosphatase to remove a phosphate group. So we're going to take fructose-1,6-bisphosphate, remove a phosphate group, and we are left with fructose 6-phosphate. And of course, this is undoing step 3 from glycolysis. And lastly, undoing the very first reaction of glycolysis, that task is left up to the enzyme Glucose 6 Phosphatase. So, remember, hexokinase, that's the first enzyme of the glycolytic pathway, and it's going to act on glucose as soon as it enters the cell and phosphorylate it. Again, we're undoing the action of a kinase with a phosphatase here and going to remove that phosphate group from glucose 6-phosphate and turn it back into glucose. And it's worth noting that this enzyme, glucose-6-phosphatase, is actually only present in liver cells. And that's very important because gluconeogenesis is one of the big functions of the liver. And if you get more into anatomy and physiology, you'll see that the liver plays a huge role in maintaining blood sugar levels. So it makes sense that gluconeogenesis is one of the main jobs of liver cells and so it should also make sense that the enzyme needed to complete this process and turn our glucose 6-phosphate into actual glucose, ready to hit the bloodstream and be delivered to the cells that need more glucose, that enzyme would only be present in the liver where this process is being carried out. Alright. So, just to recap, we are reversing reactions 10, 3, and 1 of glycolysis with the 4 enzymes presented on this page. And reaction 10 of glycolysis takes 2 enzymes to actually undo that step. All right. Let's flip the page.

Here’s what students ask on this topic:

What are the key enzymes involved in gluconeogenesis?

The key enzymes involved in gluconeogenesis are pyruvate carboxylase, PEP carboxykinase, fructose 1,6-bisphosphatase, and glucose 6-phosphatase. Pyruvate carboxylase converts pyruvate to oxaloacetate using ATP. PEP carboxykinase then transforms oxaloacetate to phosphoenolpyruvate (PEP) using GTP. Fructose 1,6-bisphosphatase converts fructose 1,6-bisphosphate to fructose 6-phosphate. Finally, glucose 6-phosphatase converts glucose 6-phosphate to glucose, a reaction exclusive to liver cells, crucial for maintaining blood sugar levels.

Why does gluconeogenesis require two steps to reverse the action of pyruvate kinase?

Gluconeogenesis requires two steps to reverse the action of pyruvate kinase because the reaction catalyzed by pyruvate kinase is highly favorable and cannot be simply undone in one step. Pyruvate kinase converts phosphoenolpyruvate (PEP) to pyruvate, generating ATP. To reverse this, pyruvate carboxylase first converts pyruvate to oxaloacetate using ATP. Then, PEP carboxykinase converts oxaloacetate to PEP using GTP. These two steps ensure the energy barrier is overcome, allowing the pathway to proceed.

What role does glucose 6-phosphatase play in gluconeogenesis?

Glucose 6-phosphatase plays a crucial role in gluconeogenesis by converting glucose 6-phosphate to glucose. This reaction is the final step in the gluconeogenic pathway and is essential for maintaining blood sugar levels. Notably, glucose 6-phosphatase is only present in liver cells, which are primarily responsible for gluconeogenesis. This enzyme's activity ensures that glucose can be released into the bloodstream to supply energy to other tissues and organs.

How does gluconeogenesis differ from glycolysis?

Gluconeogenesis and glycolysis are essentially reverse processes. Glycolysis breaks down glucose into pyruvate, generating ATP, while gluconeogenesis synthesizes glucose from non-carbohydrate precursors like pyruvate, consuming ATP and GTP. Key differences include the enzymes involved: glycolysis uses hexokinase, phosphofructokinase, and pyruvate kinase, whereas gluconeogenesis uses glucose 6-phosphatase, fructose 1,6-bisphosphatase, PEP carboxykinase, and pyruvate carboxylase. Additionally, gluconeogenesis primarily occurs in the liver, whereas glycolysis occurs in almost all cells.

Why is oxaloacetate an important molecule in gluconeogenesis?

Oxaloacetate is a crucial intermediate in gluconeogenesis. It is formed from pyruvate by the action of pyruvate carboxylase, using ATP. Oxaloacetate is then converted to phosphoenolpyruvate (PEP) by PEP carboxykinase, using GTP. This molecule is not only vital for reversing the action of pyruvate kinase but also plays a significant role in the citric acid cycle, linking gluconeogenesis to other metabolic pathways. Its formation and utilization are essential for the efficient production of glucose from non-carbohydrate sources.