In these videos, we're basically going to be picking up our story where we left off last time. You might recall that we ended with glycolysis last time. That's sort of where we ended our story of cellular respiration and you can kind of think of glycolysis as act 1 of cellular respiration. In these videos, we're going to be covering act 2 which is the citric acid cycle. And in fact, next time, we will finish with the 3rd and final act which is going to be electron transport and oxidative phosphorylation. So the point is if you're not totally comfortable with what was going on in glycolysis, you might want to go and review that before jumping into these videos because we're just going to be continuing the story and we're not really going to be reviewing that material. The assumption is you already understand it by the time you're here. So let's begin with pyruvate oxidation. Now, a little recap. You might recall that glycolysis ends with, glucose being turned into 2 molecules of pyruvate and then all of this is taking place in the cytosol. Now, our pyruvate is going to be transported from the cytosol into the mitochondrial matrix. And what's, what's important that I want you to, you know, make sure you're aware of, is that we are only going to be looking at the path of 1 pyruvate. Right? Remember that there's 2 coming off of glucose. So, if you are trying to, you know, do your sort of like mental accounting of all the substrates and products and all that, be aware that, you know, if you're coming from glucose, you need to double the numbers that you're seeing now. And when we finish up our discussion of the citric acid cycle, we will be going through a grand accounting of all the substrates and products and all that sort of stuff that we have talked about. And I'll be very explicit about the numbers relating to one glucose, etc. So just for now, be aware, we're only looking at the path of 1 pyruvate, not the 2 that are going to come off of glucose. Okay. Without further ado, pyruvate dehydrogenase. Dehydrogenase. This is going to pick up pyruvate once it enters the mitochondrial matrix. And you can see from its delta G, this is a favorable reaction. And it's actually a complex of 3 enzymes, but you don't really need to worry about their names. I've included them for the sake of thoroughness but don't worry about memorizing it. It's not very important. What you do need to know are rather the substrates, products, cofactors, and how this enzyme is regulated. So let's start with the substrates. Again, we are our reaction with pyruvate. That one's kind of obvious. You should know the structure of this molecule already. Be very familiar with it and, we also use the substrate NAD+ and coenzyme A. And just take note, this is the reduced form of coenzyme A. That's what that SH is symbolizing. And, we are going to generate from this reaction. Our products are going to be CO2. This is going to come off as CO2. We're going to talk more about that, momentarily. Our NAD+ is going to be reduced to NADH and talk about more a little more about that in a second too. And lastly, our product is going to be acetyl CoA or our main product, right? What pyruvate is going to become and what's going to carry on in the, subsequent reactions is Acetyl CoA, this molecule you see here. So, the cofactors, you need to know what they are but the only note I really want to make about them is FAD because it's kind of interesting what happens here. You see, FAD is going to be in the course of the reactions. FAD, write this over here. FAD is going to be reduced to FADH2. Now the reason I've drawn my arrow kind of weird and backwards, to demonstrate this is because this is actually going to, FADH2 is actually going to reduce NAD+ and turn back into FAD, right? This is a cofactor so it's going to be, you know, we're going to be reusing this FAD, in subsequent reactions. So FAD gets reduced to FADH2. That's going to reduce our substrate NAD+ into NADH. We're going to regenerate FAD and pick up another pyruvate, do another reaction, so on and so forth. Interesting little note there. And lastly, just want to again make the point that glycolysis glycolysis was taking place that was a weird s. Glycolysis was taking place in the cytoplasm. I'm sorry. Let's call it the cytosol. And the, reactions are now pyruvate, right, has moved into the mitochondrial matrix. So that is where all of these reactions that we're going to be talking about now are taking place. Last note I want to talk about, everyone's favorite subject I know, counting carbons. And you might recall a little review here that from glycolysis, we are left with 2 Pyruvate and they will have the numbering scheme of 1
Review 3: Pyruvate & Fatty Acid Oxidation, Citric Acid Cycle, & Glycogen Metabolism
Pyruvate Oxidation