Usually, our bodies are pretty keen on breaking apart glucose molecules, like I said in my post on glycolysis, and using them for energy. However, there’s another important process concerning glucose, and it’s the conceptual opposite: the building up of glucose from pyruvate. Although lots of cells do it, the liver is especially happy to carry out this reaction; after all, it’s got to make sure that you don’t die of hypoglycemia after taking a thirty minute nap.
You might expect gluconeogenesis to be the exact opposite of glycolysis, but that’s not quite the case. It takes a skosh more energy, and it has its own fancy name: gluconeogenesis. Thankfully, though, despite its subtle differences, it’s a lot easier to understand after you’ve got glycolysis under your belt.
All right! (Monosomy X), let’s goooo!
Gluconeogenesis is the metabolism through which two pyruvate are reduced into a glucose molecule. This reaction has a lot of similarities to the formation of pyruvate from glucose, but of course, like anything in biology, it’s decided to be a bit obstinate. You can tell just by looking at the net reaction that it’s a bit of a “problem child:”
2 Pyruvate + 4 ATP + 2 GTP + 2 NADH —> 1 Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ + 6 H+
As you’ll notice, the primary difference between gluconeogenesis and what would be “reverse glycolysis” is the amount of energy it requires. If we just reversed glycolysis, we would expect that, overall, it would take 2 ATP to convert pyruvate to glucose. Instead, it takes not only 4 ATP, but also 2 GTP (another nucleotide triphosphate that stores energy). That’s three times more energy than “reverse glycolysis.” What the frickity frack?
Well, it turns out that there’s a very good reason for this. If you think back to glycolysis, you’ll remember that the whole pathway had three irreversible reactions in it. When you think that, your train of thought probably jumps to my next point: oh. That’s why you can’t just reverse glycolysis.
Yep, gluconeogenesis is different from “reverse glycolysis” because three of the reactions in glycolysis just won’t go backwards. Instead, for those three reactions, gluconeogenesis has to find its own way to do things. That’s where the energy comes in.
The first step that gluconeogenesis has to do for itself is Step 1 of glycolysis. This will be Step 11 of gluconeogenesis (the last step). If you remember, in glycolysis, hexokinase irreversibly transfers a phosphate from ATP to ADP. Well, if hexokinase can’t take that phosphate off again, what’s a cell to do?
Welp, it turns out gluconeogenesis bears a very strong resemblance to my friend Fritz, because it figured this one out in a way that’ll make you want to facepalm. In glycolysis, we used a whole ATP molecule to put that phosphate on. In gluconeogenesis, we pop it off with water. Freaking water.
Yup. Gluconeogenesis uses glucose-6-phosphatase, a hydrolase enzyme, to hydrolyze the phosphoester bond. This is spontaneous, too. Way to go, gluconeogenesis. I have to give you a slow-clap for that one.
M’kei, m’kei, so, so far, so good. However, you know that bad things are coming, because we somehow have to use six “energy molecules” between this step, the last step, and the beginning.
Well, remember that the next step that is irreversible in glycolysis is Step 3, the transfer of a phosphate group to fructose-6-phosphate using phosphofructokinase. In gluconeogenesis the reversal of this happens at Step 9. So, what are we doing here? We’re going to use energy, right? Do some fancy thermodynamic footwork? Tell energetics where to go?
Nah, we gonna use another hydrolase.
To be more precise, fructose-1,6-bisphosphatase uses a water molecule to cleave off one of the phosphates (the 1-phosphate) from the sugar. This is also spontaneous, since you’re releasing energy from a phosphoester bond.
“You’re kidding me.” Nope! Turns out, cells are pretty darn efficient!
But now, dearest reader, we do have to do some heavy lifting.
Remember the last step of glycolysis? The step in which we used that really high-energy molecule, phosphoenolpyruvate, to make ATP and pyruvate? Well, for gluconeogenesis to start, we’ll have to do the opposite—the cell will have to make that really high energy compound from pyruvate.
Now you’re looking at those six energy-containing molecules and doing something a little like this.
Yup. Not only is this where we use all of our energy, but it’s where we get the extra step that glycolysis doesn’t have (remember, I said gluconeogenesis has eleven steps!). Turns out, phosphoenolpyruvate is a pain in the neck to make.
In Step 1 of Gluconeogenesis, an enzyme called pyruvate carboxylase, which is dependent on a vitamin called biotin, uses ATP and carbonic acid (you can also think of it as CO2) to put a carboxylate group on the end of pyruvate. This must be a flux reaction, and it’s spontaneous. The product is a molecule called oxaloacetate.
Once you’ve made oxaloacetate, Step 2 uses a molecule of GTP (per each pyruvate, remember) to decarboxylate oxaloacetate and simultaneously phosphorylate it to phosphoenolpyruvate. Although the phosphorylation is really the “tricky part,” since the phosphate is what traps the molecule in the enol form, phosphoenolpyruvate carboxykinase (a lyase) cleverly uses the decarboxylation of oxaloacetate to force it to happen. See?
Step 1: 2 Pyruvate —> 2 Oxaloacetate using ATP and CO2 by the enzyme Pyruvate Carboxylase (Class 6) | -2 ATP (+2 ADP + 2 Pi)
Step 2: 2 Oxaloacetate —> 2 Phosphoenolpyruvate using GTP by the enzyme Phosphoenolpyruvate Carboxykinase (Class 4) | – 2 GTP (+ 2 GDP + 2 Pi)
Step 3: 2 Phosphoenolpyruvate —> 2-Phosphoglycerate by Enolase (Class 5)
Step 4: 2 Phosphoglycerate —> 3-Phosphoglycerate by Phosphoglycerate Mutase (Class 5)
Step 5: 2 3-Phosphoglycerate —> 2 1,3-Bisphosphoglycerate by Phosphoglycerate Kinase (Class 2) | – 2 ATP (+ 2ADP + 2 Pi)
Step 6: 2 1,3-Bisphosphoglycerate —> 2 Glyceraldehyde-3-Pi by Oxidoreductase (Class 1) | – 2 NADH (+ 2 NAD+)
Step 7: Glyceraldehyde-3-Pi to Dihydroxyacetone-Pi by Triose Phosphate Isomerase (Class 5)
Step 8: Dihydroxyacetone-Pi + Glyceraldehyde-3-Pi to Fructose-1,6-Bisphosphate by Aldolase (Class 4)
Step 9: Fructose-1,6-Bisphosphate to Fructose-6-Pi by Fructose-1,6-Bisphosphatase (Class 3)
Step 10: Fructose-6-Pi to Glucose-6-Pi by Glucose-6-Phosphate Isomerase (Class 5)
Step 11: Glucose-6-Pi to Glucose by Glucose-6-Phosphatase (Class 3)
This is consistent with our net equation, eh? Funny how things work out like that.
Now that we’ve talked about gluconeogenesis for a bit, let’s talk about how our cells decide when and where to use it. We’ll have to split our conversation into two separate parts: extrahepatic (nonliver) cells, and hepatic (liver) cells.
In extrahepatic cells, a high energy charge means that the cell should be storing glucose rather than trying like a crazy person to get energy out of it. Insulin, that famous hormone released by the pancreas, makes fructose-1,6-bisphosphatase more active, which makes gluconeogenesis move faster and produce more glucose-6-Pi. Insulin also inhibits phosphofructokinase, preventing glycolysis from moving very quickly.
Note that I said that that gluconeogenesis makes glucose-6-phosphate, not glucose. Why is that? Well, it’s the same reason that insulin modulates fructose-1,6-bisphosphatase instead of glucose-6-phosphatase—in nonliver cells, glucose-6-phosphatase isn’t expressed. Instead, the cell takes glucose-6-phosphate and makes glycogen (animal starch). That saves it some time and energy, because, really, the only reason you’d want to make straight-up glucose is if you were going to shuttle it into the blood.
Anyway, another hormone, called glucagon, exacts the opposite effect on both of these pathways. When energy charge is low, glucagon signals some kinases that end up making fructose-2,6-bisphosphate. This acts as an allosteric activator for phosphofructokinase (hey, glycolysis, wake up!) and an allosteric inhibitor of fructose-1,6-bisphosphatase (stahp using my energy, gluconeogenesis!). Similar effects regulate pyruvate kinase and phophoenolpyruvate carboxykinase in the last step of glycolysis/second step of gluconeogenesis. In this way, in extrahepatic cells, gluconeogenesis and glycolysis are inversely regulated.
Now, what about if you’re a liver cell? A kind, benevolent, selfless liver cell? Well, then you do things a little differently, because you’re a good guy.
In liver cells, glucagon decreases the amount of that signaling molecule fructose-2,6-bisphosphate. That means that gluconeogenesis, which is repressed by F2,6BP, increases in the liver, and energy consumption (catabolism) increases. Gluconeogenesis proceeds, and the cell makes lots of glucose. (And yes, that is glucose—hepatic cells have glucose-6-phosphatase, because they dump their glucose in the blood.)
This might seem a bit contrary, but, like I said, liver cells (and kidney cells and intestinal cells) are really quite nice guys. When blood glucose gets low, instead of pulling a muscle cell and going, “#$%@! I don’t have ATP! Better make some!” they think, “Oh, man, if I’m running out of glucose, so is the rest of the body—better make some for it, with the energy I have left!” They’re quite selfless like that. It’s really a lovely thing.
As you probably guessed, hepatic cells also act quite a lot like Grandma when it comes to making themselves ATP. Once blood glucose levels are normal (or when they’re high), glucagon signalling stops, and fructose-1,6-phosphatase slows down a bit. In its place, phosphofructokinase starts up glycolysis, and the liver cell finally helps itself to some of that energy that it was so carefully preparing for the rest of the body.
[Ahem] Okay. Were you creeped out by my apparent affection for hepatic cells? Don’t worry, I kind of worry myself a bit there, too. I’ll tell you the reason for my newfound appreciation for them, but that’ll have to wait until my next post, which will wrap up my test material. (Aww. I’m sure you’re gonna miss me. Right? … Right?) Yep, next time, we’re going to look in greater detail at glycogen degredation and biosynthesis.
You think I’m kidding about that “affection for liver cells” thing. I see your skepticism, and I raise you a, “Bro. Bro.”