Chitin synthase

Transcription

Kevin Ahern: Should we just call off class today? What do you think? Student: Yeah! Kevin Ahern: And then go twice as fast on Monday? [laughter] Until we get to Monday, right? I'm going to get through party pretty much carbohydrates today. I'm not sure if I'll finish it completely, but if I do, great. If not, then we'll... go twice as fast on Monday! [laughter] Either way, you lose, right? Last time, I said just a few words about steric hindrance and its effect on the conformation that sugars may have. I talked about the boat form. I talked about the chair form. There are similar conformational considerations on five-membered rings, as well as for six-membered rings. You are not responsible for them—I'll show them to you because they're a little harder to conceptualize, but I think of them as sort of like an envelope. There's the top part of the envelope there, and where the top part of the envelope is up or down, but that's really, we need space models, I think, to have a better understanding of that, so we won't mess with those guys. As I alluded to earlier when I talked about carbohydrates, in general, we discovered that they come in a variety of forms and some of those forms include chemical modification to the carbohydrates. One of those common modifications is an oxidation that can occur. Oxidation is actually used in some assays to measure the presence of certain carbohydrates. One of these is an oxidation reaction, and the oxidation reaction is shown on the screen. This oxidation reaction happens in the presence of what are called "reducing sugars." So reducing sugars are sugars that are easily oxidized. Why do we call them reducing sugars? Well, because they are reducing something else. They are, in fact, reducing, in this case, a copper ion from the +2 state to the +1 state, and that means an electron has to have moved from the reducing sugar to the copper ion as we see here. That change of copper's configuration actually changes the color of the solution that it's in, so we can actually monitor the color and the amount of color change that occurs as a measure quantitatively of the amount of a reducing sugar that is present. Now, reducing sugars are, as I say, called that because they are easily oxidized. Notice this guy is an aldehyde, and I hope you remember from your organic chemistry that aldehydes are oxidized more readily than ketones are. In general, we'll see sugars like glucose will be much stronger reducing sugars than sugars like fructose, for example, which is a ketose. Fructose does or can, to a limited extent, act as a reducing sugar, but for all practical purposes it's not a reducing sugar, certainly not in comparison to glucose. The oxidation of an aldose, like glucose, creates a carboxyl group as a result of that oxidation and makes an acid where there was an aldehyde. That is obviously a fundamentally different structure than what we have with a standard aldose. Notice that in order for this oxidation to occur, the sugar must be in the straight-chain form because in the ring structure we don't have an aldehyde, we have a hemiacetal, and that hemiacetal is not oxidizable, whereas the aldehyde is. One of the things I did not tell you last time about the straight-chain form, I showed you how that a sugar could go from ring to straight-chain and back to ring and so forth, but I only very briefly mentioned the fact of something that prevents that conversion, and that things that prevent that conversion are alterations to the hydroxyl on that anomeric carbon. So if we alter this hydroxyl right here, in any way, then we will not be able to go to the straight-chain form, and, as a consequence, we would not have a reducing sugar. There are quite a variety of modifications that can happen to sugars, and, no, I'm not going to ask you to know all of these in any stretch of the imagination. But I do point them out to you because you'll see them in a variety of biochemical molecules. Fucose is a modified sugar and you'll noticing that it's lacking in OH or a CH2OH on this last terminal carbon, up here. You'll also notice that it's in the L configuration. I said that we predominately see things in the D, but we do see some things in the L configuration. Fucose is one of those that we find in the L configuration, and you'll notice that the L configuration means that that last carbon goes down instead of going up. So the D carbons are always going up, the L carbons are always going down, in that ring structure, as you see there. The addition of N-acetyl groups to the ring of sugars does, in fact, commonly occur, and we'll see a couple of examples of that today. There are polymers that we will talk about later that occur in nature of modified sugars, like N-acetylglucosamine, and these polymers have interesting properties. The exoskeleton of insects, known as chitin, for example, is a polymer of this guy right here. You'll notice that these alterations that have occurred in both of these cases did not affect the anomeric carbon. So if I were to ask you if this guy would be a reducing sugar, I would hope that you would tell me "yes," because the anomeric carbon is still intact and still could go back to the aldehyde form. Whenever we alter the anomeric carbon, in any way, we create a compound known as a glycoside. So the alteration of the anomeric carbon creates a glycoside, and glycosides have many shapes, many forms. They are natural compounds. Some of them are very nasty. Some of them are very innocuous. So the term "glycoside," per se, doesn't have any negative connotation, it's just that some really nasty compounds are glycosides and others are not. A common non-nasty compound that we see is when we link together sugar subunits. When we link together two different, or in some cases hundreds of different or thousands of different sugar subunits together, most commonly the linkage will occur through the anomeric carbon, creating a glycosidic bond. So a glycosidic bond occurs when we have a glycoside, and you'll notice that we call this an alpha-1,4-glycosidic bond. It's named from left to right. Here's the alpha configuration. This is, obviously, carbon number 1. So it's alpha 1, and the "alpha" means that that's in the down position, going over here to position number 4, over here. Another modification that we see on sugars, and we'll see this especially as we start talking about their metabolism, is that of phosphorylation. The addition of a phosphate to a sugar has the effect of actually increasing its energy. So sugars and modified sugars that have phosphates on them will have higher energies than those that don't. That's partly because the phosphate itself is fairly negative and really likes to be released away from all these hydroxyl groups. We'll talk more about those in a bit. So those are the modified monosaccharides. The disaccharides, as its name implies, contain two sugar residues. Your book has one of the most idiotic possible structures it could conceive of for sucrose, so don't even look at this sucrose. It will confuse you. I'll show you in a second a much better figure for that. The reason it's stupid is that they had to flip this guy out to actually draw it in this way. The normal configuration of this has, or the normal way to draw it, actually has the fructose underneath the glucose, but they're saving ink by doing this. It's a really dumb structure. Well, the important thing that we look at in this is we see, again, we have glycosidic linkages, in this case, of sucrose. Sucrose is comprised of one glucose unit linked to one fructose unit. You say, "Whoa! "Am I going to have to learn all that nomenclature?!" Well, not really. I mean, alpha-D-glucopyranosyl-1-2-beta-D-fructofuranose. Whoo! A mouthful, right? I think if you know that glucose and fructose are joined together to make sucrose, that's pretty good, and I think that you will need to know the structure of sucrose, so I'll show you a much better structure of that in a second. So sucrose is one of the sugars that you'll need to know the structure of, but this structure will definitely confuse you. There are other disaccharides that are important in nature. Lactose is one of them. Lactose is also known as milk sugar. It's also a sugar that's very commonly confused by students with a molecule known as lactate. Lactate is not a disaccharide. Lactate is a byproduct of metabolism. We'll talk about it later. Don't confuse lactose and lactate. They are very different molecules. Lactose is a disaccharide comprised of one unit of galactose, seen on the left, linked to one unit of glucose. We can see that the linkage here is a beta-1,4, meaning this bond would go up to an oxygen and then link to the down on the glucose, as shown over here. Would you say that lactose is a reducing sugar or not a reducing sugar? It is. One glycosidic bond, but one anomeric carbon remains open. This guy could still go to straight-chain, still get to the aldehyde form and still become oxidized. Another compound, maltose, is comprised of two molecules of glucose, linked alpha-1,4, as we can see here, and I do think that you should be able to at least illustrate an alpha-1,4, or an alpha-1, whatever, bond, if I ask you such a thing on an exam. Yes? Student: So the first alpha or beta refers to the linkage, and the second, in, like, in the really long word... Kevin Ahern: Mm-hmm. Student: so it's alpha-D-glucopyranosyl, so that first alpha refers to the bond going down, correct? Kevin Ahern: This is referring to the bond going down. That's correct, and, as I look at this, you know, there's another error? That is missing a hydroxyl. Student: Yeah, that's what I was wondering. Kevin Ahern: Yeah, yeah, yeah. Student: Is that supposed to be ... Kevin Ahern: That should be a hydroxyl, right there, instead of an H. Student: Which is what makes it a beta. Kevin Ahern: Yeah, yeah, yeah. Student: Okay, that's why I was confused. Kevin Ahern: So this is a beta. She's exactly right. This is a beta. That should be an OH there. The book is really bad. They decided to randomly change all your figures for you. That's why I say, don't even look at that structure! I hadn't even noticed that part. Let me show you a much better structure for sucrose... a very expensive drawing. [laughter] I didn't save any ink doing this, but I will tell you that I think it actually much better depicts the way that sucrose actually looks. Here is the fructose. There is the beta. Well, actually, no, I take it back. You know, that other structure is right. Hold on. Let me go back to... This is why that structure is stupid. The structure is right but I see why you're confused. Notice that this is 1, 2. This is not carbon-2. This is carbon number 6. So that's why, and you say, "Well, it's going down." Well, they had to flip it to make it go. So that's why I say, and you see the confusion that arises from that structure. This actually shows it much more accurately. It's beta because the OH is going up. This OH is going down. Now, sucrose is interesting. Is sucrose a reducing sugar or not a reducing sugar? It's not a reducing sugar. It's one of the very few sugars that I refer to as a diglycoside, meaning that both of the anomeric carbons are tied up in making this molecule. It's not a reducing sugar. So we see that, and we see versus that. You might wonder about the reducing sugar part, and you think about this, you think about, well, we sweeten drinks with sucrose, right? We sweeten drinks with sucrose. Sucrose is not a reducing sugar, meaning it's not readily oxidized. If I put sucrose in solution and I don't put a bunch of bacteria there to eat it, it'll stay there quite a while. It's not going to chemically oxidize. You think, "Oh, but if I put glucose in there, it will chemically oxidize," and the answer is, yes, it will. One of the reasons that soft drink manufacturers use high fructose corn syrup is because fructose is chemically stabler. It's not necessarily better for your body, but it's certainly chemically stabler. That's why you see high fructose corn syrup being used. So if you're going to draw the structure of sucrose, use the one I have on the board. You'll be much better off than if you use the idiot one that your book uses. So much for disaccharides... moving on to polysaccharides. As the name suggests, polysaccharides are molecules that contain many sugar subunits. One of these is glycogen. Glycogen we'll talk a lot more about in about the last week of the term. Glycogen is a very important polysaccharide. It is the primary storage form of glucose in your body. Your body doesn't store free glucose, as such. The reason? As I will say many times this term, glucose is a poison. Glucose is a poison. If you don't believe me, ask somebody who cans and makes preserves. Why do they put so much sugar and so much glucose, or sucrose, for that matter? Why do they do it? You can take a jar of jelly and leave it laying out on the counter for a long time before anything will happen to it, because it's a poison. That's why. So your body doesn't want to keep much free glucose around. It stores it in the form of a polymer. The polymer that we store it in the form of is glycogen. Our liver is full of glycogen. Our muscles are full of glycogen. Glycogen is not a poison. Our body tolerates glycogen very well. So we can release glucose in small quantities. In small quantities, glucose doesn't kill us. In large quantities, glucose nails our kidneys, glucose nails our eyes. That's why diabetes is such a nasty problem. Our body is telling us, "This stuff's a poison." So we've got to find a way to keep it. Now, this is a polymer of glucose subunits. It may have thousands of glucose subunits, and the linkages between the glucoses that we see is interesting. Let's not focus on the top sugar at the moment. Let's just focus down here at the bottom. If I take and I make a polymer of glucoses and I make them only with alpha-1,4 linkages, I create something that we call amylose. Amylose is a component of starch. So amylose is a polymer of glucoses with only alpha-1,4 linkages... just long, straight chains. Glycogen has a polymer of glucose alpha-1,4 chains, but about every 10 glucoses, we see a 1,6 branch. So now, instead of having something that's very long and linear, we have something that's forked, and forked, and forked, and forked. About every 10 residues, we find another fork. That fork turns out to have tremendous implications for life as an animal compared to life as a plant. Amylose we find in plants. Glycogen we find in animals. I'll talk about those later when we talk about glycogen metabolism, but I want to plant that idea in your head. Both glycogen and amylose contain only glucose. They contain only glucose. Actually, since I'm talking about this, let's go here. When you hear the term "starch," starch is a very common term used to describe carbohydrates in plants. Potatoes are full of starch. Corn is full of starch. Starch is really a mixture of compounds. It's a mixture of amylose, that I described to you, and a branched form of glucose of its own. So plants have a little bit of branched glucose molecules. It's not called "glycogen." It's called "amylopectin." A-M-Y-L-O-P-E-C-T-I-N. It's similar to glycogen, but instead of having branches about every 10 residues, which is what glycogen has, amylopectin has branches about every 30 to 50... not nearly as branched. So when we use the term "starch" that's a mixture of those two compounds: amylose and amylopectin. If we look only at the alpha-1,4 bonds, we see these guys can form sort of nice, circular helices. They're kind of cool looking. These are a little bit like a related compound known as cellulose. Cellulose is plant polymer. It's a polymer, again, only of glucose. But instead of having alpha-1,4 linkages, cellulose has only beta-1,4 linkages. That very slight difference, of having an alpha-1,4 linkage versus a beta-1,4 linkage, converts cellulose from being something we can digest into something that we can't digest. We can digest amylose. We can digest amylopectin very readily in our digestive system. We have enzymes that will break them down because they're full of alpha-1,4 bonds. Cellulose has beta-1,4 bonds. We can't touch 'em. The reason that roughage is roughage is because we can't digest the cellulose that they're full of. So we can't go out and eat grass. We can't go out and eat things like that and get energy out of them. To do that, we'd have to have an enzyme that allows us to break down beat-1,4 bonds. We don't have such an enzyme, but animals known as ruminants, like cows, don't contain the enzyme either. You thought I was going to say they had the enzyme, didn't you? They don't contain the enzyme either. They actually contain, in their rumen, which is a modified stomach, they contain a bacterium that has an enzyme that will do it. So the bacteria in the rumen of ruminants contains an enzyme known as cellulase. Cellulase breaks down beta-1,4 bonds between glucoses and converts grass, green things, et cetera, from just being roughage and things that kind of go shooting through you, into something that actually breaks down into glucose units and gives them energy. So that's why a cow's out there eating a field all day, because it tastes good and because, of course, they're getting energy from that. Those are the simple polysaccharides. There are many polysaccharides. As I mentioned, chitin is a polymer of N-acetylglucosamine and it forms the exoskeleton of insects, and there are other polymer examples, as well. There are some modified polysaccharides that we can take a look at. The first group is known as the glycosaminoglycans. Even though I stumbled on saying the word, it's not a difficult word. Glycosaminoglycan tells you what the structure of this guy is: "glyco" meaning "sugar"; "amino" meaning it has amine groups "glycan" referring to the polymerization of the sugar. So glycosaminoglycan is a polymer of sugars that contain at least one amine group. Now you can see by looking at the structures on the screen that, in addition to containing at least one amine group—there's an amine group for this guy, there's an amine group for this guy, there's an amine group for this guy, et cetera, et cetera, et cetera inaddition to containing these amine groups, we see oxidation. We see molecules or we see portions of molecules that have negative charges. Here's a carboxyl group. Here's a sulfate that's been put onto here. Here's another sulfate, sulfate, carboxyl. There's a sulfate on the amine. There's a sulfate there. There's a carboxyl. We see that every one of these guys has at least one negatively charged group. You're not going to have to draw the structures of these. Don't worry. You're not going to have to memorize which ones have which there. But you should know some common things about the glycosaminoglycans. They all contain amine. They all contain at least one negatively charged section and these are the repeating subunits. These are polymers. So the repeating subunit, in each case, is a disaccharide, and that repeating subunit may be repeated thousands of times. So because we have a polymer that goes on thousands of times, and each subunit contains at least a couple, or one or more of the negative charges, the glycosaminoglycans are what we refer to as "polyanionic," "anionic" referring to negative charge, "poly" referring to many. These polyanionic substances turn out to have some really interesting properties, chemically. We dissolve them in water. Our body uses them in several ways. One of the ways in which it uses them is as a lubricant. Hyaluronic acid, for example, or hyaluronate, is the lubrication material of our joints. Dissolved in water, these compounds get very slippery and slidey. Snot is full of glycosaminoglycans. It gives you an idea about the slippery, slideyness of it. These polyanionic substances don't like to interact with each other. They all repel each other because they're all full of negative charges. You put enough of them together in solution, you really will alter the chemistry of that solution and create something that's like we think of as an oil or something, in terms of how slippery it is, but in fact it's an aqueous solution. That's a pretty cool property of our system. Heparin up here is an interesting compound. Heparin is an anticoagulant. Student: Are most mucuses in biological organisms composed of glycosaminoglycans? Kevin Ahern: They contain at least a significant Component of that, yes. Heparin is a very powerful anticoagulant. Looking at this compound, any ideas how it might work as an anticoagulant? I'll give you a hint. It doesn't look like Vitamin K, so it has nothing to do with that. What are those negative charges? Do you suppose those might be useful for something? Do we have any negative charges we talked about in blood clotting that had any significant impact on a process? Kevin Ahern: Calcium! Why was calcium important? Calcium was important because it was what the prothrombin modified side chains grabbed ahold of at the site of the wound. If this guy is grabbing calcium, what do you suppose is going to happen to the availability of calcium at the site of the wound? It ain't gonna be there. Heparin is very useful in that sense, and these negative charges can help sequester calcium. Proteoglycans. What the heck's a proteoglycan? If I talk about a glycosaminoglycan, what's a proteoglycan? Let's imagine I take a glycosaminoglycan and I attach it to a protein or multiple proteins. That's what's happened here. When I do that, I create a proteoglycan, meaning that it's a combination between a protein and a glycosaminoglycan. I told you that the glycosaminoglycans were polyanionic. I said that altered their chemistry. I said they really didn't like each other. They repelled each other, and you can look and see how they are staying as far away from each other as they possibly can. A visual representation of what I just told you about the properties of the glycosaminoglycans. The protein is anchoring everything there, on the inside, but we see these guys, the polyanionic sides of these, are getting as far away from each other as they can. So a proteoglycan is a protein linked to a glycosaminoglycan. There's a schematic representation of what I just showed you. Saccharide hybrids. We'll actually talk about this later in the term. We sometimes see sugars linked to other things. I'm going to talk about them being linked to proteins more specifically in just a minute. Here we see a molecule of glucose being linked to a nucleotide. It's being linked to a nucleotide. UDP is a nucleotide. UTP, of course, being uridine triphosphate; UDP being uridine diphosphate. We'll see that in the process of making glycogen, cells make this intermediate, right here, UDP glucose. The reason they make this intermediate right here is this guy is full of energy. It's a very high energy intermediate that we call an "activated intermediate." I'm going to define that term for you. An activated intermediate is a molecule that has a high energy bond... so an activated intermediate is a molecule that has a high energy bond and it uses the energy of that bond to donate 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 donate a part of itself to something else. What we'll see when we talk about glycogen metabolism is that, in order to add a glucose to a growing glycogen chain, it takes energy, and the energy for doing that comes right there. This guy is an activated intermediate because part of itself is becoming donated to a growing glycogen chain. This guy down here will become donated to a growing glycogen chain. The glycogen chain will get one unit larger in the process of making that glycogen. Questions about that? I'm sailing through things today. Let's see, glycoproteins. Glycoproteins, as their name suggests, are combinations between sugars and proteins. You might say, "What's the difference between a glycoprotein "and a proteoglycan?" A glycoprotein versus a proteoglycan. Well, let's think back to proteoglycan. A proteoglycan was a protein linked to a glycosaminoglycan. The "glycan" names are common to the two. A glycoprotein doesn't have the glycosaminoglycan. It has a relatively simple sugar on it... or oligosaccharide, as we will see, meaning it only has a few sugars, and they're not glycosaminoglycans. So a glycoprotein is a protein that has a few relatively simple sugars on it. There are many examples of glycoproteins. One of the more common ones that we talk about are those that are the blood group antigens. We talk about the various blood types, O versus A, AB, et cetera, and these characteristic blood types arise from the presence of specific glycoproteins on the surface of blood cells. So a person who has O-type blood will have this, A-type blood will have this, et cetera. And there's my little bouncing ball. Get out of here, bouncing ball. We can see that the composition—here's the protein part down here, here's the carbohydrate part up here—the composition of these are not very different. Galactose, N-acetylglucose, galactose and fucose. This guy over here, to have an A antigen, has galactose, N-acetylgalactose up there. This guy up here has galactose up here. But the immune system recognizes these differently. The immune system recognizes these differently, so you have to careful, obviously, which blood you put into which person, because if you put blood into a person that their immune system recognizes as foreign, they will attack that blood and kill the person. So understanding what type of blood that one has is important, and that typing occurs as a result of these signatures that are on the surface of the red blood cells. The signatures themselves turn out to be quite important not just for blood typing but also for tissue typing. When we, for example, give a transplant, we transplant an organ—maybe we're transplanting a liver from one person to another—tissues have their own identity markers that are on them that say, "Hey, here's what I am," and the immune system says, "Oh, that's what you are. "You're part of me." You take a tissue from a person that has a different kind of marker on their liver than what the recipient has and you're going to create incompatibilities and you're going to see rejection of that organ. So a major consideration in success of transplantation is matching the antigens that are on the surface of donated tissues. That helps immensely in the immune system leaving it alone, not attacking it as a foreign invader. Again, these arise from the composition of oligosaccharides on the surface of cells. Student: So what's the O-positive and O-negative? What's the positive and negative? Kevin Ahern: The positive and negative refer to the Rhesus factor and that's something separate from this, so I won't go into that here. Student: Well, I see that we have an antigen for A and for B separately, but what about people that are AB blood type? Kevin Ahern: They're going to have an immune system that's going to recognize combinations. When we look at the glycoproteins, we see that they typically contain two classes of linkages... two classes of linkages. One group of glycoproteins are what we call N-linked, meaning that they have a link through the nitrogen. They have a link through a nitrogen that joins the carbohydrate to the protein. The O-linked have a link through the, in this case, through an oxygen between the protein and the carbohyd—I'm sorry. I've got it down here. It's up here—the nitrogen and the carbohydrate, or the oxygen and the carbohydrate. So N-linked have a link through a nitrogen. O-linked have a link through an oxygen. It turns out that—well, actually, I should also point out that the N-linked are linked through the side chain of asparagine, whereas the O-linked are linked through the side chain of a serine. So this is the protein up here. There's asparagine side chain. There's a linkage. Serine side chain, there's a linkage. Different glycoproteins are made in different places in the cell. N-linked glycoproteins are made starting in the endoplasmic reticulum, but they get processed further in the Golgi apparatus. So there's a transport. There's a travel that's happening in the cell. O-linked glycoproteins are made solely in the Golgi apparatus. Now, this license plate, if you want to think about it, of carbohydrate residues that are there on a protein, proteins have little identity markers on them, in the form of carbohydrates, that tell the cell where this protein's supposed to go. Is this protein destined to go outside the cell? Is the protein destined to get buried in the membrane? Is the protein destined to work in an organelle inside the cell? Different tags on these proteins will tell the cell where this particular protein is supposed to go. The endoplasmic reticulum and the Golgi apparatus play a role in putting those tags onto proteins appropriately. Now, when we look at the N-linked glycoproteins, we discover that they have a common core. This common core you're not going to have to memorize, but you can see that it consists of five modified sugar residues. There's N-acetylglucose, N-acetylglucose and three mannose residues there. The ones in boxes we find commonly among all of the N-linked glycoproteins. The things out here will vary from one protein to another, because, remember, the composition of what's out here may tell the cell, "Hey, I'm destined to go into the nucleus." "I'm destined to go out to the cell membrane." "I'm destined to get kicked out of the cell altogether." "I'm destined to go to the lysosome." So these guys out here vary from one protein to another, but these are the common core that we see of the N-linked glycoproteins, the portions shown in gray. Yeah? Student: How far beyond that on the chain do those variable units usually extend? Kevin Ahern: How long, he's asking, is that, commonly, is that oligosaccharide that's out there? It's not typically overly long. What we see more commonly is we see variation of different sugars that are placed in there, and that gives a lot of different combinations and possibilities, but they're not real long, no. They're not like a long polymer, no. There's some more examples, and we see some modifications occurring there: sialic acid, galactose, N-acetylglucose, et cetera. I talked about the endoplasmic reticulum. I talked about the Golgi apparatus. We see the endoplasmic reticulum, out here. We see that basically migrating to the Golgi, transporting proteins, and the Golgi buds off, and those buds of the Golgi then transport specific proteins to specific places. All of those that are destined to go outside the cell will bud off in a specific place and go out and get kicked out of the cell, for example. One of the things that happens in the synthesis of N-linked glycoproteins is interesting. As I said, it occurs in the endoplasmic reticulum and it involves a large molecule here, known as dolichol phosphate. It's interesting what happens with it. When we look at how these carbohydrate residues on a glycoprotein are actually made, they're actually built on dolichol phosphate. So the carbohydrate portion is built on dolichol phosphate. Where is dolichol phosphate? Well, dolichol phosphate, if you look at it, it's got a long nonpolar tail and it's got a polar end. When the cell starts making that carbohydrate forked thing that's going to be on the glycoprotein, this phosphate part is sticking out of the endoplasmic reticulum. It's facing the cytoplasm side. It's sticking out, so here's this hand sticking out. So it's on this hand sticking out that this initial portion of the glycoprotein is put on. So we put several residues on the outside, up here on this phosphate, and at some point, and I almost think it's magically, at some point, this molecule inverts and comes in. So instead of projecting outwards, now this guy that has these carbohydrates on it comes to the inside. It's actually a flip that occurs so that now this is on the inside. Well, it's on the inside of the endoplasmic reticulum where the protein is found. The protein then gets a hold of this targeted carbohydrate that's on there, gets put on there, and they get modified a bit more in that process. Once it gets taken off, the dolichol phosphate flips back out and starts the whole process again. So this dolichol phosphate's critical in the synthesis of that carbohydrate portion of the glycoprotein, and it's a combination of things occurring outside the endoplasmic reticulum and then flipping in and basically delivering it to the inside. Student: [unintelligible] through some unique [unintelligible]of some type? Kevin Ahern: As far as I know, and as far as is known, there's not a particular thing it comes through. I think it's thought that there actually is a protein that facilitates its movement, but it's not coming through anything. It's actually literally just flipping itself like that. And that's kind of odd, because when we talk about membranes, later, we'll see that it's very unusual for membrane lipids to do that flip. But this guy does it and it's kind of cool. In the case of membrane lipids, one of the things we see is there's enzymes called flippases that actually help that flipping process. So it may be that there's something like that that's helping this, as well, but it's not going through anything, no. You guys look tired. Shall we sing a song? Shall we sing two songs? Okay, I have two songs. They're both relevant. They're actually both easy to sing, too. You guys liked the extra credit on the last exam, right? Students: Yes. Kevin Ahern: What was the deal on the last one? Sing loud, okay? Let's sing loud, okay? We'll start with this guy. I've never sung it in class before. It's called "Hyaluronic Acid." ["Hyaluronic Acid" to the tune of "Rudolph"] Everyone: Hyaluronic acid, acting almost magically, placed just beneath the kneecap, lubricating the debris. Better than joint replacement, simple as 1-2-3, if it can stop the aching, you will get to keep your knee. When the pain is getting bad, try not to be sad. Just go out and have a talk, with your orthopedic doc. Beg him to use the needle. To not do so would be a crime. Hyaluronic acid, working where the sun don't shine. [laughter] The next one is also very easy to sing, and it is to the tune of "Hark the Herald." [All singing "Hark the Sucrose"] Everyone: Carbohydrates all should sing, glory to the Haworth ring. Anomeric carbons hide, when they're in a glycoside. Glucopyranose is there, in the boat or in the chair. Alpha, beta, D and L, di-astere-omer hell. Alpha, beta, D and L, di-astere-omer hell. Have a good weekend! [scattered applause] [END]