Subtitles section Play video Print subtitles [Ahern laughing] Student: Do it. Ahern: Do it. [laughing] I'm guessing if I gave everybody who came to class an A today then they'd never come to class again. That's just my hunch. Which would kind of be a self-defeating thing, right? So... Student: Not necessarily. Ahern: Maybe we would, right? What she should say is, "Ahern, you're a scientist, "let's do the experiment and fight out," right? Student: Exactly. Ahern: Well, we can't do that. Student: You can give us extra credit. Ahern: I could give you extra credit. There's a lot of things I could do. I could give you money. [class laughing] We could go have beer. We could have pizza. Student: How many of us would not get in trouble for you buying beer. Some of us are still under age, so... Student: Yeah, you could get in a lot of trouble. Ahern: No, actually the way I do that is I go to a place where they can serve people underage and you have to show an ID so it's not my responsibility. A lot of energy. I hope everybody's got a big Thanksgiving planned. Wild plans? Student: Family. Ahern: Oh. Family, huh? Like I said, if any of you are in town and would like to come over, give us a holler and I'll let you know where we're going to live and everything, but if you'd like to come over, we've got plenty of turkey and other things. And no, I won't get you drunk. But we'll have a good time. Today we're going to have a good time because we're going to be thinking of the making of glucose. I know that for many of you, that's been something you've dreamed of doing and you're going to get that dream today. Happy days are here again. Glucose synthesis is an interesting process. The phenomena of course is known as gluconeogenesis and it is a pathway that is very similar, very similar to glycolysis. Very similar. It's very similar to the reverse of glycolysis. However, there are important differences and specifically, there are two reactions. I'm sorry, specifically that there are 3 reactions. There are 3 reactions in gluconeogenesis, in glycolysis that are replaced by 4 reactions in gluconeogenesis. So gluconeogenesis has 11 steps, glycolysis had 10. One of the steps takes two steps to get around. So it's 2 step. If you learned glycolysis, gluconeogenesis for 8 of the steps, Let's get that right for 7 of the steps. I can't get my head right today. for 7 of the steps is identical to glycolysis except for in the reverse. Same enzymes, same intermediates going to the opposite direction. Three of the steps that are in glycolysis as I said are replaced by 4 steps. So let's take a quick look at that. Before I take a look at that, I'll show you something your book is distracted by and that is this process here, which is the making of glucose from glycerol. Why do we care about the making of glucose from glycerol. One of the reasons we care about the making of glucose from glycerol is glycerol is a byproduct of fat metabolism and so it turns out that the only portion of the fat molecule that can be converted directly into glucose is the glycerol. We don't convert fat into glucose for the most part, with the exception of the glycerol. I just show you this, I'm not going to go through and expect you to memorize these are anything, but just show you that glycerol is a 3-carbon molecule. It gets made in a couple of reactions into an intermediate in glycolysis, dihydroxyacetone phosphate. And of course, once it's dihydroxyacetone phosphate, we can now do the upwards pathway, going into making glucose via gluconeogenesis. And we see this, this is a phenomena you've seen before. We saw how other sugars entered glycolysis and gotten broken down by being converted into glycolysis intermediates. We saw, for example, fructose got converted into fructose-6-phosphate and then got metabolized as an intermediate in glycolysis. In this case, we see glycerol being converted into an intermediate in glycolysis or gluconeogenesis, it can actually go either way, and be made into either glucose or ultimately into pyruvate. Let's focus on gluconeogenesis since that's our main topic of the day. You'll notice in looking at the screen that we oriented very much like we oriented glycolysis except that we're going upwards in gluconeogenesis where as we were going downwards in glycolysis. So we start at the bottom and the place where we will start gluconeogenesis is actually pyruvate. But again, we remember that all these designations about where something starts and stops is really arbitrary. We could just as easily start it at lactate for some types of metabolism. We can start at amino acids as well, but we're going to start right here at pyruvate. So starting at pyruvate, and that's a good place to start because that's where we finished glycolysis, starting with pyruvate, cells can convert pyruvate into glucose. Well, not surprisingly, if pyruvate is a 3-carbon molecule and we want to make a 6-carbon glucose, we need to have two pyruvates to start everything. We're going to start with 2 of everything and eventually they're going to combine into 1 as we get higher up in the pathway. What we discover in gluconeogenesis is the first instance that we see of a phenomenon I call sequestration meaning we're sequestering something. All of glycolysis occurs in the cytoplasm of the cell. All of the enzymes of glycolysis are found in the cytoplasm of the cell. In the case of gluconeogenesis, we see 2 enzymes that are not found in the cytoplasm of the cell. These are sequestered into other organelles in the cell and I'll show you those. They actually end up being the first and the last enzyme in the pathway. All the other enzymes between the first and the last are all found in the cytoplasm. Let's look at what's happening in making glucose from pyruvate. If we recall in glycolysis, in going from PEP to pyruvate, I said that was the big bang. I said that was a reaction that was extraordinarily energetic. It had a large delta-G-zero prime. And as a consequence, that, you might imagine going in the reverse direction, would be an enormous energy barrier to overcome. And in fact, that's exactly what it is. It's because of this enormous energy barrier that cells can't go directly back from making pyruvate to PEP in one step. They have to do a two step around it. And the two steps around it are these two enzymes here. Pyruvate carboxylase and phosphoenolpyruvate carboxykinase, which you are more than welcome to abbreviate as PEPCK. Let's talk about the first one first, pyruvate carboxylase is an enzyme that is found in the mitochondrion of cells. It's not found in the cytoplasm. The very first reaction of gluconeogenesis occurs in the mitochondrion, not the cytoplasm. In this reaction, as you can see on the screen, carbon dioxide in the form of bicarbonate and ATP are used to convert pyruvate into oxaloacetate. You can see the structure here. Here's a 3-carbon, here's a 4-carbon over here. We've put an additional carboxyl group onto the end of pyruvate. The carboxyl group going right here. We can see the new carboxyl group on the right side. Here it was what pyruvate looked like over here. So what we did is we took this methyl group and we added a carboxyl group to it. That takes energy to put that on there. It makes a 4-carbon intermediate and that 4-carbon intermediate you're going to hear a lot about next term because oxaloacetate is one of those molecules that appears in so many metabolic pathways. It's a very, very important molecule. It's important in amino acid metabolism. It's important in the citric acid cycle. And it's also important as you can see here in the synthesis of glucose. This is an energy requiring reaction so if we started with 2 molecules of pyruvate, it's going to take 2 molecules of ATP and 2 molecules of bicarbonate to make 2 molecules of oxaloacetate. We haven't gotten to PEP yet because even with all that energy that we've put in, we've made a 4-carbon intermediate and we have to convert that 4-carbon intermediate into phosphoenolpyruvate or PEP. To do that, the oxaloacetate which was made in the mitochondrion has to be moved out of the mitochondrion and into the cytoplasm. Next term we'll talk about how molecules move across an organelle. But it turns out there are specific proteins that will shuttle a molecule across a membrane. There are specific proteins that will transport oxaloacetate out into the cytoplasm. When it's out in the cytoplasm, oxaloacetate can be acted upon by this second enzyme that's unique. By the way, all the unique enzymes are shown in red on here. By the unique enzyme phosphoenolpyruvate carboxykinase, or PEPCK. Notice that it also takes energy and that energy in this case comes from GTP. So GTP is just like ATP, a high energy source. GTP is used in some places in the cell for energy. The most common place we actually see GTP used is in the synthesis of proteins because all proteins are made using GTP, not ATP. We'll talk about that next term. If we have 2 molecules of oxaloacetate and we want to make 2 molecules of PEP, it takes us 2 molecules of GTP and this enzyme to accomplish that. In the process, a CO2 is released. Look what we did. We put a CO2 on the form of bicarbonate and we've released the CO2 up here, so no net gain of carbons have occurred. We've done a molecular rearrangement and in the process of doing that molecular rearrangement, we've also put a phosphate onto the molecule, creating that very high energy PEP molecule. As I said, this reaction occurs out in the cytoplasm. To go from pyruvate to PEP in terms of synthesizing glucose, we had to use 2 high energy phosphates for each molecule for a total of 4. So in order to go from here 2 pyruvates to 2 PEPs, we need 4 triphosphates. 2 ATPs and 2 GTPs. From an energy perspective, GTP is exactly equivalent to ATP. There's no difference. Going down, if you recall, when we went from PEP to pyruvate, we only got a total of 1 ATP for each one, or a total of 2 ATPs. So we can see that building molecules in anabolic pathways takes more energy than we get out in catabolic pathways. That's not surprising. We're thinking okay, well we're going. Yes? Student: So how does this prevent PEP from immediately switching back to pyruvate? Professor: She's reading my mind. My next point is, her question was how does the cell keep PEP from just going back to pyruvate?