Oxidative Phosphorylation 3 - Video Tutorials & Practice Problems
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Oxidative Phosphorylation 3
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Let's take a closer look at what's going on inside the complexes of the electron transport chain. Complex 1, which is also called NADH dehydrogenase, pumps 4 protons into the intermembrane space and it has the prosthetic groups, flavin mononucleotide and iron sulfur proteins. Now NADH delivers 2 electrons to complex 1 and it passes them again to flavon mononucleotide and then they move on to the iron sulfur ubiquinone which also takes 2 protons out of the intermembrane space And then it gets reduced and it will go on to complex III. But we will get to that in a moment. 1st, let's take a look at complex 2 which is succinate dehydrogenase. So, this is actually an enzyme that is also part of the citric acid cycle if you might remember and it doesn't pump any protons. It does however, contribute slightly if you think about it because it is going to, also pass its electrons onto ubiquinone. And ubiquinone is going to pull 2 protons out of the mitochondrial matrix. So even though it doesn't pump any protons, it is going to remove protons Okay. So it actually has FAD as a prosthetic group. I know the image looks as though FAD FAD or FADH 2 is external to the protein complex but it's actually part of the protein complex. So it's a prosthetic group, right? The reaction with succinate dehydrogenase is going to pass electrons to FADH 2 and it is going to pass electrons onto those iron sulfur proteins. And, FADH 2 again will deliver its, electrons to ubiquinone which will become reduced and go on to complex 3. Now, it's interesting to note that there are actually 3 FAD entry points to ubiquinone, right? We have complex 2 which we talked about. But we can also send FAD to ubiquinone through beta oxidation and, if we use that system we talked about just previously where NADH will actually pass its, electrons onto an FAD which gives them to ubiquinone, right? So there's actually three ways for FAD to give electrons to ubiquinone. And complex 2 is only one one of them. So just something to think about. Now Complex III, or cytochrome B as it's sometimes called, pumps 4 protons into the intermembrane space and has a heme prosthetic group and iron sulfur proteins. And it actually has a pretty interesting thing that, occurs with ubiquinone called the q cycle where essentially cytochrome C which is ultimately what's going to receive the electrons from ubiquinone, cytochrome c can only accept one electron at a time. It can only move 1 electron at a time. Ubiquinone passes on 2 electrons at a time though. It actually it has to pass on 2 at a time. It can't hold on. So essentially what happens is you have and I just want to be clear that all the numbers in this figure are representing the, net reaction of what's going on here. So I'm going to talk you through how it goes, step by step. So essentially you have just one just one reduced ubiquinone come in and drop both of its electrons off. And one goes to the iron sulfur proteins and on to cytochrome c. The other takes a different route where it actually links up with another ubiquinone that gets reduced and actually comes back and drops off its electrons. So it has let me actually do it this way. I'm going to do its line in red. So this ubiquinome comes and drops an electron off that goes to cytochrome c and another one that also takes this circuitous route. And again, this cycle continues. So after this, you'd have another ubiquinone come in and this would repeat. But we're not actually being shown that in the figure. We're just being shown the net reaction for 2 electrons, right? Because we had 2 electrons coming in from complex 1 or 2, so we want to follow their path through. So the net reaction of those 2 electrons coming in and those 2 electrons leaving via 2 cytochrome c is 4 protons get pumped through. And ubiquinone while it is performing this q cycle, right? The ubiquinones that pick up that extra electron are going to end up taking 2 protons out of the mitochondrial matrix. So from here, cytochrome c is going to bring the electrons to their final stop which is complex 4 or cytochrome c oxidase. Now this is going to pump 4 protons as well in here. Let's let's be thorough. Right? So, let me jump out of the image here too so you can see better. So Complex 4 is going to pump, 4 protons as well. Essentially the electrons are going to move from cytochrome c to these 2 coppers. You have these 2 coppers in complex IV. They are going to accept the electrons from cytochrome c. What I am showing here is that significant in a second, right? So now we're talking about what would have been delivered from 2 NADH or FA DH2. And this is important to the numbers matching up to the reaction below we're going to talk about. Anyhow, you have cytochrome c dropping off those electrons to the coppers. They move through complex 4. What you wind up with is water being formed. So Complex 4 is going to take in protons and oxygen, and it is going to form water. So, oxygen is the final electron acceptor here and it enters its most reduced form as water and that is the end of Electron Transport. I mean you got to think about this for a second. It's it's pretty amazing that, you know, this oxygen right here is the reason CO2 2 that we're exhaling is the CO2 that's coming off of the reactions in the citric acid cycle. I mean that's pretty mind boggling, right? I just think it's so cool. Anyhow, with that, I'm done geeking out and let's flip the page.