Regulation of oxidative phosphorylation

- [Instructor] In talking about how oxidative phosphorylation is actually regulated inside of our cells, I find it helpful to remind myself of two basic things about this pathway. So the first is, what is the purpose of oxidative phosphorylation? So remember, this is a process that takes place inside of the mitochondria in the electron transport chain, right? And it's sole purpose is to produce lots of ATP. So remember, ATP can be produced without oxygen, through substrate-level phosphorylation which does take place in glycolysis and also the Krebs cycle, but what's cool about electron transport chain and oxidative phosphorylation is that by having oxygen and having the electrons shuttled through these electron carrier molecules like NADH and FADH, it allows the body to produce efficiently a whole lot of ATP, which is important for many of our tissues which can't survive just on substrate-level phosphorylation. For example, your brain and your heart, and some other tissues in your body, really rely on the electron transport chain to produce most of it's ATP. The second point that's important to recognize is that oxidative phosphorylation is the kind of common end pathway of aerobic respiration. What do I mean by this? Remember that there are many different types of fuels that can enter cellular respiration. We've talked about glucose and also fatty acids can enter cellular respiration as well as occasionally, under extreme starvation, amino acids can also enter, but ultimately, all of these are broken down and much of their reducing power is stored in the electron carrier molecules like NADH and FADH2 that are ultimately shuttled, as I mentioned before, to the electron transport chain to produce as ATP. Alright. So how do these two points relate back to how oxidative phosphorylation is regulated? Well this first point simply reminds me that the major form of regulation in oxidative phosphorylation is looking at the energy needs of the cells, and the way that the body does this is by looking at the levels of ADP compared to the levels of ATP. You know, specifically, it should make sense to you that if the body has a lot ATP lying around, it should essentially be a sign to say, you know what? We have enough energy. We don't need to produce more. oxidative phosphorylation can slow down. But on the other hand, if we have a lot of ADP compared to ATP, it's a sign that the cell is running out of ATP, and that more ADP can be and should be phosphorylated using the electron transport chain. This is actually just really a kind of application of Le Chatelier's Principle, which is a general chemistry principle to oxidative phosphorylation and I will actually go into a little bit more detail about this in a second, but first, I wanna kind of touch on a second point here, which is that it's a common end pathway for aerobic respiration. And I really make this point because it reminds me why there is kind of no major hormonal or allosteric, remember allosteric means there is some type of enzymatic control that's being altered, but there is no major hormonal or allosteric regulation in oxidative phosphorylation. The way I've kind of always justified that to myself is that these are very, these forms of regulations allow us to really fine tune regulation and to make sure that when we turn something on, we are turning it on with full certainty, but the fact that it's downstream of many of the entry points to aerobic respiration, such as breaking down glucose and glycolysis and the oxidation of fatty acids, means that it's probably more important for those pathways, which they in fact do have a lot of hormonal and allosteric regulation, but once those pathways are turned on, it's kind of just gonna keep rolling down the pathway and it probably may not be as important to have that level of fine-tuning in oxidative phosphorylation. So with that in mind, let's go ahead and talk about more about how the energy levels in the body are used to regulate oxidative phosphorylation. I've gone ahead and drawn out a simplified diagram of the electron transport chain and I want to remind you that it's much more complex than this, right? We know we have four protein complexes. We have ATP synthase. We have this all occurring in the inner mitochondrial membrane, but for our purposes, I just really wanted to highlight what the main reactants and products of the electron transport chain were. Let me go ahead and guide you through this. We have the entry of electron carrier molecules such as NADH, and remember, we can also be dealing with FADH too as well, but just as an example, I'm using NADH. the NADH carries two electron from the molecule, from the fuel such as glucose or it could be fatty acid or some type of fuel, and it essentially gets oxidized at the electron transport chain. It releases it's electrons into the electron transport chain and becomes itself oxidized. This flow of electrons, of course, fuels the phosphorylation of ADP and a free phosphate group into ATP. Of course this is all done indirectly through a proton gradient that's formed in the intermitochondrial membrane. Then finally, the electrons must have somewhere to go and they end up reducing oxygen. It's kind of funky to think about two electrons reducing half an oxygen but this is just so that the stoichiometry works out. You can see here that if we were to reduce one molecule of oxygen of course, we'd need four electrons but in any case, it reduces oxygen and it combines with some free protons to produce some water. At this point, I want to remind you of Le Chatelier's Principle in general chemistry which states that if you have an equilibrium, so let's say this overall reaction of the electron transport chain is our chemical reaction that's in equilibrium, and there is some type of alteration to this equilibrium, so let's say we have the addition of more reactant or we take away some product, Le Chatelier's Principle essentially says that the equilibrium will re-equilibrate to counter this change. So it turns out, that this is exactly how the electron transport chain is regulated. To make this point, let's go ahead and basically ask ourselves what would happen if we had more NADH, more ATP, more free phosphate, or more oxygen around. Remember these are all of our reactants. Indeed, if we had more of these reactants, Le Chatelier's Principle would essentially say that this reaction, so to say, would be pushed towards the forward direction and we would produce more ATP. We would say that that kind of flow of electrons through the electron chain is faster and we'd get more ATP. Now, of all three of these reactants, I just want to make a point here that practically speaking, we consider the level of oxygen to be pretty constant. If we're breathing in and out normally, normally this is not a limiting factor that essentially alerts the electron transport chain to go faster. I'm just gonna go ahead and erase that for practical purposes. But generally speaking, of these three, the NADH, the ADP, and the free phosphate group, it's really the levels of ADP in a cell that are most likely to alert the electron transport chain to produce more ATP. That's just because it's usually the limiting factor of all three. But it should make sense to you that high levels of NADH are essentially assigned from up above from the breakdown of glucose or fatty acids that it's time to make more energy for the cell. Of course, we can also use that experiment for the products of the reaction. Specifically, let's say we had elevated levels of ATP in a cell or elevated levels of the oxidized form of these electron carrier molecules. Le Chatelier's Principle would tell us that this equilibrium would essentially shift in the opposite direction, so the flow of electrons through would be slower and we would produce less ATP. Of course, you know in reality, we don't really think about electrons traveling the opposite direction down the electron transport chain, but this is just a way to kind of essentially signify that having higher levels of ATP in the body, or you know, high levels of NAD+, are essentially, by Le Chatelier's Principle, putting a break on the electron transport chain. Just as before, the ADP levels were more likely to alert the electron transport chain. ATP levels are kind of the limiting factor to alert the electron transport chain as compared to the NAD+ levels. That's really because the body usually keeps NAD+ and NADH in a pretty kind of stable ratio. The body's really looking to whether there's high levels of ADP or ATP to ultimately decide and regulate how fast the electron transport chain is.