Fatty Acid Oxidation - Part II

- Now it turns out that relative to the inner mitochondrial membrane, the outer mitochondrial membrane is a bit more permeable to many molecules, and specifically it contains specialized proteins called porins that have essentially created little tunnels or pores for molecules to kind of diffuse through, so I'm kind of just drawing representative pores to remind us of that fact and it turns out that this carnitine attached to this fatty acid which is actually commonly referred to now as an acyl carnitine molecule can readily diffuse from the cytoplasm into the intermembrane space here via one of these pores and it's actually a bit unclear to scientists whether it's the carnitine and the fatty acid, the acyl-CoA that both diffuse into the intermembrane space and then get kind of attached via this carnitine acyl transferase or whether it's more like what we've drawn up here in that everything occurs in the cytoplasm and this entire acyl carnitine molecule diffuses into this intermembrane space. So I just wanted to mention that, but in any case the ultimate endpoint is the same. We get this acyl carnitine into this intermembrane space via these pores in the outer membrane. Now at this point you might be wondering to yourself why did we go through all the trouble to create this acyl carnitine molecule if these porins are permeable to a lot of things, including even fatty acids, so why can't the fatty acids just you know go through these porins and get into the mitochondrial matrix? Well, first I want to remind you that remember we're only halfway home. We have to still traverse this inner mitochondrial membrane in order to get to the mitochondria and it turns out that this inner mitochondrial membrane does not have these nice pores in its membrane and so it's far less permeable to molecules than this outer mitochondrial membrane, but what it does have is it has this protein transporter within its membrane that's gonna help us out a little bit, and this protein transporter is called acyl carnitine, so already it's giving you a clue that it's going to recognize that molecule acyl carnitine that we worked so hard to create and it's called acyl carnitine translocase, so just like its name suggests, it's gonna essentially translocate this acyl carnitine from this intermembrane space into the mitochondrial matrix. And to show that action I'm gonna go ahead and take a quick shortcut here and actually go ahead and copy and paste this molecule here from the cytoplasm to the mitochondrial matrix right here, and I'll go ahead and extend our path right here from the intermembrane space, facilitated by this protein transporter into the mitochondrial matrix. Now of course we're not finished here because we don't want to oxidize this entire molecule. We just want to oxidize this fatty acid chain to obtain some ATP, so we need some way to remove this fatty acid from this carnitine molecule, so remember how I said that we also have a carnitine acyl transferase on our inner mitochondrial membrane? Well that's where this enzyme comes in so I'm gonna go ahead and abbreviate that now as CAT for carnitine acyl transferase and I'm gonna put the roman numeral two to indicate that it's a different enzyme located on the inner mitochondrial matrix and what this enzyme does is it catalyzes essentially the reverse reaction of what happened inside of the cytoplasm. So what I mean by this is that it take a coenzyme a molecule from the mitochondrial matrix and in a reverse reaction now our sulfur atom here is gonna serve as our nucleophile essentially and essentially displace this bond that was formed earlier so that we end up forming a carnitine molecule back to its original form with a hydroxyl group and we also end up forming back again simply our acyl-CoA group, so remember that looked exactly like it does above with our acyl group attached to an acetyl-CoA molecule via this sulfur atom. And in fact, just to make that crystal clear, I'll go ahead and actually write out the full name below it. So we're calling this is the acyl-CoA molecule. Now before moving on to the oxidation step I want to point out one more thing, which is that it turns out that this acyl carnitine translocase protein transporter is actually quite efficient because for every acyl carnitine molecule that it pumps into the mitochondrial matrix it actually exchanges it for one molecule of carnitine, so this carnitine molecule that's produced by this carnitine acyl transferase two enzyme can actually return via this protein transporter into the intermembrane space so I'll go ahead and continue its journey in the intermembrane space and then out through the outer mitochondrial membrane and back into the cytoplasm where it can be recycled to help transport another fatty acid into the mitochondrial matrix. That's kind of a pretty cool mechanism, I think. Alright, so now that we've finally gotten our fatty acid into the mitochondrial matrix in the form of this molecule acyl-CoA, what happens to this molecule next? Well, using the enzymes found in the mitochondrial matrix this molecule undergoes actually a cycle of repeating steps and each cycle consists of four main steps and at the end of each of these four steps we end up producing one molecule of acetyl-CoA and I'll remind you the structure of this molecule is a two carbon structure, an acyl group attached to a coenzyme a and each time we also produce an acyl-CoA chain that is now two carbons shorter than it was before. Of course that's because it's losing those two carbons in producing this acetyl-CoA molecule. I'm not gonna go in depth into these four steps here or the enzymes used to complete these steps, but what I do want to point out big picture are the major inputs and outputs of these four steps to get to this molecule of acetyl-CoA and a fatty acid chain that's two carbons shorter. So remember that the production of acetyl-CoA involves oxidation. So oxidation, we're losing electrons which means we need the help of electron carrier molecules to accept those electrons so they can shuttle them to the electron transport chain to produce some useful energy in the form of ATP, so remember our two electron carrier molecules are FAD and NAD and of course these are in their oxidized forms so when they accept electrons they become reduced into FADH and NADH and of course this also produces a proton as well and the kind of convenient point that I want to point out to you is that it turns out remember that the electron transport chain is not far away. The electron transport chain is located on the inner mitochondrial membrane, right, so that's quite a quick journey and so these are readily reoxidized to their oxidized form once they donate to the electron transport chain and so this cycle can continue quite conveniently in this mitochondrial matrix. Now as a couple of minor points, remember that we need some source of oxygen to be forming all of these carbons bonded to oxygen in each of these subsequent acetyl-CoA molecules that we produce, so there's actually an insertion of water somewhere along these four steps as well as an insertion of an additional coenzyme A molecule for each subsequent acetyl-CoA group that's formed. Now before we wrap things up and briefly talk about the regulation of this oxidation process I just want to mention something that I didn't get quite the chance to talk to you in depth to you about. It's something you might hear when referring to this process which is the statement beta oxidation, and it sounds super intimidating but it's actually quite simple and so I just wanted to take a minute to just explain this to you briefly, like what the big idea behind this is. So this beta oxidation simply refers to the carbon, the position of the carbon in this fatty acid chain that's being oxidized. So remember the carbon that's being oxidized is this C double bond O bond right here in this acetyl-CoA molecule and so to explain this to you I wanna go ahead and actually, I know I've been abbreviating this fatty acid chain using the letter R but I just want to draw in a couple of these carbon carbon bonds here so I'll draw in a couple of CH2 groups attached to the carboxylate group here. So it turns out that the way that chemists label these carbons is based on their position to this carboxylate group here, so the carbon closest to this carboxylate group gets the designation of being the alpha carbon and the one, the next furthest one away is called the beta carbon and of course if you're further away you also get other names as well, but just because we're talking about beta oxidation I'll stop there and so you can see that it's this carbon, this beta carbon, so in other words, every other carbon relative to this carboxyl carbon here that is going to be turned into this C double bond O to form this acetyl-CoA molecule. So, that in a way is probably pretty intuitive but this beta oxidation is just kind of a fancy way to refer to that. Alright, so we're hitting the home stretch here and the only other thing that I want to mention is how this oxidation process is regulated and it turns out that among all these reactions the rate limiting reaction is catalyzed by this enzyme carnitine acyl transferase. I'll actually go ahead and write that up. This is a rate limiting step and remember that means that kinetically it's the slowest among all of these reactions and also remember that's important to know when we're thinking about regulation because since the rate limiting step determines the overall rate of this entire reaction it's a good step to regulate, essentially to turn on or off. And it turns out that in order to figure out how this is regulated we need to remind ourselves a little bit about fatty acid synthesis, so I'm going to remind you of a molecule called malonyl CoA and if you remember this molecule was kind of a charged up version of acetyl-CoA that was used to synthesize fatty acids inside of the cytoplasm and it turns out that this molecule is actually an allosteric inhibitor of this enzyme, so I'm gonna kind of just squeeze this arrow through to indicate with a red line and a minus sign here that this inhibits this enzyme carnitine acyl transferase one. So how can we think about this? Well the way I kind of reason it in my head is, is if we have a lot of malonyl CoA lying around in our cell it means that we're making a lot of fatty acids, so if we're making fatty acids, if we're synthesizing them, then we don't want to be breaking them down, and so we can essentially inhibit the rate limiting step of this oxidation process to make sure that we have a net production of fatty acids. Now another cool thing is that I think is worth noting about this type of regulation is that by using a substrate in fatty acid synthesis to regulate an enzyme in fatty acid oxidation, the body has essentially made these processes mutually exclusive, so when one is on, so when there's a lot of malonyl CoA, the other, you know being oxidation will have to be kind of essentially turned off, right? And so that's exactly what the body wants. It doesn't want to be in a gray zone. It either wants to be essentially producing a net amount of fatty acids for storage or it wants to be breaking them down to extract all of that ATP from. And finally the last point that I would make is that, even though it kind of seems like a pain to traverse through all of these compartments to get to where we need to go, having these compartments makes this kind of regulation also pretty cool, because by kind of regulating this oxidation at this step right here which is the key step for transporting the fatty acids from the cytoplasm to the mitochondria, we can essentially keep these reactions separate from one another, literally, so we're actually keeping them in separate compartments which is a great way to regulate whether something is on or off.