Let's begin by talking about amino acid oxidation, which is how the body uses the carbon skeletons from amino acids to fuel the citric acid cycle. Now, before it can use those amino acid carbon skeletons, it needs to do something about the nitrogens. It needs to get rid of them, but it can't just cut them off and release them as ammonia to the cell because that would really mess with the cell's pH and also it could be toxic to the cell. It's a reactive species. Anyhow, what the cell is going to do is it's going to take those nitrogens and it's going to put them through the urea cycle, which is a cycle that occurs in the liver, so it's going to export this stuff to the liver, and the cycle takes in 2 nitrogens and puts out 1 urea, and it costs 3 ATP to do this. So we're going to take a look at the cycle right here, and the first step, or what you could think of as the first step, which is right here, is going to occur inside the mitochondria. So this here is inside the mitochondria matrix. And out here, we have the cytosol. And what's going to happen is Carbamoyl Phosphatesynthetase, this enzyme right here, is going to take bicarbonate and this should be an ammonium because it's dissolved in the cell. So this figure is a little mistake. But you can see that up here that it's ammonium. And it's going to take bicarbonate and ammonium and at the cost of 2 ATP form this molecule right here, which is carbamoyl phosphate. I'm just going to write carbamoyl P. It's carbamoyl phosphate. Now, ornithine is going to enter the mitochondria. And ornithine is this molecule right here. That's ornithine. It's going to enter the mitochondria and it is going to combine with carbamoyl phosphate, and that phosphate group is going to leave. You can see it's inorganic phosphate, and they're going to combine to form this molecule right here, citrulline. Now, citrulline is actually going to then exit the mitochondria. So a little back and forth in here. It's going to exit the mitochondria, and then things get a little interesting. What's going to happen is ATP is going to be broken down to pyrophosphate. Of course, pyrophosphatase is going to break this down to 2 inorganic phosphates. And citrulline is going to be combined with aspartate. What's actually happening, you see this AMP let me jump out of the image here. My head is blocking it. You see this AMP right here. Basically, the AMP gets attached to the enzymatic intermediate. The aspartate is added, and the AMP leaves. So that's why you see the AMP coming off, a little later here before that pyrophosphate. So these come together and they form this molecule right here, argininosuccinate. Alright. Oh, I'm sorry, I'm not labeling my steps. So, we had step 1, this here was step 2 and this was step 3. Okay. So argininosuccinate is then cleaved into fumarate right here, and arginine right here. That is step 4. From there, arginine has urea removed from it, right? So basically, these nitrogens right here are coming off as urea and what we're left with is ornithine again right here. So that is our final step of the cycle, Step 6. And then, of course, that ornithine will reenter the mitochondria combined with carbamoyl phosphate, and the cycle will repeat itself. So, let's turn the page.
Review 4: Amino Acid Oxidation, Oxidative Phosphorylation, & Photophosphorylation
Amino Acid Oxidation 1