Oxidative Phosphorylation 2 - Video Tutorials & Practice Problems
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Oxidative Phosphorylation 2
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You might recall that cytosolic NADH produced by glycolysis can lead to the production of 3 or 5 ATP depending and now we're going to get into the depending on what. So any DH will actually only lead to the production of 3 ATP if it passes its electrons to an FAD via mitochondrial Glycerol 3 Phosphate dehydrogenase. The way this works is basically NADH will oxidize to NAD plus in the process of reducing DHAP remember that familiar compound from glycolysis. DHAP to Glycerol 3 Phosphates. It's going to reduce DHAP and this is glycerol 3 phosphate not to be with glyceraldehyde 3 phosphate which we often abbreviated as G3P. So this is not G3P. This is a different molecule. Anyhow, glycerol 3 phosphate will then be used to reduce an FAD to FADH2 and this passes its electrons onto ubiquinone which will of course pass them on to complex 3. So essentially in this process, complex 1 is bypassed entirely. And what we're going to see momentarily is complex 1 pumps protons and because that is normally where NADH drops off its electrons and this is bypassing that, we are missing out on some of those, some of those protons getting pumped. So it's essentially as if the electrons were coming in, this way. However, it's important to note that it is not through complex 2 that they're actually entering. They're entering through this enzyme, Glycerol 3 Phosphate dehydrogenase which is, a peripheral protein that's basically going to be sitting here. So here g oops. I'm just going to abbreviate it gp inside inside of it and it's going to pass those electrons to the quinone so that they will make it to make it to complex 3. The other way that NADH can get into the mitochondria or get its electrons in there is through the Meily Aspartate shuttle. And this using this method, we yield 5 ATP. So let me make that clear. This is the 3 ATP method and this here is the 5 ATP method. Now, the malate aspartate shuttle transports NADH across the mitochondrial membrane at no energy cost. So we don't have to expend any energy to get that NADH into the mitochondria which is why it's going to yield 5 ATP just like the NADHs that are generated in the citric acid cycle which of course already takes place in the mitochondria. Now, this is a slightly confusing process so I'm going to walk you through it step by step but essentially, what you have is malate dehydrogenase in the cytosol. So here, this is the cytosolic side and here we have matrix. Now, might, Melydehydrogenase in the cytosol is going to take oxaloacetate. This molecule, OAA oxaloacetate and it's going to reduce it using NADH which will convert that to NAD plus and it's going to convert it in to malate. So here we have malate. Malate gets transported into the matrix and it actually moves via an anti porter but I want to get to that in a second. So once in the matrix, malate, this is still malate right here. Malate is going to get reoxidized back to oxaloacetate. Remember I said this is going to be a pattern. So reoxidized back to oxaloacetate and in the process, it's going to reduce NADH to I'm sorry. Reduce NAD plus to NADH. Now, oxaloacetate will actually be oh one more thing. It's worth noting that this conversion from malate to oxaloacetate will happen basically, automatically just by the pull of the, citric acid cycle Because remember that this is actually one of those readily reversible reactions of the citric acid cycle. But because the citric acid cycle so so technically, this would be a readily reversible reaction but because the citric acid cycle is running here in the matrix in that direction, it's going to pull this reaction to go from malate to oxaloacetate. So just kind of a side point there but it's worth noting. So what happens then to oxaloacetate? Well, oxaloacetate is going to be converted to aspartate. How do we do that? You hopefully remember this. We just talked about this so all we need to do is add an amino group and so we are going to convert oxaloacetate into let me jump out of the image here. Aspartate. And then while the way we do this is by actually taking glutamate which is this molecule. See that juicy amino group there. We're going to take glutamate and use its amino group to turn oxaloacetate into aspartate. And this is going to leave glutamate as alpha ketoglutarate. Okay. Now here is where I want to get back to this transporter because this is super cool. That melee transporter is actually an antiporter and it's an antiporter that moves mehlate into the matrix while it takes alpha antiporter that moves malate into the matrix while it takes alphaketoglutarate and moves that into the cytosol. So it's an antiporter that is flip flopping these 2 molecules that are both integral parts of this, little shuttle system here. So, very very smart, right? The product of this reaction to form aspartate is actually what's used in the antiporter to get melee in. Hope you can appreciate how eloquent, this system is. Alright. So then what's going to happen with that aspartate? Well, the aspartate also moves through an antiporter and it moves back into the cytosol and from there, this aspartate is going to be converted back into oxaloacetate. So it's a real cycle here, right? And guess what? Guess how aspartate gets converted back to oxaloacetate? I hope you're already thinking that alpha ketoglutarate, alpha Kg is going to pick up aspartate's amino group. Let me draw that differently. Pick up aspartate's amino group and reform glutamate. Alright. Last thing. Guess what? That aspartate transporter is an antiporter that takes glutamate in and sends aspartate out of the mitochondrial matrix. Pretty cool. Pretty, beautifully eloquent if I do say so myself. So, this system might seem fairly complicated but it allows the electrons of NADH to get into the matrix, right? So technically that NADH is not entering the matrix but the electrons are and they're getting picked up by an NAD plus once in there. So it's as it's equivalent to that NADH moving into the mitochondrial matrix. And with that, let's flip the page.