Steroid Delta-isomerase

Transcription

Kevin Ahern: I promised I'd get them back to you as quickly as I get them back from the TAs and that's what I'm waiting on at the moment. They have an exam but as I told them I still expect to have it done before now, so that's life. So we're going to move forward with fatty acid oxidation. We have fatty acid oxidation, we've got synthesis, and we've got ketone bodies and also prostaglandins to talk about so we have quite a few things to talk about now and Wednesday. So I'm fairly optimistic. I think we're actually in pretty good shape with stuff here. But we have a fair amount of material to cover. I know you guys know I go fast, and I am going kind of fast through things. So I apologize for that. But we are because of our off-day a little behind. Well last time I talked about the fact that we have the problem with fat in that fat is water-insoluble. And you've already seen the issues that the body has with respect to moving that insoluble material through the bloodstream. And we also recognized that that insolubility requires a good storage mechanism. And so as I mentioned last time there are specialized cells called adipocytes that are involved in storing that fat. What you see on the screen is a depiction of the breakdown process for a fat that is involving using a lipase and a lipase right here cleaves each fatty acid off of the glycerol backbone leaving glycerol plus three fatty acids. I told you that lipases can be extracellular. Lipases can be intracellular. And what I'm going to focus on today are all the things that are happening inside the cell. So everything I'll be talking about today will be happening inside of the cell. When we look at the regulation of fat breakdown we see, not surprinsingly, that fat breakdown is partly a function of a system like you've seen before. It's regulated through a 7TM. That 7TM is the same 7TM that you saw that was involved in the regulation of the breakdown of glycogen and also in the glucose in gluconeogenesis, or glycolysis and gluconeogenesis pathways. This is the beta-adrenergic receptor. We have binding of a hormone to the receptor which activities a G protein which activates adenylate cyclase which makes cyclic AMP, activates protein kinase. And protein kinase comes in and acts in a couple of places, and I don't think we need to know all of the detail that's in here. This is a little bit more than we really need to know. But what I would say that I think that you should know is that the phosphorylation of a target lipase activates that lipase and thus activates the breakdown of fat. So the breakdown of fat is occurring here inside an adipocyte and that adipocyte could release those fatty acids and glycerol out into the bloodstream. And again those fatty acids are going to be carried by the serum albumin, I can't pull the word out myself, carried by the serum albumin in the blood. And that serum albumin will then donate those fatty acids to target cells so the cells have what they need. Well what we saw in the case of glycogen breakdown was that phosphorylation of the enzymes involved in glycogen breakdown tended to activate them, and so phosphorylation activated breakdown in phosphorylation here is also activating breakdown. So not surprisingly, phosphorylation is favoring the breakdown of larger substance to smaller substances and of course that's what's happened, that's what's necessary for energy production. So again, cells are needing energy. This turns out to be the only regulation of fatty acid breakdown. Fatty acids themselves there's no regulation of the scheme that I'm getting ready to show you. So the fatty acid breakdown is controlled at the level of the breakdown of fat. It's controlled at the level of the breakdown of fat. Now we will see some regulation in the synthesis of fatty acids. There's one primary regulatory step. But for our purposes the breakdown of fatty acids is only regulated at the level of breakdown of fat. Glycerol itself is another component that is produced by fat, by the breakdown of fat. And glycerol turns out to be the only component, with a minor exception that we'll see later, but it's the only component that can be used to make glucose out of fat. Glycerol is a three-carbon compound. That three-carbon compound can be converted into in this case glycerol phosphate, and ultimately into dihydroxyacetone phosphate which of course once we get to dihydroxyacetone phosphate we can make glucose through gluconeogenesis and so glycerol can ultimately, if we have two of them, glycerol can ultimately be converted into glucose and that's the only component, the only significant component of fat that can be. The fatty acids are largely broken down to acetyl CoA's. And acetyl CoA I hope you learned from earlier is not a source of material for making glucose, at least in animal cells, because we don't have the glyoxalate cycle. Plant can use that but we can't do that. I don't think we need to go through that. So let's turn out attention now then to the breakdown of individual fatty acids. I gave you a schematic showing you that the other day and I wanted to just briefly show you that again to remind you. So we looked at fatty acid oxidation. This process shown here on the left. And it's a very nice scheme. I like the overall view of this picture of this process very well. And this process has a name. It's called beta-oxidation. Beta-oxidation. And the reason is because if we number the carbons as alpha/beta, all the action happens as I said between carbons two and three, it's the beta carbon where the process is happening as we shall see. So to remind you there are several steps in the process the first step being an oxidation step. And that oxidation step is known as a dehydrogenation. A dehydrogenation. You've seen dehydrogenation before as I said but this is the similar dehydrogenation that you saw in the succinate dehydrogenase reaction pulling hydrogens and associated electrons away from the two carbons leaving behind a double bond. Notice that double bond is in the trans configuration. And again that's exactly what happened in the synthesis of fumarate in the citric acid cycle. Hydration followed that. That takes a water, splits it into an OH and an H. The H goes onto carbon two and the OH goes onto carbon number three. And again that's very much like what we see in the citric acid cycle where we convert fumarate into malate. That hydroxyl group is a target for further oxidation and that further oxidation yields this compound, and that is equivalent to the oxidation that we see in the citric acid cycle for making oxaloacetate. So again these things are very very similar. And the last step in process involves what's called thiolytic cleavage, T-H-I-O-L-Y-T-I-C. And that process uses that enzyme that I'll talk more about later. That enzyme is known as thiolase, T-H-I-O-L-A-S-E. Now there's only just a couple of enzymes I'm going to ask you to know the names of. Thiolase is one of them. Thiolase catalyzes this last step where a acetyl CoA is split off and a fatty acid with two fewer carbons is left. Now in each case, this says an R, R, R, R, R. These R's are all coenzyme A's in the case of oxidation. They're all coenzyme A's. So that's the overview. Let me take you and just say a couple of words about the individual steps to consider in this process. Let's imagine I'm a cell that's taken in a fatty acid from the outside. It's been delivered to me by serum albumin and I've gotten it inside. Well that fatty acid, when it gets inside the cell, is just a free fatty acid and as I've tried to point out to you before this is a little bit of a problem because this guy can act as a detergent. We don't want it to act as a detergent and denaturing our proteins so cells basically cover it up with a CoA on the end. And that's what's happening in this process. That takes energy to do so. You'll notice it takes a lot of energy. It takes two phosphates off of there. ATP is going to AMP to yield what is called an acyl CoA, A-C-Y-L. The acyl is an acyl group. It just simply means it's a fatty acid. So fatty acid CoA is what's produced by that. Now fatty acids with a CoA on them, unfortunately, don't make it across the mitochondrial inner membrane. So the cell puts a CoA on there and then it takes it to the mitochondrion and goes, "Uh-oh, a CoA won't get me across the membrane." So the cell has an enzyme known as a carnitine acyltransferase that will take that acyl CoA. It'll take off that CoA it just put on there and it will replace it with a carnitine group. So it's basically making an acyl carnitine. It turns out that acyl carnitine is in fact something that can be transported across the mitochondrial inner membrane. There is a translocase that does that. You can see that happening here. We see that translocase is an antiport, and the antiport when it brings in an acyl carnitine kicks back out a carnitine. There's a one to one relationship and so we have accounting for all carnitines that are there. Then, and of course fatty acid oxidation as I've said before occurs in the mitochondrial, in the matrix of the mitochondria. Once acyl carnitine has gotten in, the carnitine is removed and replaced by a CoA. So it's kind of a dumb process. Fatty acid comes in, acyl group gets attached, acyl group gets removed, replaced by carnitine, carnitine comes in, carnitine removed replaced by CoA. So in essence what we've got at this point is an acyl CoA that is in the matrix of the mitochondrion and ready for oxidation. Okay, so [inaudible] oxidizing. Jodie? Student: Is this in all cells? Kevin Ahern: This is in all cells, yeah. So here we go. Oxidation. So we've seen the oxidation process. That's exactly the figure I showed you before. It's now showing you a little bit more detail and the detail is that when we have an oxidation, of course, we have to have an electron acceptor, and the electron acceptor for this first dehydrogenation is FAD. It becomes FADH2. Just again like we saw with the succinate dehydrogenation reaction. So we have the product of an FADH2. I don't care that you know the names of these guys. That's not important. Trans-delta squared-enoyl CoA, L-3-hydroxyacyl CoA, I'm not worried about that. But I think you should know their general structure. You should know that there is a double bond trans configuration between carbons two and three. You should know everything that's happening between carbons two and three. The water, when it adds, the hydroxyl group goes onto carbon three and it creates an L structure. We'll see in fatty acid synthesis that there's a D instead. That's one way in which they differ. So we have an L-configured hydroxyl group on this at this point. The oxidation of that hydroxyl group causes NAD to be converted to NADH. So at this point in this oxidation of this fatty acid we've made one FAD and we've made one NADH. We are then ready for thiolytic cleavage, and thiolytic cleavage happens right between carbons two and three and we have the products that we talked about before. Now there's only one enzyme besides thiolase on here I want you to know and it's an important one. It's the very first one. It's called acyl CoA dehydrogenase. Not a complicated name. Acyl CoA dehydrogenase. So it's working on an acyl CoA. It's removing hydrogens and electrons. It's making FADH2. Why do I want you to know that name? Well it turns out to be an important enzyme from a health perspective. Our bodies have three different forms of this enzyme, one that works on long fatty acids, long meaning longer than probably about eighteen or so carbons. One that works on medium length fatty acids and it'll typically work on fatty acids anywhere from about eight to eighteen, eight to sixteen, thereabouts. And then there's the ones that work on short fatty acids and it'll work on fatty acids generally shorter than about eight carbons in length. Three different acyl CoA dehydrogenases. And as we'll see the longer one is found in a different location. It actually is not in the mitochondrion and we'll talk about that in just a little bit. It's the medium one that's of interest from a human health perspective because it's the medium one that's frequently found to be deficient in children who die of sudden infant death syndrome. So it appears at least in some cases that sudden infant death syndrome is linked to an inability to break fatty acids down through that middle component. They're getting pretty to eat and so on and so forth but they're not breaking fatty acids down through that middle range. As a consequence probably are starved for energy at that point. Okay, let's see. That process will continue and continue and continue until finally we break everything down. Now fatty acids, when we look at them, we discover they mostly have even numbers of carbons. Mostly they have even numbers of carbons. We'll see other considerations but mostly they have even numbers of carbons. So if we start with eighteen it goes sixteen, fourteen, twelve, ten, eight, six, four, and when it gets to that four it splits it in half and you've got your final two. If we have an odd number of carbons we have other considerations and I'll show you that. There's a bunch of enzyme names that we're not going to worry about. As I said, the acyl CoA dehydrogenase, and the what I call thiolase-they call it beta-ketothiolase. You can call it thiolase and that'll be fine by me -are the two enzymes whose names I think you should know. Oxidation of fatty acids is important and it's an important source of energy, but of course what you've seen so far what I've shown you has been the oxidation of fatty acids that have saturated bonds. I haven't showed you any unsaturated bonds so we have to have some considerations for how we oxidize fatty acids with unsaturation within them. An example of it might be the one you see on the screen known as palmitoleoyl CoA. Palmitoleoyl CoA has sixteen carbons and it has a single double bond as you can see here. Well if you think back to what I showed you before you said, "Well there was a double bond "that was in that thing that you showed us. "Can't it just go from there?" and the answer is no it can't. A couple reasons. One is the double bond may not be positioned at the right position, that number two and three that I've talked about. That's one consideration. And another consideration is the double bond here as it exists in most fatty acids in our body is in the cis configuration, and the one you saw before was in the trans. So there are some considerations in how a natural cis-containing bond of an unsaturated fatty acid must be dealt with in fatty acid oxidation. That's what we're getting ready to see right here. Here's what happens. So we go through several cycles, each cycle knocking off two carbons at a time until we get to this thing that look like this. Here's carbon number one, carbon number two, carbon number three, carbon number four. First of all, the double bond we notice is not between carbons number two and three, and second of all it's in the cis configuration. Well this arrangement turns out has a very very simple solution for the cell to consider. It simply moves the double bond from position three and four in the cis to positions two and three in the trans. Bang! one enzime and this guy now can go through the rest of beta-oxidation without problem because the next thing that's going to happen to this guy is going to be what? Addition of water, right? So it'll have water across this and we'll continue along the process. It'll be oxidized just fine and everybody's fine and dandy. Well things aren't always that simple. What if we have multiple double bonds and they have other arrangements? And so that's what we have to talk about in our next consideration. I think you should know the name of this enzyme. It's known as enoyl CoA isomerase. I don't care if you know the cis part, but enoyl CoA isomerase. That's the enzyme that converts a 3,4-cis to a 2,3-trans. That's going to be an important consideration in fatty acid oxidation overall for unsaturated fatty acids. We'll see how that comes up in other places. What if we have something that looks like this guy? And with this guy we have two double bonds and they're not positioned exactly the way we need to have them positioned. So how do we deal with this? Well let's take this into consideration. We first of all have elineoyl-, linoleoyl -I can't even say that- CoA and we go through several rounds of oxidation until we get to this process. We're sitting by here we go one, two, three, four. And we see, what do we have? We have a 3,4 with a cis and we want to make it into a 2,3 with a trans. There's our friend from the last cycle, enoyl coA isomerase. It does exactly that. The next step that will happen in this is we will add water across this double bond. And we add water across that double bond, we will oxidize it, and we will ultimately break this guy, one, two, three, we're going to break this guy right here. When we do that, and there should be a couple of arrows here indicating that we've got a couple more steps to get to here, we're left with this guy. Now this guy is a little bit different. It's got one, two, three, four, five is where the double bond is and it's in the cis configuration. You might say, "Well why doesn't the beta-oxidation process "just continue and you can take away "these hydrogens and electrons?" but it turns out the enzyme won't touch this. This guy has got to be dealt with. And the way this guy deals with this is, or the way the cells deal with this is a little bit of unusual. First of all we see that there is the start of the process. The start of the process is we do remove a hydrogen and associated electrons to make this, but this is a conjugated set of double bonds and this is a different kind of character then what we had over here. These are not conjugated, these are conjugated. Notice that this guy is in the trans configuration. This is in the cis configuration. And another enzyme is necessary to deal with this unusual structure for the cell. This conjugated double bond's really a very different kind of electronic environment than we had over here. This is dealt the by the enzyme 2,4-dienoyl CoA reductase and what it's doing is two things. First of all it's adding electrons from NADPH so we know that we're going to have one double bond leftover after we finish this process, and the one double bond is not between either four and five or two and three but instead it is now between three and four. So basically we've merged these two into one double bond and that was possible because electrons and protons came from NADPH. And look what we did. We put them in the 3,4-trans configuration. Beautiful. Now we can use their enzyme and we're preceding along the way. With these two enzymes we can oxidize every unsaturated fatty acid that we have in our body. Now if you're using the sixth edition of the textbook I will caution you that this structure that they show in the sixth edition is wrong. They show it in the cis configuration and it's not. It's actually in the trans configuration like you see here. Okay, so basically now we have something again that's 2,3-trans, we can go ahead and-actually, I said that wrong, didn't I? So this is, that's not the trans. That should be in the cis, right? So it's still wrong. I just noticed that. So in the seventh edition of the book they didn't correct that error. This guy is in the cis because a 3,4 and the cis is what's needed for this enzyme and they're showing it in the trans. So the seventh edition is wrong as well as the sixth edition. This actual structure is in the cis configuration, not the trans as it shows you. Make a note of that. So that should be in a cis, not in a trans. Student: So the name is also wrong? Kevin Ahern: The name is also wrong. Again I'm not worried about the names so don't worry about the names. Well, we can now oxidize all the unsaturated fatty acids that we have inside of our cells. Well the last thing I need to deal with then, one of the last things I need to deal with on the oxidation side of things, is to deal with what happens if I have an odd number of carbons. If I have an odd number of carbons then there are other things I have to think about. So we can get down, let's say I started with a fatty acid that had seventeen carbons. When I keep chopping, chopping, chopping, I'll get down to five. I knock off two, I'll get to three. And you might think well you can just cut that three into a two and a one but the enzymes won't touch that three. So the three which is known as propionyl CoA has to be dealt with and that's what I'm going to be showing you right here. So how do cells deal with fatty acids that have three carbons? And the answer is right here. It's a very odd scheme. I can't emphasize this enough. This is a very, very odd thing that cells are doing in oxid-, not oxidizing but in metabolizing propionyl CoA. I hope to convince you of the oddness of this. Here's propionyl CoA. It's got three carbons. And the cell says, "Oh, well if it had four carbons "I could work with it." Okay, well let's put another carbon on it. We can put a carbon on it very easily and in fact can put a carbon dioxide on it very easily so it'll be something like a fatty acid that we might metabolize. Well let's put this on here. So we grab a bicarbonate and we grab some ATP and we put it on. Except, instead of putting it on the end we put it on carbon number three, or I'm sorry, carbon number two. Okay well that's fine. All I have to do is move it. Well, no not quite. Not only did we put it on carbon number two but we put it on carbon number two in the D configuration, and we really want it in the L configuration. Duhh! What were you thinking cell? Well here's the L configuration and then, once it's in the L configuration it says, "Oh, I know where I want this guy!" and it's on the end. Instead of going from here to here it goes through here, here, and then over to here. This is an odd set of reactions and it's an odd set of reactions that actually involves movement of a methyl group. This methyl group gets moved around in this process and it takes an odd system to do that. The odd system that does that-and by the way, once we get down to here at succinyl CoA of course this now can be oxidized in the citric acid cycle. It can be used in the citric acid cycle. So at this point that propionyl CoA is in usable. That's what we're trying to get to over here is something usable. Well moving this methyl group around actually requires action of vitamin B12. I think you should have to draw this for the next exam so please memorize this structure. [class laughing] If you get that cobalt in the wrong configuration you're hosed. So don't forget the cobalt. You won't have to memorize it of course. But B12 is an interesting coenzyme. [pointer batteries clattering] I'm getting violent in my old age here. [class laughing] Either that or I had too many beers before I came to class, I don't know. [class laughing] It still works. Okay, it's an unusual coenzyme. It's the only molecule that I'm aware of in our body that uses cobalt, and that cobalt actually plays a role in grabbing and carrying those methyl groups. Vitamin B12 is important for several things. This is one of the things that vitamin B12 is important for. You're not responsible for this mechanism. I'm just showing you what's happening in this process. And we're making some swaps. There's the grab of what will become actually a methyl group right there is CH2. And there's the overall process. So now we pretty much handled everything. There's only one thing I haven't told you about oxidation, and I told you about it but I haven't shown you, and that is the fact that long fatty acids actually do get metabolized in a different location in the cell. So when I showed you everything coming into the mitochondrion I lied a little bit. You guys are used to professors lying. They do that all the time, right? "Oh this is going to be an easy exam," right? Well the truth is that the long fatty acids start their oxidation process in an organelle known as the peroxisome. This is what a peroxisome looks like. It's normally not quite that large. And peroxisomal fatty acid oxidation requires that long chain acyl CoA dehydrogenase that I talked about. There it is. And this enzyme is only found in the peroxisome. So if you want to oxidize long chain fatty acids you have to have, you have to first of all get them into the peroxisome. And when they go into the peroxisome they get a CoA just like they got in the mitochondria. Now peroxisomal oxidation of fatty acids is not nearly as efficient from an energy perspective as mitochondrial oxidation of fatty acids is. The reason for that is because there's no electron transport chain in the peroxisome. There's no electron transport chain in the peroxisome so the cell's got to have a way of regenerating FAD from FADH2. The way it does it is by action of an enzyme that adds electrons to oxygen and makes hydrogen peroxide. Hydrogen peroxide breaks down to water and O2, but that means the electrons that were here are not going in any reasonable fashion into the electron transport system. This guy can then go on through further oxidation which it would continue to go through. And, again, we're not going to have as much energy realized because we've got a NADH that we'll have to deal with as well. And there are shuttles it takes to actually get that out of the peroxisome into the mitochondrion, et cetera, et cetera. And that's a pretty involved process. So peroxisomal oxidation is not generating as many ATPs as mitochondrial oxidation is generating. Yes? Student: [Inaudible] Kevin Ahern: Is that enzyme active whenever it's in the peroxisome? That's a good question and the answer is it is, yes. It is. Well once a fatty acid gets broken down to a small enough size, let's say down into the eight- to twelve-carbon length it gets moved out of the peroxisome and it gets taken to the mitochondrion where the rest of the oxidation continues. Actually it's even longer than that. It's down to about sixteen to eighteen. So once it gets down to about sixteen to eighteen it's taken to the mitochondria and is oxidized. So that's what fatty acid oxidation looks like. Before we move to the next topic I thought we'd do a song. It's to a tune that you will recognize because we used the tune I think the last time we sang, and it's to the tune of "When Johnny Comes Marching Home." [all singing "When Acids Get Oxidized"] Lyrics: The fatty acids carried by CoA, CoA Are oxidized inside the Mitochondriay They get to there as you have seen By hitching rides on carnitine Then it goes away when acids get oxidized Electrons move through membranes, yes it's true, it's true They jump from complex one onto CoQ, CoQ The action can be quite intense When building proton gradients And it's good for you when acids get oxidized The protons pass through complex V you see, you see They do this to make lots of ATP, TP The mechanism you should know Goes through the stages LTO So there's energy when acids get oxidized [end singing] Okay, that was a short one. We turn out attention next to ketone bodies. Now ketone bodies turn out to be relevant to fatty acid oxidation. It may not be apparent at first but I'll tell you why that's the case. Your body as you've seen has a very large need for glucose. Glucose is our immediate source of energy. It's made by our liver, whether by breakdown of glycogen or by gluconeogenesis. What happens in those cases where glucose is not available? We do know that we go hypoglycemic sometimes and when that happens it's really nice and important to have a backup energy source available because if we don't have that backup energy source available our brain is toast, our eyes are toast, and probably we are toast as well. Well the backup energy source turns out to be -that bouncing ball. If we could just harvest the energy of that bouncing ball... [laughter] ...life would be so much better. The backup energy source turns out to be ketone bodies. Now ketone bodies are important because we can make them from fatty acids. So we're making a readily available water-soluble set of compounds from fatty acids. That means they can get released quickly and they can go and provide energy when needed just like glucose can be made available quickly. Well how do we make them? It turns out that we make ketone bodies starting with the last enzyme of fatty acid oxidation. Thiolase you recall split off a two-carbon piece from a longer chain, right? Breaking between carbons two and three. Well what if that last piece has four carbons? It's going to make two twos, right? It's going to make two acetyl CoA's from one four-carbon acyl CoA. Well all we're doing with this reaction is going backwards. So in the normal oxidation process we'd be going right to left. In the synthesis of ketone body direction we're going from left to right. Same enzyme, thiolase. So thiolase catalyzes that first reaction. It creates this molecule called acetoacetyl CoA. And that's actually what the last molecule looked like in fatty acid oxidation. A third set of two-carbons is brought in with another CoA, and again the CoA is split off but the carbons are kept, and we create 3-hydroxy-3-methylglutaryl CoA or as we talked about it earlier, HMG-CoA. You saw this molecule when we were making cholesterol. HMG-CoA as I pointed out at the time is also an intermediate in the synthesis of ketone bodies. That six-carbon intermediate is rearranged and a two-carbon piece is removed yielding this four-carbon guy here, acetoacetate. That's one of the ketone bodies. It is a ketone as you note. We see that that acetoacetate has two possible fates, one that's useful and one that's not. The useful fate is shown going up because that's a reduction and that reduction creates a molecule called D-3-hydroxybutyrate, or you can just call it hydroxybutyrate if you want. That hydroxybutyrate is stable. Chemically it's stable. It's a nice way of moving this four-carbon piece through our bloodstream. If acetoacetate doesn't get converted to that four-carbon piece it is chemically unstable and it will spontaneously decarboxylate to yield acetone. It will spontaneously decarboxylate to yield acetone. Now this turns out also to be of critical human health importance. The important piece of this is we're making ketone bodies when we're very low in glucose. People who have some forms of diabetes have real highs and real lows of glucose, and one of the ways they discovered they were diabetic without any other indication is if you can smell acetone on their breath. If you smell acetone on one of your friends' breath, and I usually get the report of people who've claimed that happened in this class once or twice a year, if you smell acetone on your friend's breath you want to tell them to go to the doctor and get checked to see if they have diabetes. Just because they have acetone doesn't mean they have diabetes, but they could because this means that their glucose levels have gotten very low and the body is dumping ketone bodies out. Some of them are breaking apart in the lungs to produce acetone which they exhale and which you can smell on their breath. Well I've talked about why this is an important energy source. I haven't told you how we get the energy. The brain needs energy. The brain needs glucose. The brain can't make glucose. And if you're starving or there's something else going on and you're not producing enough glucose the body will start making these ketone bodies which go into the bloodstream. Let's start up here. This guy's in the bloodstream. It makes it across the brain-blood barrier. I said that, didn't I? The brain-blood barrier. And then what do we do? We reverse the whole process. We go from here back to here, from here back to here, here back to here, and finally what we have done is we have delivered to the brain two acetyl CoA's. What can acetyl CoA be used for? Synthesis of ATP in the citric acid cycle. We've just saved the brain. Clear as mud? Oh your thumbs up. You approve of this. It's an election year. "And I approve this message," right? Okay, that's good. Let's see, what else can I say? That's a bunch of blah blah blah, and this is simply showing you the reversal of that whole process that I told you except for I started with... [loud voices outside classroom] Well hello! Please come down. Let's move to fatty acid synthesis. So we've broken down fatty acids. We need to know how we make fatty acids. How do we make fatty acids? Well fatty acid's, as I said earlier, chemically is very much like the reversal of the oxidation process. We're going to see there are some differences but chemically it's pretty much the reversal. One significant change as we will see and that's the very first steps in the process. Fatty acids grow by two carbons at a time during the synthesis process. However, and this is a big however, the starting material is not a two-carbon piece. It's a three-carbon piece. So the cell is going to do something odd again in the synthesis of fatty acids. It's kind of like the odd thing you saw with the odd number of carbons. This is going to involve an odd number of carbons in the synthesis process. Here's acetyl CoA. Acetyl CoA gets converted to a three-carbon molecule known as malonyl CoA by an enzyme whose name you absolutely need to know and you're going to need to know something about how it works. It's known as acetyl CoA carboxylase. Acetyl CoA carboxylase. This guy, this enzyme, turns out to be the only enzyme in fatty acid synthesis that's regulated. It's the only enzyme in fatty acid synthesis that's regulated. By the way, fatty acid synthesis is not occurring in the mitochondrion, again with a minor exception that I'll talk about later. It occurs in the cytoplasm. It's occurring in the cytoplasm. So oxidation and synthesis are physically separated in the cell. They're physically separated in the cell. Well in that process of making fatty acid there's another difference from the oxidation and that is the carrier. We saw the very first process occurs on a CoA but that's the only place where we see CoA's involved. Everything else involves something called an acyl carrier protein. So in place of CoA during synthesis we have something called acyl carrier protein, and though it may sound very different, if we look at what's attached to the acyl group, they're identical. Look at that. The thing hanging off of this protein is the same thing that's hanging off of coenzyme A. By having a carrier protein the cell is able to recognize this is something that's being synthesized. It's a very visible difference because this thing hanging off here isn't present out here on coenzyme A. So this guy is being synthesized. Here are the synthesis reactions. So once we get it to this point we have a malonyl ACP and it turns out we have to have also an acetyl ACP because we have to have a starting material to start this fatty acid. We have to have a two-carbon piece plus a three-carbon piece to start. So this is priming the pump. This is getting everything set up. There we go. So here's the two in blue. Here's our three-carbon malonyl ACP. That's the other starting piece. What happens? The first step is what we call condensation and that simply involves attaching two carbons of that three-carbon piece to the two carbons of the acetyl ACP. Follow the colors, okay? Here we go. The CO2 that comes off is this guy right here on the end. There's your CO2. The two carbons of the acetyl ACP go onto the end. They go onto the end. So the attachment of this piece to this involves the loss of carbon dioxide. That's a condensation reaction that's occurring. Now we're home free because now, that looks just like the end of fatty acid oxidation. The first thing we're going to do is we're going to reduce that ketone to an alcohol. We do that using NADPH. That makes a hydroxyl group. Again, between carbons two and three is where all the action is. This one's in the D configuration. Previously we saw the fatty acid was in the L configuration when we were doing oxidation. The next step then involves removal of water to make a double bond. That's a dehydration. There we go. There's our trans double bond. And the next step then involves hydrogenation, that is adding hydrogens and associated electrons from NADPH, and we're right there. Now there's a whole bunch of enzymes that do this and they've got a mouthful of names that are this long. But the enzymes that do this are really interesting. When we look at cells they're really interesting. Why are they so interesting? Because they're all contained in one giant complex. One giant complex. And literally the complex works like a clock. We see it, literally the fatty acid chain is moved around in a circular fashion with each of the reactions occurring as it goes around and around and around. We give one name to that big complex. It's known as fatty acid synthase, S-Y-N-T-H-A-S-E. So you don't have to know all the names of all the enzymes that do this because they're all contained in this one big enzyme known as fatty acid synthase. Acetyl CoA carboxylase is not in fatty acid synthase. That's a separate enzyme. Keep that in mind. That's separate. This guy here has all these other reactions in here that are important for the process to occur. Okay, so now we can make fatty acid. We've gone through one cycle. Now this becomes the starting material for the next cycle. So instead of having an acetyl ACP we would have a butyral ACP and we would add another three to it. Or we'd add two of the three. We'd have another malonyl to start. So malonyl ACP will always be needed on every round of the synthesis cycle. And on every round one of those carbons is going to get lost. The newest material in the fatty acid as it's being made will be the material closest to the ACP. The newest material will be on this end of the molecule, the oldest material will be on this end of the molecule. Okay let's see. What else did I want to say here? Reaction summary, blah blah blah. There's a whole bunch of different names and you're going to call it fatty acid synthase. There's fatty acid synthase's schematic. Oh you don't want to see that, do you? [laughs] I just show you this. You don't need to know this mechanism. But this is the clock thing. You can see literally it's going around the hands of the clock. And this enzyme, this basic structure of this clock appears to be conserved across almost all of evolution. So the structure itself turns out to be very useful and very important in a variety of cells. Yes sir? Student: So the fatty acid synthase [inaudible] Kevin Ahern: So the fatty acid synthase is not regulated. Once it starts it's cranking it out. Now one last thing I'll say and then we'll call it a day. And the last thing I'll say is this fatty acid synthase works up to sixteen carbons. The endproduct of fatty acid synthase is palmitic acid or palmitoleoyl CoA. We'll take it up next time from there. [class murmur] Student: Is that ACP [inaudible] Kevin Ahern: I'm not sure I understand the question. Student: That schematic showed ACP right in the center of the four pieces of fatty acid synthase. Is that always- Captioning provided by Disability Access Services at Oregon State University [END]