before looking at each individual step of the citric acid cycle, I want to take a look at the process as a whole, you know, study the forest before studying the trees. So if you input one acetyl coa A into the citric acid cycle, you are going to get three in a D. H. One F A. D. H two and one GTP or a teepee. And we'll talk more about why it's a GTP or a teepee later, all you really need to know is that your generating one nucleotide triphosphate. Now remember that, like Hollis, ISS is going to actually yield to acetyl coa A at this point in the metabolic pathway. So if you are trying to trace all of this with glucose is your origin, you're gonna need to double your numbers here. It's also worth noting that this is also going to give off to carbon dioxides. But, um, that's really the end of their story. Whereas these molecules that's the money, right? That's what's gonna get you your at payoff. So taking a look at this figure, you'll notice that none of the chemical structures air shown that is intentional. Don't worry about those you'll get to see them in the following pages. When we look at each reaction individually. For now, I just want to sort of guide you through this diagram and, uh, show you really what? It's main purposes, which is tracking carbons and looking at the, uh, the overall, like important things that are happening during the citric acid cycle. So again, start with acetyl co. A, which you might recall, is made of either carbon one or six and two or five from glucose. And it is combined with this molecule, och Salo acetate four carbon molecule and they're gonna make citrate and notice that the steel co carbons are black, so you can follow them through the cycle. And, uh, the carbons from Excel or a state are white and gray, and those white ones are going to be the ones that come off as carbon dioxide momentarily. And just to be crystal clear, this is carbon two or five from acetyl coa. And this is carbon one or six. I'm sorry. From from glucose from glucose, obviously a steel. Coetzee's carbon numbering is it's different than that of glucose. So in the following reaction we form is a citrate, not super important. What's going on here? You remove the water and then you add it back in. Biochemist sometimes call that the Ferris wheel or this They call this enzyme the Ferris wheel because what it does, um you know, they have a particular brand of humor. Let's say, uh, me actually just jump out of the image here, so you conceit where we're at, not blocking it. So this this next reaction this is one of the important ones, right? Isso citrate to Alfa Keto. Glittery. This is where we lose our first carbon dioxide. It's this one coming off, and we also are going to generate an any D. H in this reaction which is going to go on to the electron transport chain. Uh, you know, we'll get to that story in the next exam review. And this, uh, this next reaction is also one of the one of the really big important ones where Alfa Keto gluten rate goes thio second, Al Oh, a And it's where this carbon comes off this carbon dioxide and we form another n a d h. And you might also notice that coenzyme A is gonna come back into the picture. Uhh! Now, in the following reaction where sucking ilco a form is made into hoops. Second, Nate, this is where we're gonna produce our GTP. So GDP and inorganic phosphate substrate level phosphor elation make GTP in some cells, this will actually be used to make, um, ATP, um, by substrate level foster correlation with ADP and inorganic phosphate. Not every cell is gonna do this. Some will keep this as GTP. That's why it's an or situation, right? GTP or a teepee? Um, it doesn't really matter for us, though, because in biochemical terms, uh, you know these air just nucleotide triphosphate. They're worth the same amount of energy. Let's put it that way. In terms of currency, these air, both dollar bills or whatever the same same value. Anyways, what is important about this part of the figure that I do want to point out is we have a change in color scheme. Notice that the colors here are different from the colors here, and there's a very important reason for that. And that is that suck in it is a symmetric molecule. So it's It's almost like a it has a mirror running through its center. Right? It's our Each half is like a mirror mirror of the other half. Um, now, because of this because of this, this carbon right here this'll carbon right here can wind up in this position or this position. So, uh, that's why I'm changing the coloring scheme from here on out. Alright. What I don't want you to confuse Don't think that these two great carbons are now these two great carbons. That is wrong. No, that is not what's happening here. What's happening is I'm representing the color coding of this molecule in terms of what's going to be coming off in the following cycle. Um, and the reason I'm changing the colors like this is again because, uh oh Canada symmetric. So it's orientation in the active site of Theo Enzyme that carries out this reaction is randomized meaning, you know, it basically doesn't matter, or it's not that doesn't matter. It's that it's random whether this again, you know, this carbon here winds up either up here or down here. So just to be crystal clear, that means that this carbon can wind up up here or down here as well. You know they're gonna be opposites. So they can either kind of, like, go over like this or okay, like that. And again, it's random, we don't know. So I'm changing the color scheme. Sorry to harp on that. I just wanna make sure that's crystal clear what's going on here? And I don't want anyone to get confused and think that these gray ones are the same on both sides. OK, moving on Sirkin eight will be turned into humor rate, and we're going to generate our one and only f a D h two. We only produce one in citric acid or per turn of the citric acid cycle. It's right here. And from here, FEMA rate will be converted into may late and then mail It will be reduced, uh, into box alot acetate. Oh, and just to be again sorry. Just to be super super clear, it's on Lee Sook innate that is symmetric and can have that random orientation in the enzymes active site. Just because these look, um, the same a second, you know, in terms of color scheme, don't think that their orientation is randomized to so anyways, we end by regenerating ox, Iowa state produced our final and a D. H. And we're ready for another asset Eel co A. To enter our cycle now, A really important thing, I wanna point out, while we were still looking at this image, let's go ahead and just number all the steps. Right. So here's 67 and eight. Okay, steps one, three and four are the major drivers of this process because they have a negative Delta G. These reactions here, 8765 thes can be reversed, actually, pretty easily. And we're gonna talk about that. Uh, at the end of this exam review, we're gonna talk a little bit more about that. But these reactions air readily reversible. Um, as is reaction number two. And the reason for that is because they're Delta. Jeez, air close to zero. So reactions 13 and four are the reason this process is driven this way. Consistently there the drivers, uh, this cycle and again, last note, um, about labeled carbons. Basically, what I'm saying is, if we put in are labeled carbons here, right, let's say that our black carbons are radio labeled. That's what the black carbons mean, right? Uh, let's just pretend for a moment that those air radio labels, like with you know, uh, Carbon 14 or something like that and they go through the cycle. What? I mean by the note that Onley half the labeled carbons come off due to the random ization of second. It's orientation is you know, uh, basically what we were talking about down here. This in terms of this Uccello acetate after our radio labeled carbons go in. Either one of these could be our carbon 14 right? We're either gonna have our carbon fourteen's here or here, and we won't know which we don't know which is which. But either way, Onley Onley. These white carbons are going thio be the ones that come off in the following cycle. So, uh, you know, depending on which orientation we have this one or this one Oops. One, not 1 14 14. There we go, depending on which orientation we have. Rather regardless of which orientation we have on Lee, one of those carbon fourteens will come off in the next turn of the cycle, meaning that if we put in one batch of radio labeled carbons, um, for example, like one mole Onley half will come off the first time, and the next time on Lee, a quarter will come off next time on Lien, Eighth will come off. Don't worry, we're going to revisit this concept again in case you're not totally 100% clear on it right now. But the point is, just where is all of this stuff positioned in terms of, you know, what's what's happening? That reaction with sucking eight. All right now with that, let's actually turn to some specifics and take a look at the nitty gritty of citric acid cycle.
