Fatty Acid Synthesis - Part II

- [Voiceover] To return to some basic principles and to put everything in perspective for you, I wanna remind you that fatty acid synthesis is like any other anabolic reaction that occurs in our body. Remember that the common theme with anabolic reactions is that we take some type of monomer, generally, and we link these together to form a polymer. The challenge with these types of reactions is, of course, that generally these reactions have a positive delta g value, or in other words, they are not spontaneous. The require an input of energy to occur. Obviously this is a challenge. But our body resolves this challenge by coupling this reaction with a reaction that has a favorable delta g value, that is, a delta g value less than zero. In this case, our body uses the energy currency of our cell ATP. And of course we know its reaction with water has a very favorable delta g value, very negative. So when we break it down, we form ADP and a free phosphate group. Now, just to remind you here that our monomer, when we're talking about fatty acid synthesis here, is acetyl-coA, so these two carbon subunits. We're gonna link these together to form a large fatty acid chain. Now, before we get into the specific steps of fatty acid synthesis, I wanna kind of take a bird's eye view first and write out what the overall chemical reaction would be for fatty acid synthesis occurring in our body. Essentially what I'm doing is, I'm adding these two equations together. Just to make this super clear, I wanna remind you that, in order for this reaction to occur, obviously we need to have a favorable delta g value. So overall, this is going to have a negative delta g value, which tells you, of course, that the hydrolysis of ATP is much more negative than this polymerization reaction is positive. Of course, if we add these two delta g values together, we get an overall negative delta g value. All right, so the overall chemical reaction that I wanna write out for fatty acid synthesis is for the product of a 16-carbon fatty acid chain that is called palmitic acid. palmitic acid happens to be the primary product of fatty acid synthesis in our body. So we're gonna focus on this. But just so you know, our body can essentially use this kind of basic product of fatty acid synthesis to synthesize longer chain fatty acids if need be. Remember that we wanna start off with our monomer, which is acetyl-coA. I'll remind you that this is a two-carbon molecule. So if we need to make a 16-carbon fatty acid, we would need eight acetyl-coA molecules. Of course, since this is an anabolic reaction, we need ATP to make an appearance as well. It turns out seven ATP molecules are required for this reaction. In addition, we also actually use a molecule called NADPH. You might recognize this from the pentose phosphate pathway. But remember that we've also produced NADPH when we've shuttled this acetyl-coA over to the cytoplasm from the mitochondria, because remember, that was done by a molecule called citrate. And citrate broke up into acetyl-coA as well as oxaloacetate. Oxaloacetate produces NADPH as it was being recycled back to pyruvate. And this is really important because it's a source of electrons that will be able to reduce any oxidized groups, any groups that are oxidized on this acetyl-coA molecule. What I will show you in a minute is that acetyl-coA has a carbonyl group in its molecule. And of course, we wanna make just carbon-carbon bonds that are essentially attached to hydrogen only. So in order to reduce this bond, we need a source of reducing power, and NADPH is that source of reducing power. It turns out that we need 14 NADPH molecules. So that's all of the reactants that we require, and we form this palmitic acid. Of course, we also form the byproducts of the rest of these reactants. We form, if we started off with seven ATP, we end up with seven ADP and, of course, seven free phosphate groups, and 14 NADP plus, because we've removed these electrons and of course we're oxidizing this now. We also end up losing some water molecules as well during the course of this reaction, end up losing six water molecules. I'll actually go ahead and move the 14 down here so it doesn't confuse you. I also forgot just one more thing here. It turns out that these coA, coenzyme A functional groups here, on this acetyl-coA molecule, will also be removed in the process of making these carbon-carbon double bonds. So we'll also form, as a byproduct, eight coenzyme A molecules. All right, I'm gonna go ahead and scroll down so that we can return to the cytoplasm and figure out how acetyl-coA is polymerized into this palmitic acid. Now, because I know that it's quite easy to get kind of lost in the details, especially when talking about metabolic pathways, my goal here is not to provide a detailed reaction mechanism for every step of this reaction, but rather to just cover the high-yield steps, the steps that are kind of the most important, I think, for you to be able to take away. So once we're in the cytoplasm and we have acetyl-coA at our disposal, turns out that the first step of fatty acid synthesis is to charge up this molecule to a higher energy molecule called malonyl-coA. And just by the fact that I'm saying charging up this molecule should clue you into the fact that this is thermodynamically unfavorable. So we need something that is thermodynamically favorable to be coupled with this reaction. And indeed, that is that hydrolysis of ATP, which I'll just write shorthand here as ATP going to ADP plus a free phosphate group. Now, it might be tempting to think that we're charging up this acetyl-coA molecule with this phosphate group here. But what we're really doing, actually, is, we're charging it up with a carbon dioxide molecule. And the breakdown of hydrolysis of ATP is really what's fueling this reaction forward. To give you a sense of what this looks like chemically, let me go ahead and draw out the chemical structure of these two molecules. Acetyl-coA, we began with this two-carbon backbone structure that we call an acyl group. So, carbon double-bonded to oxygen and also bonded to this carbon CH3 group here, is attached to a coenzyme A group via a sulfur atom here. I'm gonna leave this co-A group abbreviated like this, just for simplicity. Our malonyl-coA group looks very similar. We have a carbon double bond to oxygen, our coenzyme A group is untouched, but we end up plopping off a hydrogen here and essentially adding on this carbon dioxide molecule, like such. I'll go ahead and put a hydrogen here, just for now. But it might be a proteinate or it might not be, depending on the pH of the solution, obviously. But just to kind of give you a sense here, this is what malonyl-coA looks like. Now, after this activation step, we're ready to polymerize, we're ready to put these malonyl-coA molecules together to form the 16-carbon palmitic fatty acid. To do this, it turns out that this carbon dioxide molecule that we added to this malonyl-coA group, it plops off once again. It might kind of seem weird to you that we're adding a carbon dioxide molecule and it plops off again. But what this is really doing, as I'll touch on in a bit is, it's making this overall reaction thermodynamically favorable, because otherwise, it wouldn't be able to occur. Notably, we also happen to lose some water molecules, as well, in the process. This is also where the addition of NADPH comes in, because we need the NADPH to reduce these carbon double bonds, as I mentioned earlier, so that we can create that nice string of carbon-carbon and carbon-hydrogen bonds. That is what forms the bulk of our fatty acid. Now, I do wanna get this stoichiometry right here and remind you that what I've written out here, this loss of one water molecule and this loss of the carbon dioxide group, and the addition of these two NADPH molecules is for each subsequent addition of these two carbon subunits. So every time we make another cycle, we're gonna have another two NADPH and another loss of carbon dioxide and another loss of water. And that will essentially continue until we've built this 16-carbon palmitic acid chain. Before I touch more on this polymerization, let me just actually scroll down here and talk a little bit about the enzymes that are carrying out these two major steps. The first enzyme that carries out this activation step has a very nice name that's easy to remember. It's called acetyl-coA carboxylase. Essentially, it tells us exactly what it's doing. It's adding a carboxy group to this acetyl-coA molecule. What's important about this particular enzyme, let me scroll down a little bit more here, is that it ends up being a very highly regulated part of this entire sequence. Now, the reason this is a really good step to regulate is because it happens to be the rate-limiting step of this entire fatty acid synthesis pathway. In other words, out of all the chemical reactions that occurred, this one is the slowest kinetically. So, because the entire rate, the overall rate, how fast this fatty acid synthesis pathway can occur, is limited by this one step, it makes sense that we should essentially regulate whether this step is on or off. The way we do this is by allosteric regulation, which, if you remember, is a specific molecule binding to a non-active site portion of this enzyme to make it work better or worse, or can also regulate it with hormones, as well. So I'm gonna say hormonal. I just wanna mention a couple of these that are going to be relevant. First off, we have citrate, which is an allosteric activator of this enzyme, makes it work, makes this enzyme work better, and hence, increases the flow through this pathway. This kind of makes sense to me because I think of the citrate shuttle that's shuttling all this acetyl-coA to the cytoplasm. More acetyl-coA means that we should take advantage of this acetyl-coA and make more fatty acids, hence, kind of moving this pathway forward. In terms of hormones, the hormone that activates this pathway is insulin. This makes sense to me again because I know that, right after, you know, chowing down on that cheeseburger, we know that our levels of glucose in our blood are also gonna rise. So if we have a lot of extra glucose around, we're gonna have a lot of extra acetyl-coA. Hence, again, once again we wanna move this acetyl-coA down this pathway to produce more fatty acids. Now, in terms of inhibitors, allosterically, it turns out that fatty acids, especially, I'll say, long-chain fatty acids, can inhibit this enzyme. The way I like to think about this is, it's just a form of product inhibition. If we had too many fatty acids being produced, this process is basically, the cell is saying, "Hey, acetyl-coA carboxylase, "you can slow down a little bit. "We have more than we need." And in terms of hormones, the hormone that inhibits this enzyme is glucagon, as well as a couple of others. But this is one of the main ones. The way I like to think about this is twofold. Remember that glucagon levels rise several hours after a meal, once we have a low blood glucose level. If we don't have an excess of glucose, we're not gonna have an excess of acetyl-coA. So we won't wanna be forming all these fatty acids. But I also remember that glucagon, the rising levels of glucagon, also signal those adipose cells to release those fatty acids into the blood stream for all of our cells to break down to produce more ATP. So if we're in a state of net breaking down of fatty acids, we don't wanna be simultaneously building them back up because ultimately, we wanna be able to, in this phase at least, extract the energy by breaking those fatty acids down. And now I wanna mention what the enzyme is called that polymerizes these malonyl-coA subunits together to form this palmitic acid. The name of this enzyme is also quite easy to remember. It's called fatty acid synthase. So we're synthesizing a fatty acid. Now, as you probably saw from this arrow, there is a lot of things going on here. We're adding NADPH molecules, the carbon dioxide molecule's falling off, and a water is falling off, too, and we're not gonna go for all of the reactions that occur in this enzyme, but I think, if we scroll down here a little bit, it will be helpful to just get a sense for how these carbon-carbon bonds are forming. Now, to save us some time, I've pre-drawn a couple of things that I wanna go ahead and explain to you right now. Starting here on the left, what I've drawn here is the fatty acid synthase enzyme. And what I wanna draw your attention to, obviously this isn't what the enzyme actually looks like, but a representative drawing, is the fact that it has two identical subunits, shown here, and they each have a thiol group, or a sulfur-hydrogen group, located here in the active site region. The very first step of this polymerization reaction is actually to start off with both a molecule of acetyl-coA and malonyl-coA. And one acetyl-coA molecule attaches to one of those sulfur groups, and the malonyl-coA molecule attaches to the other sulfur group, so that they're in close vicinity of one another. But I do wanna point out that obviously, these are no longer called acetyl-coA and malonyl-coA, because you can notice here that there's no coenzyme A anywhere. And indeed, when these carbons here formed a bond with the sulfur atom on these enzymes, they lost their coenzyme A group. So that's where we lose our coenzyme A group, right here. Now, in terms of carbon-carbon bond formation, I want us to put on our organic chemistry hats for a moment. Now, I've already told you, up above here when we were looking at this reaction mechanism, that we lose this carbon dioxide molecule that we've added in this malonyl-coA molecule. But how does this impact the carbon-carbon bond formation? Here I want you to notice that, when this de-carboxylates, when these electrons decide, "Hey, let me form a double bond here "and let me plop off of this molecule," that these electrons now make this carbon very nucleophilic. In other words, these two electrons want to bond with something. So what they'll do is, they'll see the nearest carbon that has a suitable leading group around, and it happens to be this carbon right here, because this enzyme is nicely putting us in the vicinity of another carbon bond, and when it makes this carbon bond, this carbon will say, "Hey, "I like that bond a little bit better. "I'll plop off and give you my extra electrons." So what we've done, essentially, is, we've made a carbon-carbon bond and we've freed a space here to allow the input of another malonyl-coA molecule. And this cycle will continue to repeat as making carbon-carbon bonds until we form the 16-carbon palmitic acid. Now, I know we haven't really covered where the NADPH comes in to reduce these carbons bonded to oxygen or where there's a loss of water. But I will actually mention that it's quite similar to the reverse of beta oxidation, which you can take a look at, as well. But ultimately, after all of that is done, after we make this carbon bond, which I think is the most crucial part of this fatty acid synthesis, and we reduce those carbons bonded to oxygens, ultimately we form this 16-carbon palmitic acid, which I've drawn the structure of below. It's a completely saturated fatty acid that we can use to make longer-chain fatty acids if need be. And I wanna remind you that the ultimate fate of these fatty acids, remember, they're going to be attached to a glycerol backbone to form a triacylglycerol molecule, and those are gonna be sent off by the liver via VLDL particles to essentially deliver these fats to the rest of our tissues.