Understanding Electron Transport Chain and ATP Synthesis

Welcome to lecture 12 of biochemistry for 23 where we'll be looking at chapters 20 and 21 going over the electron transport chain and oxidative phosphorylation through ATP synthase. Just let me move this out of the way. Okay, so learning objectives for chapters 20 and 21 describe the key components of the electron transport chain and how they're arranged. So Here you should know the enzyme complexes and the electron carrier molecules of the electron transport chain. Second objective is explain the benefits of having the electron transport chain located in a membrane. Okay, so largely the reason that having it located in a membrane is important is this allows for buildup of a proton gradient. So movement of electrons to the chain pumps protons directionally across the membrane. And so as we pump those protons, because there's compartmentalization, Those protons are trapped on one side of a membrane, and it creates this ability of flow of protons to generate proton motive force through ATP synthase and generate ATP. Third objective, describe how the proton motive force is converted into ATP. So you should, for this, generally understand the composition of ATP synthase, F0 and F1, and the subunits of each. We'll look at a video that came out of Harvard, and I likely won't play it. over the lecture here, but it's, I believe, the last slide of the lecture links to it, and my suggestion is to watch it and to take notes on it. And I may show it in class and review it if people would like me to and go over some of the details that you may need to know. Also, how conformation change allows for the ADP and phosphate binding and then catalyzes ATP synthesis. So this is something that ATP synthase does, right? This is the reaction it catalyzes. through the energy provided by the proton motive force that spins it. You should also know how the C-ring rotates from proton motive force. So this is what I'm talking about when I say proton motive force. So one of the subunits, the C-ring of ATP synthase, is caused to turn sort of like a water wheel through proton movement through it across the membrane. And so this C-ring rotating rotates the gamma subunit, and that rotating gamma subunit has an effect. by which it changes the alpha and beta conformations, and the alpha and beta subunits have the actual active sites for catalysis. So those conformational changes, you can sort of think of this as the potential energy difference causing the movement of protons changes that potential energy into kinetic energy, and we change that kinetic energy into chemical energy through catalysis by the active sites of the alpha and beta subunits. And then finally, identify the ultimate determinant of the rate of cellular respiration. And this is really the need for ATP. So cellular respiration is a way for us to make ATP primarily, right? It has other functions. We've learned about some of the anabolic processes that can break off it based on intermediates of cellular respiration, particularly in the citric acid cycle. But most of what drives this metabolic... pathway is the need for ATP. So what is the electron transport chain and oxidative phosphorylation? So to do an overview slide, if we look at this, right, so this is sort of a general overview where what we see is on the left, right, we have these protons being pumped outside based on the electrons, okay? So from the TCA cycle, we make a whole bunch of electron transport characters, right? So the TCA cycles at the center... center of this. Most of the electron carriers are generated in the TCA cycle. We've learned about other places they're generated, right? We generate two NADHs in glycolysis from a single glucose molecule. We also generate two NADHs from pyruvate dehydrogenase from a single glucose molecule, right? One per pyruvate conversion to acetyl-CoA. And then we generate three NADHs, one FADH2 from each turn of the TCA cycle. For every glucose molecule, it turns twice. So from a glucose molecule from the TCA, we net six NADHs, two FADH2s. All right. So if we add all that up, we get some number of electron carriers. And then those electron carriers, when generating ATP, transport their electrons to a series of complexes that the electrons move through. And through movement through those complexes, we pump protons out. So this yellow. actually represents four separate complexes and they're not a single channel. The ETC is very simplified representative here through this yellow cylinder. But this represents the whole ETC. And so when we've pumped these protons out, using the force provided by movement of these electrons or the energy provided by movement of these electrons, our ultimate electron acceptor is oxygen. And along with our electrons come hydrogens. And so along with the electrons. being accepted by oxygen with hydrogens, we convert oxygen into water. So when we talk about combustion of glucose through respiration, we talk about the products being CO2 and water. The CO2 comes from the CO2 lost from the different decarboxylation reactions throughout the PDH and citric acid cycle, and then the water comes from oxygen as the final electron carrier, then combined with hydrogen to generate water. So now we've pumped these hydrogens out, and through pumping these hydrogens out, we've created this electrostatic gradient, right? So we've created this attraction whereby the protons want to move inside. Not only are we generating a concentration gradient where hydrogen ions are outside, but there's also an electrostatic force being generated where the protons want to move inside back into the matrix where it is more negative. And so ATP synthase, represented in pink here, acts as a valve to allow those hydrogens through. And through movement of hydrogens, we generate the potential energy of hydrogens moving across the membrane into motive force energy through hydrogens turning part of ATP synthase. And that turning generates chemical energy by converting, so confirmation of the enzyme, the enzymatic portion of ATP synthase, right? catalyzes this conversion of ADP to ATP, okay? So this is summarizing what I just said in words, right? Glycolysis in the citric acid cycle generates high-energy electron carriers, NADH and FADH2, which transfer those electrons to oxygen through the ETC, okay? And then exergonic energy released from these redox reactions powers the movement of these protons across the gradient, okay, between the inner atmosphere. and the outer mitochondrial membrane. So we're not actually pumping outside of the mitochondria. The mitochondria has an outer membrane and an inner membrane. Within the inner membrane is the matrix, so that's the sort of central most compartment of the mitochondria. And then there's a compartmental space between the matrix membrane and the outer membrane of the mitochondria, and that's where we get this proton gradient buildup is in that inner membrane space. So this proton gradient is then used to power ATP synthase by oxidative phosphorylation. And that's the conversion of ADP to ATP. So what's the primary catabolic function of the citric acid cycle? Harvesting of high energy electrons from carbon fuels in the form of NADH and FADH2. So we learned about the citric acid cycle last time. Its main function as a catabolic process is to generate NADH and FADH2, which ultimately take their electrons to the ETC, which ultimately pumps these protons out. okay, through the generation of water or conversion of O2 into water. And then that gradient is what drives that chemiosmosis, okay? And then that is turned into proton motive force, which is converted into chemical energy to provide the energy to convert ATP into ATP. So what is the electron transport chain? Well, it's really just a series of redox reactions. And so there's a way that we can look at this, right? We can look at it as the free energy to oxygen, right? So if oxygen is sort of the ultimate electron acceptor, then we can generate a hierarchy of things that can accept electrons. So oxygen accepts it the most readily, and in the ETC, NADHQ oxidoreductase, or complex I, represented here, is sort of the highest free energy relative to O2, okay? So what happens here? Okay, so electrons from NADH and FADH2 are used to reduce molecular oxygen to water through these series of electron carriers. Okay, so complex I, complex II, complex III, and complex IV, represented with Roman numerals here, all have names, okay, and we'll talk about the names of them, but those are all membrane-embedded complexes, okay? There are also electron carriers that take electrons from each complex to the next complex, okay? And so those are not membrane-bound, those are free, and those move electrons from one complex to another. Okay, so a strong reducing agent, such as NADH, okay, is poised to donate electrons and has a negative reduction potential, whereas a strong oxidizing agent, right, O2 is a very strong oxidizing agent, is ready to accept electrons and has a positive reduction potential, okay? So electron flow through the ETC creates this proton gradient. If you want to watch another video, Okay, this is actually, I think, a very good explanation of the electron transport chain. So we can watch this video in class. I'm not going to play it here. But let's walk through this chart or graph on the right. So we've generated NADH from different oxidation steps throughout glycolysis, pyruvate dehydrogenase, and the citric acid cycle. We've also generated FADH2. So NADH donates its electrons. electrons first to complex one. So NADH reduces complex one. And so we're going to regenerate NAD plus here. So when NADH donates its electrons, we're going to regenerate NAD plus. So why we do fermentation to regenerate NAD plus for glycolysis here is in the absence of oxygen, we can't do respiration, right? And so these oxidative steps... that can happen through PDH and the citric acid cycle are also ways for us to generate NADH and then through the ETC regenerate NAD+. But in the absence of oxygen, we can't use the electron transport chain in order to regenerate NAD+. This is why oxygen is necessary to regenerate NAD+, in a non-fermentation fashion. If we have oxygen present, we don't typically have to do much fermentation. Okay, we will only need to do fermentation and immediate needs where we lacked, you know, maybe there's sort of a rate limiting step where we're using our oxygen as fast as we can. Right. And so we get into a oxygen depleted state. And so we do some fermentation. Right. But in the presence of oxygen, we have no problem regenerating NAD+. This is just to relate to you fermentation and why we use fermentation in the absence of oxygen. When oxygen is present, very easy for us to regenerate NAD+. When oxygen is not present, we have no way to regenerate NAD-plus through the ETC. Okay, so NAD-plus dropping its electrons off at complex one, which is called NADH2Q oxidoreductase. So NADH2Q, Q is coenzyme Q, also known as ubiquinone, or sometimes just Q. So this means that this is an oxidoreductase. So complex one is an oxidoreductase. that transfers electrons from NADH to coenzyme Q. Okay, now coenzyme Q isn't a complex. Coenzyme Q is going to transfer electrons to complex three. Notice we bypassed complex two completely when I said that. Okay, so let's just say that again. NADH transfers its electrons to complex one, and complex one transfers those electrons to you. ubiquinone or coenzyme Q. Technically what complex I does is it catalyzes the conversion of NADH to NAD+, and it transfers electrons to coenzyme Q, right? So the electrons technically never stay at complex I. They go directly from NADH to coenzyme Q through complex I when catalyzing that conversion. And complex, sorry, and coenzyme Q transfers those electrons to complex III. And complex III catalyzes a reaction that takes the electrons from coenzyme Q and transfers them onto cytochrome C. Again, we've skipped complex II completely here, and we'll come back to it. So complex III's name is, once again, pretty easy to figure out. It's an oxidoreductase that... that transfers electrons from Q, or coenzyme Q, to cytochrome C, hence its name, Q-cytochrome C oxidoreductase. Cytochrome C, once again, is transferring electrons, so it's a non-membrane bound component that transfers electrons from complex III to complex IV. Complex IV is cytochrome C oxidase because cytochrome C gets oxidized. and reduces oxygen into water. Okay? So if we follow that, the electrons from NADH go from complex I to coenzyme q to complex three to cytochrome c to complex four ultimately to oxygen to make water that is the path of electrons from nadh the path of electrons from fadh2 starts at complex two so nadh cannot donate electrons to complex two instead fadh has its electrons transferred to coenzyme Q through complex II. This is the real difference, okay, is that FADH2 donates its electrons through complex II, whereas NADH donates its electrons through complex I. Both of them donate those electrons to coenzyme Q, and coenzyme Q then transfers it down to the rest of it. The path after coenzyme Q for both NADH and FADH2-derived electrons is identical, it's just their starting point that's different. So notice the name of complex 2 is sucinate to Q reductase. So why is this? Well, if we note, we generate FADH2 from the conversion of sucinate into fumarate. So if you go back to the citric acid cycle, sucinate to fumarate generates an FADH2. Really, this FADH2 is never a free-floating FADH2. This reaction of succinate's conversion to fumarate generating FADH2 is coupled with the transfer of the electrons for FADH2 through complex II onto coenzyme Q. This is why it's called succinate Q reductase, because the electrons that succinate donates to FAD are going to be transferred onto coenzyme Q. And in fact, complex II is the same enzyme from the citric acid cycle that converts succinate into fumarate. So this is why sometimes you'll see succinate Q reductase as the name for the enzyme in the citric acid cycle. So again, if you want to watch another video, this is a great YouTube video explaining it in a slightly different manner than I'm explaining it here. and it's a decent one to watch. It's simplified. It also adds some inhibitor information that you do not need to know for my class, but is interesting to know for medical school, for applications in medicine. Okay. Okay. So I'm going to show you this in multiple different ways. So I have another slide here, which gives more detail, and I'm going to run through it similarly to how I ran through the previous slide. Okay. But it's the same information that I gave in the previous slide, just shown in a slightly different manner. So here, one of the big differences is we've shown the compartmentalization where we have the matrix side of things on the bottom here in beige, and we have the inner membrane space of the mitochondria. So this is the matrix membrane that all of these complexes are embedded in, and this is the space between the matrix membrane and the outer membrane of the mitochondria. So let's run through it. So the first complex, complex one, is where NADH donates its electrons, right? So we have our conversion of NADH to NAD+. Electrons are donated onto coenzyme Q, and those electrons on coenzyme Q, why is this QH2? Well, it's because you don't just have free electrons, okay? This is why when we add electrons to NAD+, they actually come along with hydrogen here, okay? So we typically have hydrogen or hydrogens coming along with electrons. So Q has these electrons represented by two hydrogens that are actually helping carry those electrons. And you notice this dotted line past complex two. It's because the electrons on coenzyme Q that came from complex one are going directly to complex three. Whereas if we look at complex two, even though coenzyme Q is the electron acceptor here, they're also going to go from two to three. So the electrons from complex one never go to complex two. And when we transfer those electrons, the energy generated through going through this electron transport chain from the first complex is enough to pump four protons from the matrix into the inner membrane space. If we go to complex two. We have this complex 2 is succinate Q reductase. Remember, it's also the same enzyme that converts succinate into fumarate. So this catalyzes the conversion in the citric acid cycle, which happens in the matrix, of succinate to fumarate, which generates FAD into FADH2. I have a caveat here. Technically, this FADH2 conversion to FAD, when we're adding the electrons from FADH2 onto coenzyme Q, It doesn't happen in the intermembrane space. It basically happens altogether, right? So conversion of succinate to fumarate generates an FADH2, which immediately donates electrons to coenzyme Q through the same enzyme, okay? Then those electrons go to complex III, which is Q, cytochrome C oxidoreductase, because it is transferring electrons from coenzyme Q to cytochrome C, okay? And in the complex III... the transfer of those electrons to cytochrome c is enough to generate four protons pumped into the inner membrane space. And then finally with complex IV, we have pumping of two hydrogens based on the transfer of electrons from cytochrome c onto oxygen. So notice this chemical formula. Technically we transfer two electrons, oxygen accepts two electrons, but it splits the two electrons into two parts, right? In order to combine. with each oxygen molecule will combine with two hydrogens as well as an electron to form water. And so we actually generate two waters per transfer of two electrons. That's why it's half an oxygen here. If we transfer two electrons onto oxygen, it would be four hydrogens plus one oxygen plus the two electrons is going to generate two waters. So you should know this or some version of this, whether it be from the last slide or this. Okay, one thing to note here, and it's an important note. In total, we pump four hydrogens from complex 1, four hydrogens from complex 3, and two hydrogens from complex 4. A total of 10 protons are pumped, and the only complex that doesn't pump protons is complex 2. Complex 2 does not pump protons. It just serves as the acceptor for electrons from FADH2, but it does not pump any protons. So once again, in words, oh, so sorry, before we get to that, why am I emphasizing the 10 hydrogens get pumped? Okay, so if NADH pumps 10 hydrogens, right? So one NADH molecule provides two electrons. And those electrons move from complex I to complex III to complex IV. And just the movement of electrons from NADH generates IV plus IV plus II, so 10 hydrogens pumped. Okay, NADH has the energetic equivalent of 2.5 ATPs. The reason for this is because of the 10 hydrogen it pumps. So the electrons from NADH pumping 10 hydrogens, right, is the equivalent energy generation to catalyze the making of 2.5 ATPs. So how many protons does it take to catalyze production of a single ATP? Simple math says one ATP means four protons. So what that means is the protons moving back through ATP synthase at the end and doing oxidative phosphorylation, generates one ATP through four protons moving through. So if NADH is pumping 10 protons, that's where we get the equivalent of 2.5 ATPs, because four will come through, generate one ATP, four more will come through, that's eight total, that'll be two ATP total, and then we have half of an ATP worth of protons. Okay, we're not going to actually generate half an ATP, but all we need is two more hydrogens. right? So you could really think of this as two NADHs pumps 20 hydrogens, which is equivalent to five ATPs, right? Now FADH2 only pumps six electrons total. And when I say six electrons, I'm going to edit this here while you're watching. Pumps six H plus total. So it should be protons total. Okay. So FADH2 pumps six protons total. Why is that? Well, moving back here, right, we skip complex one. FADH2 doesn't donate electrons through complex one, so four of the protons that NADH pumps never get pumped when we donate electrons from FADH2. Instead, the first protons that get pumped are through complex III, and it's four of them. And then finally, we also pump them through complex IV. So we get six hydrogens pumped from FADH2, which is why FADH2 is energetically equivalent to 1.5 ATPs. It's a lower energy molecule than NADH based on where they donate electrons in the electron transport chain. And we get that 1.5 because if every four generates an ATP, we have one and a half fours here, right? One and a half times four is equal to six. That's where we get 1.5 ATPs. So in the video where they go over how many ATPs are generated from glucose, they give a range. This range has to do with the NADHs that are produced in glycolysis. Okay. I want to make sure that you guys understand that I will typically use the range of... I will say 32 are generated because we haven't learned why there's a range in the NADHs generated in glycolysis. This largely also has to do with the conversion into FADH2 versus NADH, which is where the 3 to 5 comes from. I apologize. My mic turned off. So give me just a moment here, and I'm going to pull up some background noise to play, which will hopefully prevent it. My headset automatically turns off if there's no sound going on. So now I have some background sound going on. OK. So in the video that I showed previously a link to on this slide here, they talk about that range of ATPs generated. It has to do something with the NADHs generated in glycolysis, can also be FADH2s. So if they were FADH2s, we'd have three ATP equivalencies, right, instead of five, because two NADHs are worth five ATPs, whereas two FADH2s are worth three ATPs. But I don't care too much about that. We talk about them as NADHs from glycolysis. So we're going to assume that they're NADHs. So we generate a total of 32 ATPs from lung glucose. If we go through all of glycolysis and all of oxidative phosphorylation of all of the electron carriers, as well as whatever ATPs get generated through each cycle. Right. So I'll run through this with you guys in class. But glycolysis generates two ATPs directly. Right. We input two, we get out four. So our net from glycolysis is two. We also generate two NADHs from glycolysis, which is equivalent to five ATPs. We generate two NADHs from pyruvate dehydrogenase or pyruvate metabolism, which is equivalent to five ATPs. We generate two ATPs directly from the citric acid cycle. If it goes twice, it's one per cycle. And then if we go for two cycles of the citric acid cycle, it's six NADHs times 2.5 is equivalent to 15. And we have two FADH2s, one per round of the cycle, which is equal to three. So 15 plus three is 18. We have these two, 19, 20, plus five is 25, plus five is 30, plus two is 32. So glucose, going all the way through oxidative phosphorylation, can generate a maximum of 32 ATPs. This is also showing the same information in a chart. So you can see NADH to Q oxidoreductase, what it accepts electrons from, what it converts them to. Okay, this also indicates, based on this chart, what is on the cytoplasmic side and what is not. Sucinate Q reductase, right, means that it transfers electrons from sucinate to Q. So the matrix side, the accepting side, is electrons from sucinate. Technically, it's electrons from sucinate. transferred to FAD to make FADH2, and then FADH2 transferred onto Q, but those are coupled reactions, okay? And then we have Q to cytochrome C oxidoreductase or COMPOX3, which transfers from coenzyme Q to cytochrome C. Cytochrome C is on the cytoplasmic side, and then cytochrome C transfers electrons to oxygen on the cytoplasmic side to make water, okay? So this is, again, saying everything I just said. but in words. So there's no new information here. I'm not going to say it again. I just have a summary in words to everything I've said on the last few slides. Okay. So I'll leave this up here for just a moment and you can stop it and move on when you're ready. Okay. So proton motive force or chemiosmosis. Okay. Let's just run through it. Proton motive force. So The transfer of electrons through the electron transport chain leads to the pumping of protons from the matrix to the cytoplasmic side of the inner mitochondrial membrane. I've already gone over that with you. OK, now proton concentration becomes lower in the matrix as well as the generation of electric field. Right. With the matrix side being negative and the cytoplasmic side being positive. When I say cytoplasmic side. Remember, I'm still talking about we're not in the cytoplasm. We're just on the side of the inner membrane that is closer to the cytoplasm. So it's called the cytoplasmic side. Now protons. So what is the proton motive force? This is that valve I talked about. Protons flow back into the matrix to equalize the distribution. And we use that flow to turn the C ring of ATP synthase. which ultimately turns the gamma subunit because they're attached, which ultimately causes conformational changes in the alpha and beta subunits, and those conformational changes catalyze the conversion of ADP into ATP. So I'm not going to go over this in this lecture because this YouTube link right here is the one that you should go to and you should watch the video on it. And essentially, everything in the video is what you should know. If you have questions on the details of this video in class, I'm happy to talk about it with you. So this is a suggestion that I have for you to go through. And if you need help and need to go through as a group, we can do this in class as well. But make a diagram of the following, okay? for the extraction of ATP from a carbohydrate food source. Indicate the enzymes required for the digestion of this food source, the resulting products of each, okay, and their digestion, and ultimately the ways we've learned that the body extracts ATP through anaerobic and aerobic respiratory cycles, okay? So I actually haven't chosen anything that you really need to know digestive enzymes for, okay, but you could do this, all right? I've actually just chosen things that are within many of the cycles that we've talked about. Okay, so all of these are actually within glycolysis, A through E, but we can choose some that are in the citric acid cycle or elsewhere. Okay, so assuming each NADH molecule that goes through the electron transport chain produces 2.5 ATP molecules through the electron transport chain and ATP synthase, how much ATP is generated by each of the following molecules? Assuming each one is a starting point for respiratory ATP generation. We can roll through some of these in class just so you can sort of see what I'm talking about with calculations from different things, and we can choose some that are within the citric acid cycle. Okay, and that will end this lecture.

So Here you should know the enzyme complexes and the electron carrier molecules of the electron transport chain. Second objective is explain the benefits of having the electron transport chain located in a membrane. Okay, so largely the reason that having it located in a membrane is important is this allows for buildup of a proton gradient.

So movement of electrons to the chain pumps protons directionally across the membrane. And so as we pump those protons, because there's compartmentalization, Those protons are trapped on one side of a membrane, and it creates this ability of flow of protons to generate proton motive force through ATP synthase and generate ATP. Third objective, describe how the proton motive force is converted into ATP.

So you should, for this, generally understand the composition of ATP synthase, F0 and F1, and the subunits of each. We'll look at a video that came out of Harvard, and I likely won't play it. over the lecture here, but it's, I believe, the last slide of the lecture links to it, and my suggestion is to watch it and to take notes on it. And I may show it in class and review it if people would like me to and go over some of the details that you may need to know.

Also, how conformation change allows for the ADP and phosphate binding and then catalyzes ATP synthesis. So this is something that ATP synthase does, right? This is the reaction it catalyzes.

through the energy provided by the proton motive force that spins it. You should also know how the C-ring rotates from proton motive force. So this is what I'm talking about when I say proton motive force. So one of the subunits, the C-ring of ATP synthase, is caused to turn sort of like a water wheel through proton movement through it across the membrane.

And so this C-ring rotating rotates the gamma subunit, and that rotating gamma subunit has an effect. by which it changes the alpha and beta conformations, and the alpha and beta subunits have the actual active sites for catalysis. So those conformational changes, you can sort of think of this as the potential energy difference causing the movement of protons changes that potential energy into kinetic energy, and we change that kinetic energy into chemical energy through catalysis by the active sites of the alpha and beta subunits. And then finally, identify the ultimate determinant of the rate of cellular respiration.

And this is really the need for ATP. So cellular respiration is a way for us to make ATP primarily, right? It has other functions.

We've learned about some of the anabolic processes that can break off it based on intermediates of cellular respiration, particularly in the citric acid cycle. But most of what drives this metabolic... pathway is the need for ATP. So what is the electron transport chain and oxidative phosphorylation? So to do an overview slide, if we look at this, right, so this is sort of a general overview where what we see is on the left, right, we have these protons being pumped outside based on the electrons, okay?

So from the TCA cycle, we make a whole bunch of electron transport characters, right? So the TCA cycles at the center... center of this.

Most of the electron carriers are generated in the TCA cycle. We've learned about other places they're generated, right? We generate two NADHs in glycolysis from a single glucose molecule. We also generate two NADHs from pyruvate dehydrogenase from a single glucose molecule, right?

One per pyruvate conversion to acetyl-CoA. And then we generate three NADHs, one FADH2 from each turn of the TCA cycle. For every glucose molecule, it turns twice.

So from a glucose molecule from the TCA, we net six NADHs, two FADH2s. All right. So if we add all that up, we get some number of electron carriers. And then those electron carriers, when generating ATP, transport their electrons to a series of complexes that the electrons move through.

And through movement through those complexes, we pump protons out. So this yellow. actually represents four separate complexes and they're not a single channel. The ETC is very simplified representative here through this yellow cylinder.

But this represents the whole ETC. And so when we've pumped these protons out, using the force provided by movement of these electrons or the energy provided by movement of these electrons, our ultimate electron acceptor is oxygen. And along with our electrons come hydrogens. And so along with the electrons.

being accepted by oxygen with hydrogens, we convert oxygen into water. So when we talk about combustion of glucose through respiration, we talk about the products being CO2 and water. The CO2 comes from the CO2 lost from the different decarboxylation reactions throughout the PDH and citric acid cycle, and then the water comes from oxygen as the final electron carrier, then combined with hydrogen to generate water. So now we've pumped these hydrogens out, and through pumping these hydrogens out, we've created this electrostatic gradient, right?

So we've created this attraction whereby the protons want to move inside. Not only are we generating a concentration gradient where hydrogen ions are outside, but there's also an electrostatic force being generated where the protons want to move inside back into the matrix where it is more negative. And so ATP synthase, represented in pink here, acts as a valve to allow those hydrogens through.

And through movement of hydrogens, we generate the potential energy of hydrogens moving across the membrane into motive force energy through hydrogens turning part of ATP synthase. And that turning generates chemical energy by converting, so confirmation of the enzyme, the enzymatic portion of ATP synthase, right? catalyzes this conversion of ADP to ATP, okay? So this is summarizing what I just said in words, right?

Glycolysis in the citric acid cycle generates high-energy electron carriers, NADH and FADH2, which transfer those electrons to oxygen through the ETC, okay? And then exergonic energy released from these redox reactions powers the movement of these protons across the gradient, okay, between the inner atmosphere. and the outer mitochondrial membrane.

So we're not actually pumping outside of the mitochondria. The mitochondria has an outer membrane and an inner membrane. Within the inner membrane is the matrix, so that's the sort of central most compartment of the mitochondria.

And then there's a compartmental space between the matrix membrane and the outer membrane of the mitochondria, and that's where we get this proton gradient buildup is in that inner membrane space. So this proton gradient is then used to power ATP synthase by oxidative phosphorylation. And that's the conversion of ADP to ATP. So what's the primary catabolic function of the citric acid cycle?

Harvesting of high energy electrons from carbon fuels in the form of NADH and FADH2. So we learned about the citric acid cycle last time. Its main function as a catabolic process is to generate NADH and FADH2, which ultimately take their electrons to the ETC, which ultimately pumps these protons out. okay, through the generation of water or conversion of O2 into water.

And then that gradient is what drives that chemiosmosis, okay? And then that is turned into proton motive force, which is converted into chemical energy to provide the energy to convert ATP into ATP. So what is the electron transport chain? Well, it's really just a series of redox reactions.

And so there's a way that we can look at this, right? We can look at it as the free energy to oxygen, right? So if oxygen is sort of the ultimate electron acceptor, then we can generate a hierarchy of things that can accept electrons. So oxygen accepts it the most readily, and in the ETC, NADHQ oxidoreductase, or complex I, represented here, is sort of the highest free energy relative to O2, okay?

So what happens here? Okay, so electrons from NADH and FADH2 are used to reduce molecular oxygen to water through these series of electron carriers. Okay, so complex I, complex II, complex III, and complex IV, represented with Roman numerals here, all have names, okay, and we'll talk about the names of them, but those are all membrane-embedded complexes, okay? There are also electron carriers that take electrons from each complex to the next complex, okay? And so those are not membrane-bound, those are free, and those move electrons from one complex to another.

Okay, so a strong reducing agent, such as NADH, okay, is poised to donate electrons and has a negative reduction potential, whereas a strong oxidizing agent, right, O2 is a very strong oxidizing agent, is ready to accept electrons and has a positive reduction potential, okay? So electron flow through the ETC creates this proton gradient. If you want to watch another video, Okay, this is actually, I think, a very good explanation of the electron transport chain.

So we can watch this video in class. I'm not going to play it here. But let's walk through this chart or graph on the right. So we've generated NADH from different oxidation steps throughout glycolysis, pyruvate dehydrogenase, and the citric acid cycle. We've also generated FADH2.

So NADH donates its electrons. electrons first to complex one. So NADH reduces complex one. And so we're going to regenerate NAD plus here. So when NADH donates its electrons, we're going to regenerate NAD plus.

So why we do fermentation to regenerate NAD plus for glycolysis here is in the absence of oxygen, we can't do respiration, right? And so these oxidative steps... that can happen through PDH and the citric acid cycle are also ways for us to generate NADH and then through the ETC regenerate NAD+.

But in the absence of oxygen, we can't use the electron transport chain in order to regenerate NAD+. This is why oxygen is necessary to regenerate NAD+, in a non-fermentation fashion. If we have oxygen present, we don't typically have to do much fermentation. Okay, we will only need to do fermentation and immediate needs where we lacked, you know, maybe there's sort of a rate limiting step where we're using our oxygen as fast as we can. Right.

And so we get into a oxygen depleted state. And so we do some fermentation. Right.

But in the presence of oxygen, we have no problem regenerating NAD+. This is just to relate to you fermentation and why we use fermentation in the absence of oxygen. When oxygen is present, very easy for us to regenerate NAD+. When oxygen is not present, we have no way to regenerate NAD-plus through the ETC. Okay, so NAD-plus dropping its electrons off at complex one, which is called NADH2Q oxidoreductase.

So NADH2Q, Q is coenzyme Q, also known as ubiquinone, or sometimes just Q. So this means that this is an oxidoreductase. So complex one is an oxidoreductase.

that transfers electrons from NADH to coenzyme Q. Okay, now coenzyme Q isn't a complex. Coenzyme Q is going to transfer electrons to complex three.

Notice we bypassed complex two completely when I said that. Okay, so let's just say that again. NADH transfers its electrons to complex one, and complex one transfers those electrons to you. ubiquinone or coenzyme Q. Technically what complex I does is it catalyzes the conversion of NADH to NAD+, and it transfers electrons to coenzyme Q, right?

So the electrons technically never stay at complex I. They go directly from NADH to coenzyme Q through complex I when catalyzing that conversion. And complex, sorry, and coenzyme Q transfers those electrons to complex III.

And complex III catalyzes a reaction that takes the electrons from coenzyme Q and transfers them onto cytochrome C. Again, we've skipped complex II completely here, and we'll come back to it. So complex III's name is, once again, pretty easy to figure out. It's an oxidoreductase that... that transfers electrons from Q, or coenzyme Q, to cytochrome C, hence its name, Q-cytochrome C oxidoreductase.

Cytochrome C, once again, is transferring electrons, so it's a non-membrane bound component that transfers electrons from complex III to complex IV. Complex IV is cytochrome C oxidase because cytochrome C gets oxidized. and reduces oxygen into water.

Okay? So if we follow that, the electrons from NADH go from complex I to coenzyme q to complex three to cytochrome c to complex four ultimately to oxygen to make water that is the path of electrons from nadh the path of electrons from fadh2 starts at complex two so nadh cannot donate electrons to complex two instead fadh has its electrons transferred to coenzyme Q through complex II. This is the real difference, okay, is that FADH2 donates its electrons through complex II, whereas NADH donates its electrons through complex I.

Both of them donate those electrons to coenzyme Q, and coenzyme Q then transfers it down to the rest of it. The path after coenzyme Q for both NADH and FADH2-derived electrons is identical, it's just their starting point that's different. So notice the name of complex 2 is sucinate to Q reductase. So why is this?

Well, if we note, we generate FADH2 from the conversion of sucinate into fumarate. So if you go back to the citric acid cycle, sucinate to fumarate generates an FADH2. Really, this FADH2 is never a free-floating FADH2.

This reaction of succinate's conversion to fumarate generating FADH2 is coupled with the transfer of the electrons for FADH2 through complex II onto coenzyme Q. This is why it's called succinate Q reductase, because the electrons that succinate donates to FAD are going to be transferred onto coenzyme Q. And in fact, complex II is the same enzyme from the citric acid cycle that converts succinate into fumarate.

So this is why sometimes you'll see succinate Q reductase as the name for the enzyme in the citric acid cycle. So again, if you want to watch another video, this is a great YouTube video explaining it in a slightly different manner than I'm explaining it here. and it's a decent one to watch.

It's simplified. It also adds some inhibitor information that you do not need to know for my class, but is interesting to know for medical school, for applications in medicine. Okay.

Okay. So I'm going to show you this in multiple different ways. So I have another slide here, which gives more detail, and I'm going to run through it similarly to how I ran through the previous slide. Okay. But it's the same information that I gave in the previous slide, just shown in a slightly different manner.

So here, one of the big differences is we've shown the compartmentalization where we have the matrix side of things on the bottom here in beige, and we have the inner membrane space of the mitochondria. So this is the matrix membrane that all of these complexes are embedded in, and this is the space between the matrix membrane and the outer membrane of the mitochondria. So let's run through it. So the first complex, complex one, is where NADH donates its electrons, right? So we have our conversion of NADH to NAD+.

Electrons are donated onto coenzyme Q, and those electrons on coenzyme Q, why is this QH2? Well, it's because you don't just have free electrons, okay? This is why when we add electrons to NAD+, they actually come along with hydrogen here, okay? So we typically have hydrogen or hydrogens coming along with electrons.

So Q has these electrons represented by two hydrogens that are actually helping carry those electrons. And you notice this dotted line past complex two. It's because the electrons on coenzyme Q that came from complex one are going directly to complex three.

Whereas if we look at complex two, even though coenzyme Q is the electron acceptor here, they're also going to go from two to three. So the electrons from complex one never go to complex two. And when we transfer those electrons, the energy generated through going through this electron transport chain from the first complex is enough to pump four protons from the matrix into the inner membrane space. If we go to complex two.

We have this complex 2 is succinate Q reductase. Remember, it's also the same enzyme that converts succinate into fumarate. So this catalyzes the conversion in the citric acid cycle, which happens in the matrix, of succinate to fumarate, which generates FAD into FADH2.

I have a caveat here. Technically, this FADH2 conversion to FAD, when we're adding the electrons from FADH2 onto coenzyme Q, It doesn't happen in the intermembrane space. It basically happens altogether, right? So conversion of succinate to fumarate generates an FADH2, which immediately donates electrons to coenzyme Q through the same enzyme, okay? Then those electrons go to complex III, which is Q, cytochrome C oxidoreductase, because it is transferring electrons from coenzyme Q to cytochrome C, okay?

And in the complex III... the transfer of those electrons to cytochrome c is enough to generate four protons pumped into the inner membrane space. And then finally with complex IV, we have pumping of two hydrogens based on the transfer of electrons from cytochrome c onto oxygen.

So notice this chemical formula. Technically we transfer two electrons, oxygen accepts two electrons, but it splits the two electrons into two parts, right? In order to combine. with each oxygen molecule will combine with two hydrogens as well as an electron to form water.

And so we actually generate two waters per transfer of two electrons. That's why it's half an oxygen here. If we transfer two electrons onto oxygen, it would be four hydrogens plus one oxygen plus the two electrons is going to generate two waters. So you should know this or some version of this, whether it be from the last slide or this. Okay, one thing to note here, and it's an important note.

In total, we pump four hydrogens from complex 1, four hydrogens from complex 3, and two hydrogens from complex 4. A total of 10 protons are pumped, and the only complex that doesn't pump protons is complex 2. Complex 2 does not pump protons. It just serves as the acceptor for electrons from FADH2, but it does not pump any protons. So once again, in words, oh, so sorry, before we get to that, why am I emphasizing the 10 hydrogens get pumped?

Okay, so if NADH pumps 10 hydrogens, right? So one NADH molecule provides two electrons. And those electrons move from complex I to complex III to complex IV.

And just the movement of electrons from NADH generates IV plus IV plus II, so 10 hydrogens pumped. Okay, NADH has the energetic equivalent of 2.5 ATPs. The reason for this is because of the 10 hydrogen it pumps.

So the electrons from NADH pumping 10 hydrogens, right, is the equivalent energy generation to catalyze the making of 2.5 ATPs. So how many protons does it take to catalyze production of a single ATP? Simple math says one ATP means four protons. So what that means is the protons moving back through ATP synthase at the end and doing oxidative phosphorylation, generates one ATP through four protons moving through.

So if NADH is pumping 10 protons, that's where we get the equivalent of 2.5 ATPs, because four will come through, generate one ATP, four more will come through, that's eight total, that'll be two ATP total, and then we have half of an ATP worth of protons. Okay, we're not going to actually generate half an ATP, but all we need is two more hydrogens. right? So you could really think of this as two NADHs pumps 20 hydrogens, which is equivalent to five ATPs, right? Now FADH2 only pumps six electrons total.

And when I say six electrons, I'm going to edit this here while you're watching. Pumps six H plus total. So it should be protons total.

Okay. So FADH2 pumps six protons total. Why is that? Well, moving back here, right, we skip complex one.

FADH2 doesn't donate electrons through complex one, so four of the protons that NADH pumps never get pumped when we donate electrons from FADH2. Instead, the first protons that get pumped are through complex III, and it's four of them. And then finally, we also pump them through complex IV. So we get six hydrogens pumped from FADH2, which is why FADH2 is energetically equivalent to 1.5 ATPs.

It's a lower energy molecule than NADH based on where they donate electrons in the electron transport chain. And we get that 1.5 because if every four generates an ATP, we have one and a half fours here, right? One and a half times four is equal to six. That's where we get 1.5 ATPs.

So in the video where they go over how many ATPs are generated from glucose, they give a range. This range has to do with the NADHs that are produced in glycolysis. Okay. I want to make sure that you guys understand that I will typically use the range of... I will say 32 are generated because we haven't learned why there's a range in the NADHs generated in glycolysis.

This largely also has to do with the conversion into FADH2 versus NADH, which is where the 3 to 5 comes from. I apologize. My mic turned off.

So give me just a moment here, and I'm going to pull up some background noise to play, which will hopefully prevent it. My headset automatically turns off if there's no sound going on. So now I have some background sound going on. OK. So in the video that I showed previously a link to on this slide here, they talk about that range of ATPs generated.

It has to do something with the NADHs generated in glycolysis, can also be FADH2s. So if they were FADH2s, we'd have three ATP equivalencies, right, instead of five, because two NADHs are worth five ATPs, whereas two FADH2s are worth three ATPs. But I don't care too much about that. We talk about them as NADHs from glycolysis.

So we're going to assume that they're NADHs. So we generate a total of 32 ATPs from lung glucose. If we go through all of glycolysis and all of oxidative phosphorylation of all of the electron carriers, as well as whatever ATPs get generated through each cycle.

Right. So I'll run through this with you guys in class. But glycolysis generates two ATPs directly. Right.

We input two, we get out four. So our net from glycolysis is two. We also generate two NADHs from glycolysis, which is equivalent to five ATPs. We generate two NADHs from pyruvate dehydrogenase or pyruvate metabolism, which is equivalent to five ATPs.

We generate two ATPs directly from the citric acid cycle. If it goes twice, it's one per cycle. And then if we go for two cycles of the citric acid cycle, it's six NADHs times 2.5 is equivalent to 15. And we have two FADH2s, one per round of the cycle, which is equal to three. So 15 plus three is 18. We have these two, 19, 20, plus five is 25, plus five is 30, plus two is 32. So glucose, going all the way through oxidative phosphorylation, can generate a maximum of 32 ATPs.

This is also showing the same information in a chart. So you can see NADH to Q oxidoreductase, what it accepts electrons from, what it converts them to. Okay, this also indicates, based on this chart, what is on the cytoplasmic side and what is not. Sucinate Q reductase, right, means that it transfers electrons from sucinate to Q.

So the matrix side, the accepting side, is electrons from sucinate. Technically, it's electrons from sucinate. transferred to FAD to make FADH2, and then FADH2 transferred onto Q, but those are coupled reactions, okay?

And then we have Q to cytochrome C oxidoreductase or COMPOX3, which transfers from coenzyme Q to cytochrome C. Cytochrome C is on the cytoplasmic side, and then cytochrome C transfers electrons to oxygen on the cytoplasmic side to make water, okay? So this is, again, saying everything I just said. but in words.

So there's no new information here. I'm not going to say it again. I just have a summary in words to everything I've said on the last few slides.

Okay. So I'll leave this up here for just a moment and you can stop it and move on when you're ready. Okay. So proton motive force or chemiosmosis.

Okay. Let's just run through it. Proton motive force.

So The transfer of electrons through the electron transport chain leads to the pumping of protons from the matrix to the cytoplasmic side of the inner mitochondrial membrane. I've already gone over that with you. OK, now proton concentration becomes lower in the matrix as well as the generation of electric field.

Right. With the matrix side being negative and the cytoplasmic side being positive. When I say cytoplasmic side. Remember, I'm still talking about we're not in the cytoplasm. We're just on the side of the inner membrane that is closer to the cytoplasm.

So it's called the cytoplasmic side. Now protons. So what is the proton motive force? This is that valve I talked about.

Protons flow back into the matrix to equalize the distribution. And we use that flow to turn the C ring of ATP synthase. which ultimately turns the gamma subunit because they're attached, which ultimately causes conformational changes in the alpha and beta subunits, and those conformational changes catalyze the conversion of ADP into ATP. So I'm not going to go over this in this lecture because this YouTube link right here is the one that you should go to and you should watch the video on it.

And essentially, everything in the video is what you should know. If you have questions on the details of this video in class, I'm happy to talk about it with you. So this is a suggestion that I have for you to go through. And if you need help and need to go through as a group, we can do this in class as well. But make a diagram of the following, okay?

for the extraction of ATP from a carbohydrate food source. Indicate the enzymes required for the digestion of this food source, the resulting products of each, okay, and their digestion, and ultimately the ways we've learned that the body extracts ATP through anaerobic and aerobic respiratory cycles, okay? So I actually haven't chosen anything that you really need to know digestive enzymes for, okay, but you could do this, all right?

I've actually just chosen things that are within many of the cycles that we've talked about. Okay, so all of these are actually within glycolysis, A through E, but we can choose some that are in the citric acid cycle or elsewhere. Okay, so assuming each NADH molecule that goes through the electron transport chain produces 2.5 ATP molecules through the electron transport chain and ATP synthase, how much ATP is generated by each of the following molecules? Assuming each one is a starting point for respiratory ATP generation. We can roll through some of these in class just so you can sort of see what I'm talking about with calculations from different things, and we can choose some that are within the citric acid cycle.

Okay, and that will end this lecture.

Transcript for:Understanding Electron Transport Chain and ATP Synthesis

Transcript for:
Understanding Electron Transport Chain and ATP Synthesis