#25 Biochemistry Glycogen Metabolism I Lecture for Kevin Ahern's Bb 450/550

Captioning provided by Disability Access Services at Oregon State University. Kevin Ahern: Okay, folks, let's get started! How was your Thanksgiving break? Student: Good. Kevin Ahern: I bet everybody was really dying to get back and get into biochemistry? Student: No. Kevin Ahern: No? Student: I was, a little bit. Kevin Ahern: Look at this way, you go from one turkey to another. Student: Oh. Ha, ha, ha. Kevin Ahern: The old lead balloon, that's as good of a joke as I have for you today, folks. Believe it or not, we're in the home stretch. We have, counting today, three lectures. Whoo! The end is near. I did not bring note cards with me today. I said I was going to. I did not bring note cards, so I apologize for that, number one. I've got number two and number three, I think, here, also. I will bring them on Wednesday, so on Wednesday you will have note cards, for sure. If you're really desperate and you really have to have your note card, you can come by my office and pick one up, but realistically it's not going to change things an awful lot if you go 48 hours different without your note card. But it's a 5-by-8, I can tell you. If you want to get a 5-by-8 and practice on it, you can see it's a pretty good-sized note card. Student: Wow! Kevin Ahern: But you have to get the note card from me, remember that. So the note cards have to be gotten from me, and you don't have to use it, but you have to turn in a note card, with your name on it, that you got from me, with your final exam. If you don't, you will lose points. So make sure that you get a note card from me and you turn it in with your final exam. Are we clear? Okay, that's number one. Number two, I had a big presentation this morning and I was working my presentation last night and forgot to send you guys an email saying the exams are graded! So the exams are graded, and let me just say a few words about the exam. I was totally delighted with the exam. The performance on the exam was one of the highest averages I've ever had. The average was 76.5. I have a curve. I will post it as soon as class is over today. I will post the curve on the website so you can see it for the overall sum of your grades for the first two exams, so you can see exactly where you stand. The low score on the exam was 15. Student: Ugh. Kevin Ahern: The high score was 103. I had, I think, two or three people who had a perfect 103, including the extra credit questions. So I was very impressed and I was looking, as I looked at the grades, themselves, I saw that that average went up because a lot of people in the low-to-mid jumped quite a bit, and so it was very, very satisfying to me. I just felt very, very good about that and I really respect what people did with that, so that's kind of cool. So it was worth the wait, hopefully, for you to get your exam back. As always, if you have questions, let me know and we'll go with that. Number three is we have a final exam coming up. That final exam is in here, on Monday, at 9:30. I will do a review session for it. In fact, I have put in for, I believe it's Friday evening at 6:30, I have put in a request for a room. I will announce that when I get the room for sure, but the review session will almost certainly be Friday at 6:30. That gives you a chance to get dinner and then get review session. As before, I will videotape that. Now, let's see. What else do I want to say here? I want to say we finished almost everything about gluconeogenesis. The last thing I did not talk about on Wednesday of last week, which seems like a long time ago, by the way... Student: I know, it does. Kevin Ahern: The last thing I did not talk about there I wanted to save for today because it's kind of involved and so I wanted to make sure everyone had the same opportunity to see it and ask questions and so forth about it, and it's the combined regulation of glycolysis and gluconeogenesis. I'm going to start out by showing you a complicated figure. Actually, no, I'm not going to start with that. I'm going to start by telling you the sort of philosophy of glycolysis and gluconeogenesis regulation. The philosophy is that glycolysis is a catabolic pathway. Gluconeogenesis is an anabolic pathway. These pathways, for the most part, occur in the same place, which is the cytoplasm. Gluconeogenesis only has two reactions that aren't in the cytoplasm. One's in the endoplasmic reticulum and one is in the mitochondrion. All the other enzymes of both pathways are in the cytoplasm. Moreover, many of the enzymes are the same enzymes in both pathways, which means that many reactions are driven by concentration, which side of the equation has necessarily large enough amounts to drive a reaction one way or the other. That means that we have to be careful to regulate these pathways. If we don't control these pathways, we're going to have that futile cycle that I talked about before, where, imagine, let's imagine the following scenario. Let's imagine I had glycolysis and gluconeogenesis going at the same time. What would happen? I would start with pyruvate. I would put in six triphosphates to get to glucose. I would burn glucose and get two triphosphates and be right back at pyruvate. I wouldn't have gained anything, but I would have lost four triphosphates. And then I start it again and I go up, I go down, and each time I turn that cycle I lose four triphosphates. That's futile because it doesn't give the cell anything but heat. So it's important the cell not waste its energies, and the cell doesn't waste its energies by controlling pathways like that in what we call a "reciprocal" fashion. Reciprocal regulation is something you're going to hear a lot about today and you're going to hear a lot about it on Wednesday, also. Reciprocal regulation. Well, we start to see it right here. Here's a schematic, going down for glycolysis on the left, going up for gluconeogenesis on the right. Some of these things we've talked about already. Let's look at the regulators of this pathway. What you see on the screen are the allosteric regulators, the allosteric effectors of the important enzymes. In glycolysis, we know there are hexokinase, which is not shown, PFK, and pyruvate kinase. In gluconeogenesis, there are these two enzymes. They are this guy and also glucose-1,6-phosphatase, which is also not shown. So we just sort of throw out the first one up here. We just throw it out. These guys and these guys, we're very interested in. As I said when I talked about glycolysis earlier, the most important pair are these two, right here. PFK and FBPase-1, or fructose 1,6-bisphosphatase, if you want to call it that, PFK and FBPase-1 are regulated reciprocally. F2,6BP we talked about before. Notice that in very tiny amounts it turns this enzyme on. In the same tiny amounts, it turns this enzyme off. It has opposite effects on the two enzymes. Look at AMP. AMP turns this guy on. AMP indicates low energy. With low energy, we want glycolysis to go. PFK is activated. We look over here. PFKóI'm sorry, AMP turns off FBPase-1. Citrate turns off this guy. Citrate turns on this guy. It's reciprocal. It's not perfectly reciprocal. There are things that affect this one that don't affect this one. But when we look at the thing as a whole, F2,6BP is a reciprocal regulator. AMP is a reciprocal regulator. It has opposite effects on catabolic and anabolic enzymes. To a lesser extent, we see some of that down here. ATP turns this guy off. ADP turns this guy off. But we don't see the same kind of reciprocal regulation that we saw with PFK and FBPase-1. Now, reciprocal regulation turns out to be very, very important when we have pathways occurring in the same place, at the same time, or that can occur at the same place, at the same time. Cells generally regulate them so that they don't occur that way, for the most part. Well, it gets even a bit more complicated than that, because the question arises, I told you earlier when I talked about PFK and I said the most important regulatory effector for PFK was fructose 2, 6-bisphosphate, right? And I just showed you that it was a very important regulator for FBPase-1, as well. So, unfortunatelyóand you're going to say this as well as I doóunfortunately, it means we need to understand how do cells make and break down fructose 2,6-bisphosphate. That, you're going to see, it's going to look much more complicated than it is, so I'm kind of conditioning you for what I'm going to show you, and I'm also going to tell you that I can throw a million words at it. It's kind of like the mechanisms of serine protease action. We can throw a million words at it, but until you sit down with it and look at it yourself, it's going to seem like a million words. So let's take a look at the overall pathway by which fructose 2,6-bisphosphate is made and regulated. Remember, this is the reciprocal regulator of PFK and FBPase-1. This looks complóoh, Jesus, yeahóthis looks complicated. That's always the first reaction. It's not as bad as it seems. There's a lot of information on here, and the guts of its right here. All this is showing us is, on the side we're breaking it down. On this side over here, we're making it. There's an enzyme that makes it, and there's an enzyme that breaks it down. An enzyme that makes it, and an enzyme that breaks it down. Now, let's take a look at this enzyme. This enzyme is one of the most fascinating enzymes in biochemistry, because this enzyme is actually two enzymes. The same protein molecule catalyzes the synthesis and the degradation of fructose 2,6-bisphosphate. It's the same protein. This protein has two activities. One activity makes it. It's called PFK2. PFK2 catalyzes the synthesis of fructose 2,6-bisphosphate. FBPase-2 is the other half of it, and it breaks it down. Make it, break it down. Here's the enzyme. Here's the two activities. Well, as we can see, at any given time, only one portion of the enzyme is active. Only one portion of this enzyme is active. Oo-ooh! Good job! Not my day today. What's the difference between these two? The difference is a phosphate. If we put a phosphate onto this enzyme, we flip the activities. That turns FBPase-2 on. That turns PFK2 off. If we take the phosphate off, we favor the reversal of that. Well, that's not surprising. You've seen before how covalent modification of enzymes can affect enzyme activities. We're simply putting a phosphate on, we're taking a phosphate off. It has opposite effects. This causes the PFK2 to become active. Going to the right causes the FBPase-2 to become active. What catalyzes these things? Well, protein kinase Aóthere's our friendóprotein kinase A, when it's activated, catalyzes this enzyme getting a phosphate on it and FBPase-2 being active. Let's think about what that means in terms of the cell. If FBPase-2 is active, not looking at the screen what's going to happen? We're going to break down F2,6BP, right? When we break down F2,6BP, what's going to be the effect on FBPase-1 and PFK1? PFK is activated by this molecule, so if I take the molecule away, what's going to happen? Less active, right? If I go to the right, PFK1 is going to become less active. F2,6BP is an allosteric inhibitor of FBPase-1. If I take it away, what's going to happen to FBPase-1? It's going to be active, right? Well, since those are the critical enzymes controlling whether we're running glycolysis or gluconeogenesis, now you can look at this and say, in general, what's going to happen to glycolysis and gluconeogenesis if I phosphorylate this guy, right here. Well, I'm going to go over here. I'm going to break this guy down. I'm going to favor gluconeogenesis. And look, when glucose is scarce, that's exactly what I want to be doing. I want to be making glucose. Remember the flight or fright? Remember the grizzly bear chasing me and my adrenaline starts flowing, and I said that we had that kinase cascade, and the kinase cascade activated protein kinase A? There's our protein kinase A. And I said that the result of activation of protein kinase A resulted in production of glucose. This is one of the ways in which we make glucose. Not surprisingly, if we're making glucose, we don't want to be breaking down glucose, so we inhibit glycolysis, because we're no longer activating PFK with fructose 2,6-bisphosphate. So in one simple step, depending on how you look at it, of course, but in one simple step, we've reversed those two pathways. Well, what happens now when I've got my glucose stores back up? I've escaped the grizzly bear and I'm sitting around and eating pizza. I've got plenty of glucose around, and glucose is a... poison. So I've got to deal with that glucose. I've got two things I can do with glucose. I can break it down. I can turn it into glycogen. We'll be turning it into glycogen later in the week. Today, we're going to break it down. So when we no longer are activating protein kinase A, we are no longer phosphorylating. Phosphoprotein phosphatase becomes active, and, by the way, phosphoprotein phosphatase is activated by insulin. Insulin is causing this process to go to the left. Why? Glucose is a poison. We've got to do something with that poison. We're going to take phosphates off. We're going to activate PFK2. We're going to inhibit fructose bisphosphatase-2, FBPase-2. What's going to happen? We're going to start making fructose 2,6-bisphosphate, activate PFK1. Glycolysis is going to run. When fructose 2,6-bisphosphate is present, FBPase-1 is inhibited and gluconeogenesis stops. Insulin favors going to the left. Epinephrine favors going to the right. That, in a nutshell is what's happening. Now, I want you to lay this out yourself. I'll be happy to answer any questions, but I want you to just sit down, lay it out, and you'll discover it's really not that complicated. Yes, back there? Student: What about non-strenuous activity, where you happen to have an abundance or scarcity of glucose? Kevin Ahern: So what if you have the in-between situation, basically, is what you're saying. We have an in-between response. The body will generally modulate glucose levels to provide glucose, as needed, as much as possible. So maybe we'll phosphorylate, in this case, we'll burn some of our glucose. We'll phosphorylate some of this, but not all of this. Does that make sense? Student: Yeah. Kevin Ahern: Thanksgiving took all the questions out of you guys. Yes? Student: So the glucose production, that's happening in the liver, only, right? Kevin Ahern: Glucose production, gluconeogenesis, is happening primarily in the liver and a portion of the kidney. That's correct. Okay. So look it over. If you have questions, see me, but that's basically what's up with that. That is the last of what I want to say. Oh, here's the enzyme, by the way. There's the enzyme that's there. There's the part that puts the phosphate on. There's the part that puts the phosphate off, and there's that tiny little ribbon that connects the two of them. It's an amazing enzyme, absolutely amazing enzyme. We turn our attention now to something that is an easy metabolic pathway. It's going to concern us for the rest of this week. So you say, "Well, it's not an easy pathway!" Well, I'm going to convince you, I hope, that glycogen metabolism is actually one of the easiest metabolic pathways to learn. Its regulation is complicated, but the pathway itself is extraordinarily simple. Let's talk about glycogen. We talked about it earlier in the term, and glycogen is a storage form of glucose that animals use. It's a storage form of glucose that animals use. We talked about how plants use amylose and amylopectin. We combine those and we get starch, right? But plants don't have glycogen. What's the difference between glycogen and amylopectin? Anybody remember? Student: The linkages between [unintelligible] Student: There's more branches? Kevin Ahern: There's more branches in the glycogen than there is in the amylopectin. So they're all polymers of glucose. Amylose has only alpha-1,4 bonds, so it's just a long linear chain. Glycogen has alpha-1,4 linkages, but every now and then it has 1,6 branches. There's a 1,6 branch. About every ten residues or so, glycogen has a 1, 6 branch, which means that glycogen, even though it's full of glucose just like amylose is, is structurally very different. It has a lot of ends. The more branching you have, the more free ends we have at the non-reducing end. You remember what the non-reducing end is. Is this a reducing sugar or not a reducing sugar? How many say it's a reducing sugar? How many say it's not? I'm sorry but the person who said it was a reducing sugar was right. The very first one has a free aldehyde. The very first one has a free... it's alpha-1,4 linkages. There's 1,4. That means if this is the end of the molecule that would actually be an OH there and that could become an aldehyde. Student: So the last one on the right is the reducing sugar? Kevin Ahern: In this case, it would be, yeah. Now, that's not important. I'm just throwing that out at you, just to see what you remembered after all that turkey. The difference between glycogen and amylopectin, they're both branched. Amylopectin also has 1,6 branches, but it only has them about every 50 residues or so. I'm going to tell you in a second why that's the case, but that's the structural difference between amylopectin and glycogen. Did you have a question? Student: Yeah. Isn't that initial glucose subunit before all the branching takes place, on the very internal chain, wouldn't it be non-reducing because it's covalently attached to that little seed molecule that starts the whole thing off? Kevin Ahern: That is the seed molecule, right there. So if this is the end, then that's going to be an OH, right there. That OH makes it, a free anomeric carbon on an aldehyde on an aldose will always make it a reducing sugar. I'll show you the structure of that, if you'd like to see it. Come see me. Now, amylopectin's chemically different from glycogen in just the extent of the branching. Why is that important? Well, the reason it's important, and this is why you're able to be an animal, and I'm not talking about in any sense except for walking around, you people... [laughter] I know where your minds are! How are you able to be an animal? One of the most important ways in which you can be an animal is thanks to glycogen. Glycogen is stored in our muscles. It's also stored in our liver. It's in muscles for very quick energy. It's in our liver for providing that buffer to keep our glucose levels balanced, hopefully, over time. The reason that the structure of glycogen is so important to being an animal is because glycogen has so darned many ends. All those branches, all those ends, are important, because, as you will soon see, the way that glycogen is broken down is from the ends. More ends, more breakdown, more quick release of glucose. Animals have to run. They have to escape. They have to catch prey. They have to take notes in biochemistry. All those things require quick energy. Having a system that has a lot of ends allows for a lot of glucose to be released very quickly, when necessary. Plants don't have those needs. Plants don't go running away from their prey. If they could, they might evolve into something different. But they never made anything of themselves. They just kind of sit around like plants, right? "If only we had thought of making glycogen," plants say to themselves, "where would we be now?" But, no. You guys are really quiet today. Student: It's a Monday. Kevin Ahern: It's a Monday. Student: Thanksgiving, we had a four day break. Student: Yeah. Kevin Ahern: So do you see the fundamental difference? That chemical difference really plays out as a very important thing. Well, let's look at the metabolism of glycogen. Actually, this is whatóthere you go. Is that the figure you were referring to? Student: Wasn't the very, very internal molecule not a made-of-glucose molecule, though... Kevin Ahern: It is. It's a glucose, yeah. Everything in it is a glucose. What's that? You want to draw it on the exam, you said? Student: No! Student: What?! Student: You could just draw a bunch of lines. Kevin Ahern: No, you've got to draw it, we'll line it up and we'll put it on top and see. Nope. No partial credit. Sorry. [laughter] Student: Oh, god. Kevin Ahern: Let's look at the breakdown of glycolysis. There's what glycogen looks like. That's these little black guys here. Fates of glycogen. Glycogen turns out to be important as a source of glucose. But, of course, we know glucose is not the end of the story because glucose, by itself, doesn't do anything except poison us. We want to have the energy from glucose, which is why we have glucose around in the first place, and what this is showing you is what happens when we break down glycogen and how it's converted into energy, the glucose in it. I'm going to show you in a second an unusual reaction. It's a really cool reaction. The glycogen isn't broken down directly into glucose, for the most part. Ninety-nine percent of it, or, ninety percent of it is broken down into this guy, right here, glucose 1-phosphate. Where did we see glucose 1-phosphate before? Anybody remember? Student: Glycolysis? Kevin Ahern: Not glycolysis, no. Galactose metabolism. Do you remember when we had the UDP glucose and it got released, and it was released as you don't remember thatóglucose 1-phosphate. I told you, at the time, glucose 1-phosphate would be important in glycogen metabolism because it can readily be converted into glucose 6-phosphate. This enzyme phosphoglucomutase allows this interconversion. It can go up, It can go down. It's pretty much equal in terms of which direction it goes. Student: Is it "phophoglucomutase"? Kevin Ahern: Ha-ha-ha-ha! What are they doing this in this textbook? "Phophoglucomutase." [laughter] That is now an acceptable name for this enzyme. If you want to call it "phosphoglucomutase," you can. If you want to call it "phophoglucomutase" [laughing] or "phuphuglucomutase," I don't care. [laughter] Now, glucose 6-phosphate can go to glycolysis. That's important. Glucose 6-phosphate can get released as glucose and go into the bloodstream, if this happens in the liver. Glucose 6-phosphate can be converted by the pentose phosphate pathwayówe'll briefly talk about that next termóinto ribose, and ribose is very important for making nucleotides. So this molecule is central to a lot of different pathways. How do we get glucose 1-phosphate? Let's take a look at that. Here's the end of a glycogen molecule. One of those ends that we talked about, one of those millions or thousands of ends that are on the end of a glycogen, we're sitting at it right now with an enzyme that breaks it down. The enzyme that breaks this down, that catalyzes this reaction, is known as "glycogen phosphorylase," P-H-O-S-P-H-O-R-Y-L-A-S-E, unless you're a textbook publisher, in which case it's called "phophorylase." [scattered laughter] Now, this is a reaction like you haven't seen before. It looks very straightforward. Here, we've got a glycogen molecule. Here, we've clipped off a glucose 1-phosphate, and here's the glycogen that's lost one of its residues. Very straightforward, right? Well, not quite. Look what's happened. We put a phosphate on there, in the process. How did we put that phosphate on there? We didn't use ATP. When we talked about putting ATP onto glucose before, we said that took energy, right? Where did the energy come to put this phosphate on here? Any thoughts? Wild ideas? Yes, sir? Student: It's energetically favorable? Kevin Ahern: Why is it energetically favorable? It is energetically favorable, but why? Student: Negative Delta G zero prime? Kevin Ahern: Why is the Delta G zero prime negative? Student: Is there energy in breaking that bond? Kevin Ahern: There's energy in breaking this bond. This bond has some energy in it. The energy in breaking this bond is transferred to making glucose 1-phosphate. So it tells us that that alpha-1,4 bond has some energy in it and that we can use that energy to make something. Well, why do we want to do that? Well, it turns out, whenever we can save energy, that's good, just in general, right? Insulate your glycogen, right? So that you don't... no. Alright. You don't waste energy, you see, if you insulate your glycogen. Alright, Anyway. We got a phosphate onto here and we didn't have to invest ATP energy. We just saved a triphosphate. Muscle cells, if I am running and jumping, I don't want to burn my ATP breaking down my glycogen. I want to burn my ATP using the energy from glucose. This allows me to put a phosphate on there without using any ATP energy. This is really cool because now I can isomerizes this guy to make glucose 6-phosphate andó***! I'm in glycolysis without investing any ATP to start. Very good. So this saves a reaction. The enzyme is called a phosphorylase. The name, again, tells us what it does, meaning it uses a phosphate to break a bond. It uses a phosphate in breaking a bond. It's different than a hydrolase, which uses water to break a bond. So instead of using water, we're using phosphate to break that bond. We're almost done, okay? We're almost done. There's only one other thing I have to tell you, and that is the fact that glycogen phosphorylase is a finicky enzyme. Of course it's a finicky enzyme. It has to be, right? Glycogen phosphorylase will only work to within about four residues of a branch. It gets to that point. It starts up here. It keeps chewing, chewing, chewing. It takes these red guys off here, and it says, "I ain't going any further." It will not work any closer than about four residues to a branch, the branch being a 1,6, right there. Then, something else has to happen. Well, the something else that has to happen is another interesting enzyme that has two activities associated with it, but we branch them into one name. We could memorize that it's called a transferase and we could memorize that it's called an alpha-1,6-glucosidase, but we, being biochemists, are kind of lazy. We like to call both of these activities "debranching enzyme." I'm going to tell you what debranching enzyme does, but these two reactions are catalyzed by the same enzyme known as "debranching enzyme." What happens? Well, let's look to see what this enzyme is doing. Follow the blue guys. Here's the three blue guys here. The three blue guys get transferred from this branch down to this branch. That leaves behind one green guy. They're all glucoses, by the way. So they're all glucoses. They're not different. The difference being this guy is linked by an alpha-1,6. The enzyme, debranching enzyme, uses water to break that guy off and we get free glucose. This is the only place we get free glucose in glycogen metabolism. Student: And then the glycogen phosphorylase will then be able to... Kevin Ahern: Then glycogen phosphorylase now has a new template it can work on and it can go chewing back until it gets back to another branch. Student: The free glucose, the green one, is that [unintelligible]? Kevin Ahern: The green one is the only free glucose that's released in the process. Student: [unintelligible] Kevin Ahern: What's that? Student: [unintelligible] Kevin Ahern: Right. So you might wonder, well, why in the other case does it use glucose 1-phosphate it used glucose to make glucose 1-phosphateówhy, in this case, is it releasing free glucose? It's not being consistent. No, there's something that's different here. What's different here? Student: water Kevin Ahern: It's using water, but why doesn't the other one use water? Why doesn't this one use phosphates, is my question? Student: It's not a high energy Kevin Ahern: It's not a high enough energy bond. An alpha-1,6 does not have as much energy as an alpha-1,4 does. It doesn't have the option. Well, fortunately, there's only one of these per branch that's made, so the cell says, "Okay, I'll take and use some ATP and put you into glycolysis." ***! You got it. Student: So the debranching enzyme requires ATP? Kevin Ahern: Debranching enzyme? No. There's nothing here that requires ATP. Getting that into glycolysis requires ATP. Okay, questions? Now, believe it or not, with the exception of the phosphoglucomutase that's needed oop, turn that guy off the phosphoglucomutase that's needed to convert the glucose 1-phosphate into glucose 6-phosphate, you've just seen how you break down glycogen. ***! What enzymes did we see? Phosphoglucomutase interconverts glucose 1-phosphate and glucose 6-phosphate. It's a mutase, so what does that tell you? It has a 1,6 intermediate, and, yes, that can get released as a free molecule. It does get released as a free molecule. The second enzyme was glycogen phosphorylase, that broke 1,4 bonds close to a branch, and the third enzyme was debranching enzyme, which changed the branch and released free glucose. Three enzymes in the entire pathway. Cool! Glycogen breakdown is very simple. I'm going to talk about glycogen synthesis in a second and you're going to see it's almost as simple. Here's the phosphoglucomutase. This is the glucose 1-phosphate. There's the intermediate. There's the product, glucose 6-phosphate. This is a reversible reaction, either direction. If we have excess glucose 1-phosphate, it'll go to the right. If we have excess glucose 6-phosphate, it'll go to the left. When would we have excess glucose 6-phosphate? What conditions would give us excess glucose 6-phosphate? What metabolic pathway...hint, would give us excess glucose 6-phosphate? Student: Gluconeogenesis. Kevin Ahern: Gluconeogenesis, right? So if a cell is building glucose, it's going to be building glycogen, too. We'll see in a second that glucose 1-phosphate is needed to make glycogen. So if we're making things in gluconeogenesis, we're going to the left. If we're breaking things down in glycogen breakdown, in glycolysis, we're going to the right. Yes, ma'am? Student: Which one did you say is reversible? Kevin Ahern: The entire reaction is reversible. Student: Oh. What are the yellow things? Kevin Ahern: That's just part of the enzyme. So there's the active site of the enzyme. There's the rest of the enzyme. There's the serine residue that's involved. That's really all it is. It's just showing you that side chain. Alright. DIPF would to do what to this enzyme? Student: Inactivate it. Kevin Ahern: Inactivate it, right? Okay. I should have asked you what the molecule was that'll do it. Okay. I'm going to jump down to glycogen synthesis, because I think if we talk about the metabolism and then we save the regulation for later we'll be better off. So let's talk about the synthesis of glycogen. It's just about as simple as the breakdown is. There's one extra enzyme, one extra enzyme. So, the enzyme, again, we think "phosphoglucomutase" for interconverting. Now we want to make glucose 1-phosphate, because we want to make glycogen. But it turns out that glucose 1-phosphate can't be added to a growing glycogen chain. Why? Well, remember that alpha-1,4 bond had some energy in it? Right? If it has energy in it, then we have to put some energy into making that bond, and there's not enough energy in water, essentially, to make that bond. So we have to use a high-energy intermediate in order to make that alpha-1,4 linkage. The high-energy intermediate we use is this guy, right here. You saw it before. You saw it when we talked about galactose metabolism. This was a molecule I described as an "activated intermediate." An activated intermediate is a molecule that has a high-energy bond, and there is the high-energy bond. It's a molecule that has a high-energy bond that uses the energy of that bond to transfer a part of itself to something else. So an activated intermediate is a molecule that has a high-energy bond and it uses the energy of that bond to transfer a part of itself to something else. Well, the part of itself it's transferring is this guy, right here, glucose. What it's going to do is attach it to position 4 of a glucose on the end of a growing glycogen chain. If we're going to talk about the enzymes of glycogen synthesis, we have to talk, first of all, about how do we make this molecule. Once we know that, everything else is pretty much like glycogen breakdown. Let's take a look at how we make that. Here's the reaction that makes UDP-glucose. Glucose 1-phosphate, okay, you know how that's made now. Glucose 1-phosphate we combine with UTP. We make UDP-glucose and we make what's called pyrophosphate. Those are two phosphates joined to each other. Let's count the phosphates. One, two, three, four. One, two, three, four. We haven't lost any phosphates, but they've reorganized. Now we have this guy and we have this guy, over here. Student: What did you say the name was? Kevin Ahern: It's called "pyrophosphate," P-Y-R-O-P-H-O-S-P-H-A-T-E. Pyrophosphate means two phosphates covalently linked to each other. Well, we've just made an activated intermediate. What did it take to do it? It took a triphosphate. UTP has the same energy as ATP does. It has the same energy as GTP does. That triphosphate is high energy. The cell is having to invest some energy into making this bigger molecule. That's a fundamental principle of anabolism. Building bigger things takes energy. It took energy to make glucose. It's now taking energy to make glycogen. We're nearing the end, believe it or not. UDP-glucose. What's the next step in the process? Well, the next step in the process is adding that glucose to a growing glycogen chain. This is the reaction that's catalyzed, here. There's the UDP-glucose that we just made. Here's carbon number 4 of the end of a glycogen chain, right there. In this reaction, this glucose gets transferred over there. The energy of this bond is used to make this high-energy bond. We've now made a glycogen that has one more glucose on it. The enzyme that catalyzes this reaction has a very simple name. It's called "glycogen synthase," S-Y-N-T-H-A-S-E. Glycogen synthase catalyzes the addition of glucose to a growing glycogen chain. The product is UDP, of course, and UDP can be converted into UTP and then reused again. Now, we're only missing one thing. What are we missing? How do we get branches? Well, for branches, we've got a really complicated enzyme name that's used to do it, but I prefer to call it "branching enzyme," as I'm sure you will, too. There is, believe me, it's a mouthful of a name. It's about that long, okay? But, in essence, branching enzyme will create alpha-1, 6 branches about every ten residues. Here's an alpha-1,4 linkage. Here's a branching enzyme. ***! Got it! So branching enzyme is creating the branches. So what enzymes have we seen in glycogen synthesis? Well, we saw phosphoglucomutase, as before. I didn't give you the names of the UDP-glucose synthesizing enzyme, did I? Student: No. Kevin Ahern: Do you really want it? Student: Nope. Kevin Ahern: Should we give it a name? I'll tell you what the real name is and then you can tell me perhaps a more humorous name. The real name is UDP-glucose pyrophosphorylase. Student: Steve! [laughing] Kevin Ahern: Steve. These are all male names. Do we have any female...there's never a female...it's true, every year when I ask for names people always give me male names. Student: Helga. Kevin Ahern: Ursula! Student: Tina. Kevin Ahern: Tina? Student: Amaryllis. Kevin Ahern: Amaryllis? Student: Shaniqua. [laughing] Kevin Ahern: I'm sure they'd like to spell that one. So you may call it either UDP-glucose pyrophosphorylase, which is the real name, or... I'm going to vote on this. I don't know. I think the best names I've heard were Steve... Tina... Student: Lucy. Kevin Ahern:...and Ursula...Lucy! And Lucy. Okay. Steve, Tina, Ursula, Lucy. Steve? Tina? Ursula? Lucy? Lucy is the simplest one, I think. People wanted Lucy. "Lucy in the Sky with Diamonds," right? Student: What was the real name? Kevin Ahern: It's the enzyme that catalyzes this reaction right here. Its real name is UDP-glucose pyrophosphorylase. UDP-glucose pyrophosphorylase. That's the breakdown. That's the synthesis of glycogen. I'm going to cut short early today but I'm not going to finish quite yet. I just want to say one last thing, and that is, on Wednesday I'm going to talk in detail about the regulation. The regulation is reciprocal, but it's also complicated. It involves both covalent modification and allosteric regulation. If you want to look over a lecture material before you come to lecture, next time might be a good one. See you Wednesday. [indistinct conversation] Kevin Ahern: Yes, sir? Student: [unintelligible], why was that a pyrophosphate instead of a bisphosphate? Kevin Ahern: What's that? Student: [unintelligible] Why is that a pyrophosphate instead of a bisphosphate, when it's free floating? Kevin Ahern: I think the term's interchangeable. Student: Okay. Kevin Ahern: Yeah. Student: So "pyro -" means "bond"? Kevin Ahern: Just bond, yeah. Yeah. Student: Okay. Thank you. Student: I didn't catch where you said we could pick up our exams. Kevin Ahern: Yes, they're at the BB office, in ALS-21. Student: Okay. Thank you. Kevin Ahern: Sure. [indistinct conversations] [no audio] [END]