Biology: Oxidative Phosphorylation

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So now we're ready to put it all together and make some ATP. Let's do a quick overview of where we've been and where we're going. Remember that we started this whole process way out here in the cytoplasm, back when we were mere biological children, and we were taking glucose and we were breaking it down from a C[6] to 2 C[3]'s, and we called that process glycolysis. And then glycolysis took the 2 three-carbon substances and converted them with a decarboxylation to acetyl CoA, a high-energy compound that was your entrance key into the Krebs cycle. Within the Krebs cycle, we took these 2 carbons, this acetyl group, from the leftover glucose, and we put it through a series of changes, at each step changing the energy level of many of those electrons, releasing energy and using that to do a bunch of biochemical activities, one of which was to reduce some coenzymes, another one was to actually do a little bit of substrate level phosphorylation of ADP.
And now we come to the final step. Let's really use all this energy and let us make some ATP. And you can see from this that we're still in the mitochondria, and we're going to call it the electron transport chain and oxidative phosphorylation. That's a mouthful; let's see what it means. Let's take a look at the general structure of the mitochondria. Remember the mitochondria is a two-membrane system. Remember that that two-membrane system has an outer compartment, and the inner compartment, which we can call the matrix of the mitochondria. Remember that we've already established that there are ways to work molecules. And the way to make molecules work quite often is through phosphorylated intermediates. This has been a point we've established since the beginning - phosphorylated intermediates can be used to make things happen.
Well, in this discussion, we're going to see the opposite of that. Stay tuned, you're going to enjoy that. Well, anyway, here's my double membrane, and what we're hoping to be able to do is to concentrate hydrogens, somehow pump hydrogens across this membrane. Maybe we can use the NAD's as the source of these hydrogens, and certainly the source of the energy to pump them. And that brings us to the inner mitochondrial membrane, the place where all of this is going to happen.
So let's take a good up-close look at that. The inner mitochondrial membrane, as you know, is loaded, absolutely loaded - I don't know why I keep putting this upside down, but I do - it's absolutely loaded with membrane proteins. And if you remember when we discussed the electron transport chain, it is important to visualize these proteins next to each other, embedded in the membrane; and one lying next to each other may pass an electron in a downhill fashion, from high-energy level to lower-energy level, to still lower, to very much lower, to a whole lot lower - in other words, passing these electrons along an energy cascade, if you will.
Well, each time we drop from one energy level to the next, to the next, to the next, to the next, we are hopefully going to capture that energy. And now you see what we can do. We could actually use that energy to pump hydrogens. Yes, that's right. We are going to pump hydrogens and make a battery. So what you are seeing happening here is by virtue of those proteins that are passing the electron from level to level to level to level. We are giving the proteins in the membrane the ability to do work. How are we doing that? It's very simple.
Just take a look at this. If I were to take - let's just draw that - let's call this the bilipid layer, and let me embed a protein in there. Well, very simply stated, what we can do is we can take that NADH+H and we can give this high-energy molecule - we can pass these electrons onto here, and let those electrons enter the electron transport chain. Now in doing that, what does that do to NAD? Well, think about it. If NADH loses its electrons, that is going to cause hydrogens to be lost from the NADH+H.
So now we're accomplishing two things. We are actually taking hydrogens and giving them - and actually coming up with a place to put the hydrogens, but even more importantly, we're putting the electrons someplace.
Let me show one more thing you can do. You can actually get these hydrogens to this side by pumping them through this membrane, and by pumping them through this membrane now, the hydrogens can actually form this gradient, and we can have a battery with a positive side here and a less positive side here. What's positive versus less positive? We're going to call that negative. So the positive's out here, the negative's in here; life is going to be good. Why is life going to be good? Because now we have the option - we have the ability - forget it, it's not an option - we have the ability - we have a current - we have the ability to pass current across a membrane.
And now we come to an extremely important complex of molecules - a complex of proteins - something called ATP synthase. Now, this is where I'm going to want you to think. Ready? Watch this: I'm going to bring you back to your just-beginner days in learning biology and the biochemistry of proteins. And you remember this kind of interesting situation where we could take ATP, put it on a protein, and use that protein to pump hydrogens. You knew all about that, right? By forming the phosphorylated intermediate, or a phosphorylated protein, we could use that to pump proteins; the phosphate would actually get onto this protein, cause it to do a shape change, and allow it to move a proton across a membrane.
In this case, we're going to do the opposite. Now we are going to take proteins. We're going to take those proteins and shoot them through an enzyme complex, and that enzyme complex is going to use the energy of the protein - of the protons - to make ATP. Let me restate that, because I might have used the word protein incorrectly there. We're going to take protons - did I saw protons? - and where are the protons? The protons are out here, these charged hydrogen ions. These protons, we are going to allow those to come through the ATP synthase, and in passing through the ATP synthase, the energy of these protons is going to be used to generate ATP. How? This process is called chemiosmosis. "Chemi" - chemical; "osmosis" - passing through a membrane. And in this process of chemiosmosis, we're going to do something called oxidative phosphorylation. Chemiosmosis - in chemiosmosis, we're going to shoot those hydrogens across the membrane, and in doing that, we're going to put it through the ATP synthase.
Now, the ATP synthase - this complex series of proteins right there, of enzymes - the ATP synthase is going to take ADP and an inorganic phosphate, and generate ATP. Why are we calling it oxidative phosphorylation? Well, number one, we're contrasting it. Remember substrate level phosphorylation? Remember I defined that as the "energy for the phosphorylation comes from the substrate"? Well, now the energy is going to come from not the substrate, but from oxidation. You see? And that's why this is called oxidative phosphorylation. This whole oxidative process is what's driving the ATP formation, and so it's called oxidative phosphorylation.
Well, I know a lot of you are saying, "Okay, but just how much ATP is made?" I've got to tell you, that's important, maybe. But right now I want you to get this; I want you to get the big picture; the big picture's what's key here. The NADH loses its electrons to the series of proteins embedded in the membrane. In losing its electrons to the series of proteins embedded in the membrane, the NADH loses its hydrogens. The protons will be able to be built up on this side of the membrane. And in building up these protons, these positive charges, we have a potential energy difference; we have a battery. And now, just like that electric line, we can zip those protons through the ATP synthase, do an oxidative phosphorylation, and there, my friend, is the ATP.
Respiration
The Electron Tansport Chain and Oxidative Phosphorylation
Oxidative Phosphorylation Page [1 of 2]