BIOCHEMISTRY DISCUSSION 7
Leahh
19HarvestingElectronsfromtheCycle.pptxBiochemistry: A Short Course Fourth Edition CHAPTER 19 Harvesting Electrons from the Cycle
Tymoczko • Berg • Gatto • Stryer
© 2019 Macmillan Learning
Created by Brett Barbaro
All right - so, now that we have gotten our acetyl CoA from the pyruvate dehydrogenase complex, we'll talk about how that gets processed further to generate more high energy electrons and ultimately ATP.
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CHAPTER 19 Harvesting Electrons from the Cycle
Created by Brett Barbaro
Roundabouts, or traffic circles, function as hubs to facilitate traffic flow. The citric acid cycle is the biochemical hub of the cell, oxidizing carbon fuels, usually in the form of acetyl CoA, and serving as a source of precursors for biosynthesis. [(Left) Lynn Saville/Getty Images.]
Kind of a strange feature of your book - the diagram makes it look like ATP is coming off from the backwards direction on the circle. That's just incorrect. It starts with acetyl CoA and releases two carbon dioxides and eight electrons and then some ATP. And what's remaining from that continues on in the citric acid cycle.
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Chapter 19: Outline
19.1 The Citric Acid Cycle Consists of Two Stages
19.2 Stage One Oxidizes Two Carbon Atoms to Gather Energy-Rich Electrons
19.3 Stage Two Regenerates Oxaloacetate and Harvests Energy-Rich Electrons
19.4 The Citric Acid Cycle Is Regulated
19.5 The Glyoxylate Cycle Enables Plants and Bacteria to Convert Fats into Carbohydrates
Created by Brett Barbaro
We'll talk about two stages of the citric acid cycle. In the first stage, two carbon atoms are oxidized and high energy electrons are released and harvested. And in the second stage, oxaloacetate is regenerated, and along with that more high-energy electrons are harvested. We'll talk a little bit about the regulation of the citric acid cycle. But we're going to skip the part with the glyoxylate cycle because that only applies to plants and bacteria and we're going to focus on animals.
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https://en.wikipedia.org/wiki/Citrate
Citrate
citrate = citric acid = tricarboxylic acid
Citric Acid Cycle
=
Tricarboxylic Acid Cycle
=
TCA Cycle
=
Krebs Cycle
Citrate = Citric Acid = Tricarboxylic Acid
DID YOU KNOW?
The manuscript proposing the citric acid cycle was submitted for publication to Nature but was rejected. Dr. Hans Krebs proudly displayed the rejection letter of June 1937 throughout his career as encouragement for young scientists. His work was subsequently published in Enzymologia.
Created by Brett Barbaro
So the central molecule of the citric acid cycle is shown here in diagram form. It's called tricarboxylic acid because it's essentially three carbons with three carboxylic acid groups attached to them and one alcohol group. You may have heard the Citric Acid Cycle, the Tricarboxylic Acid Cycle, the TCA Cycle, and the Krebs Cycle. All of these are equal. They’re all the same thing. It's all this cycle that we're studying right now. It's just different names for it.
Krebs, himself, actually was the guy who first proposed the citric acid cycle (or the Krebs cycle). And it was initially rejected by Nature {the top scientific journal}, which he found kind of humorous. So he put his rejection letter on the wall of his office, so that students in the future would not feel discouraged by being rejected every now and then.
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Diagram of the Citric Acid Cycle
Created by Brett Barbaro
Figure 19.6 The citric acid cycle. Notice that because succinate is a symmetric molecule, the identity of the carbon atoms from the acetyl unit is lost.
Well, here's just an overall picture of the cycle. Starting at the top with citrate - you create the citrate by removing the acetyl group from acetyl CoA and attaching it on to oxaloacetate. Then there is an isomerization, changing it to isocitrate. Then there is the release of a carbon dioxide and harvesting of NADH. And that creates α-ketoglutarate. That happens again, and also makes succinyl CoA - it’s the same coenzyme A, but this time it's stuck to a succinate molecule. That bond is broken and ATP is generated. And then it's processed further, producing FADH2, and on further to make malate, another NADH, and finally regenerating oxaloacetate which is ready to accept another acetyl group for the cycle.
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Citric Acid Cycle Oxidizes the Acetyl Fragment of Acetyl CoA
The citric acid cycle oxidizes the acetyl fragment of acetyl CoA to CO2.
In the process of oxidation, high-energy electrons are captured in the form of NADH and FADH2.
These high-energy electrons power the majority of ATP synthesis in aerobic organisms.
The citric acid cycle also generates building blocks for amino acids and other important biomolecules.
Created by Brett Barbaro
So overall what the citric acid cycle does is oxidize the acetyl fragments that are attached to acetyl CoA - oxidizes them to carbon dioxide and releases that. And in that process it releases high energy electrons which are captured in the form of NADH and FADH2. These high energy electrons then make their way to the electron transport system - and there, in aerobic organisms at least, they are used to create a great deal of ATP. The citric acid cycle is also something that is used to generate building blocks for amino acids and other important biomolecules, so we'll discuss that to some extent.
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ftp://resources.rcsb.org/motm/tiff/154-CitricAcidCycle_citricacidcycle.tif
Citrate Synthase
Aconitase
Isocitrate
dehydrogenase
Alpha-Ketoglutarate
dehydrogenase
complex
Succinyl CoA Synthetase
Succinate
dehydrogenase
Fumarase
Malate
Dehydrogenase
The Citric Acid Cycle Illustration
Created by Brett Barbaro
Here is a diagram/illustration showing the enzymes that are involved in the citric acid cycle. And you can see once again - they’re highly symmetrical. A couple of things stand out. α-ketoglutarate dehydrogenase looks a lot like our good friend pyruvate dehydrogenase. And in fact, yes, they are very similar in their structure and their function. Another thing is succinate dehydrogenase, which you can see has a gray bar across it. That gray bar represents the … membrane of the mitochondria, and it's because this particular enzyme is embedded in the {mitochondrial} membrane.
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Diagram of Cellular Respiration
Created by Brett Barbaro
Figure 19.2 Cellular respiration. The citric acid cycle constitutes the first stage in cellular respiration, the removal of high-energy electrons from carbon fuels (left). These electrons reduce O2 to generate a proton gradient (red pathway), which is used to synthesize ATP (green pathway). The reduction of O2 and the synthesis of ATP constitute oxidative phosphorylation.
So the overall process starts on the left with fatty acids, glucose, and amino acids all getting broken down in various ways to create acetyl CoA. That acetyl CoA then introduces the acetate molecule into the citric acid cycle, creating carbon dioxide, ATP, and electrons. Those electrons are then passed down the electron transport chain where they are able to oxidize some hydrogens and drive the formation of ATP. We'll talk about how that happens in the next chapter.
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Section 19.2 Stage One Oxidizes Two Carbon Atoms to Gather Energy-Rich Electrons
Learning objective 3: Identify the primary catabolic purpose of the citric acid cycle.
Learning objective 4: Explain the advantage of the oxidation of acetyl CoA in the citric acid cycle.
The first stage generates two molecules of CO2 by oxidative decarboxylation.
Created by Brett Barbaro
All right, so focusing on the first stage of the citric acid cycle, we will show how it produces two carbon dioxides by oxidative decarboxylation.
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Citrate Synthase Forms Citrate from Oxaloacetate and Acetyl Coenzyme A
Citrate synthase catalyzes the condensation of acetyl CoA and oxaloacetate to form citrate.
DID YOU KNOW?
A synthase is an enzyme that catalyzes a synthetic reaction in which two units are joined usually without the direct participation of ATP (or another nucleoside triphosphate).
Created by Brett Barbaro
So citrate synthase is the first step in this process, and it combines oxaloacetate with acetyl CoA to create citryl CoA, which is immediately hydrolyzed into citrate - which is our molecule that we have named this whole process after, the citric acid cycle. Note that's also the citric acid that you find in lemons and oranges and other citrus fruits. The word citrus is related to this molecule, which tastes a little sour and has a great deal of energy in it.
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IN DETAIL:
http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/Articleimage/2008/NP/b600517a/b600517a-s1.gif
IN DETAIL:
Details of Oxaloacetate and Acetyl CoA Mechanism and Citrate Formation
Created by Brett Barbaro
Now, I don't know about you, but when I saw this reaction, I thought, “Wow, how does that happen?” It seems kind of odd that the methyl group of acetyl CoA would actually be attacked. But we show here on the bottom, and if you take a close look at it, you can see that the methyl group is first destabilized by an aspartate acidic residue in the active site of citrate synthase. The electrons from that carbon-hydrogen bond go back up and create a double bond, which is stabilized by water and a histidine residue, and that double bond’s electrons then attack the oxaloacetate, kicking up a couple of electrons to make a bond with a hydrogen from the positively charged arginine residue. So this is just a good example of how these highly unlikely reactions are created by making a favorable environment in the active site of the enzyme.
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The Mechanism of Citrate Synthase Prevents Undesirable Reactions
DID YOU KNOW?
Citric acid is stored in vacuoles in citrus fruits where it can sometimes reach a concentration of 0.3 M. It has a tart taste and provides some of the flavor of citrus drinks.
Citrate synthase exhibits induced fit.
Oxaloacetate binding by citrate synthase induces structural changes that lead to the formation of the acetyl CoA binding site.
The formation of the reaction intermediate citryl CoA causes a structural change that completes active site formation.
Citryl CoA is cleaved to form citrate and coenzyme A.
Created by Brett Barbaro
Citrate synthase is an example of induced fit. So at first it really doesn't have the active site that it needs, but when oxaloacetate binds, then it causes structural changes that then form the acetyl CoA binding site. The reaction intermediate, citryl CoA, then causes a structural change that completes the active site. Citryl CoA is cleaved to form citrate and coenzyme A, and then the coenzyme A can go along and pick up some more acetyl groups.
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Structures of The Conformational Changes in Citrate Synthase on Binding Oxaloacetate
Created by Brett Barbaro
Figure 19.3 Conformational changes in citrate synthase on binding oxaloacetate. The small domain of each subunit of the homodimer is shown in yellow; the large domains are shown in blue. (Left) Open form of enzyme alone. (Right) Closed form of the liganded enzyme. [Drawn from 5CSC.pdb and 4CTS.pdb.]
And here's a diagram showing the rather dramatic change in conformation that occurs when oxaloacetate binds to citrate synthase.
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Citrate Is Isomerized into Isocitrate
Aconitase catalyzes the formation of isocitrate from citrate.
Created by Brett Barbaro
The next step is the isomerization of citrate into isocitrate - a very simple idea, just taking the hydroxyl group from the central carbon and shifting it one carbon up. This is actually accomplished by removing the hydroxyl group and hydrogen, making water, and formation of a double bond, creating cis-aconitate. And then this intermediate, the double bond, is hydrolyzed by another water molecule (potentially the same water molecule), and forming the isocitrate product.
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Created by Brett Barbaro
Aconitase performs a classic stereospecific reaction that is often used as an example in biochemistry textbooks. It extracts a hydroxyl group and a specific hydrogen atom from citrate, and replaces them in a geometrically precise way to form isocitrate. This process is revealed in two crystal structures, but you need to use a little imagination when you look at them, since the crystal structures do not contain the hydrogen atom positions. PDB entry 1c96 , shown on the left, has citrate bound in the active site. In the normal form of the enzyme, the oxygen atom shown in pink will be extracted by the iron sulfur cluster and a hydrogen atom will be extracted by a serine at the top (both of these reactions are shown with green arrows). This structure, however, has mutated the serine to alanine, so the oxygen atom in the serine is missing. In the second step of the reaction, shown on the right from PDB entry 7acn , the molecule flips upside down (notice the different location of the labels A-B-C) and the hydrogen and hydroxyl are added back in different places to form isocitrate.
Now, here's a closer look at that mechanism which is kind of interesting, so that's why I wanted to show it to you. You see on the left, the citrate is interacting with a sulfur-iron cluster in the aconitase enzyme. When the hydroxyl group has been removed, then the remaining molecule changes its orientation in the active site. As you can see on the left (the A, B, and C), the hydroxyl group is attached to the B carbon, and on the right it is now attached to the A carbon. The hydrogen that has been removed, that was originally attached to the A and then becomes attached to the B, is shown in outline form. And remember there are other hydrogens present in this molecule that are not being shown at all.
This is a kind of an interesting point. You see on the left, there is an oxygen shown in outline form, but on the right, it is shown in solid form. That oxygen is always there in the actual molecule, in the actual enzyme, but when they crystallized this enzyme to get the structure, they removed that hydroxyl group, basically substituting an alanine for a serine, and that was probably to prevent the reaction from occurring, so that they could catch it in this first step. That's a common technique used in protein crystallography. Small changes are made to proteins to get them into the state that they want to have them in.
Take special note of the iron-sulfur cluster in this diagram. We'll be running into more of those later.
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Isocitrate Is Oxidized and Decarboxylated to Alpha-Ketoglutarate
Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate, forming α-ketoglutarate and capturing high-energy electrons as NADH.
Created by Brett Barbaro
Isocitrate dehydrogenase then removes the hydrogens from isocitrate as the name would imply, and along with those hydrogens it takes electrons. These electrons are attached to a NAD+ molecule to form NADH, and a hydrogen proton is released. This is an oxidation of that carbon, and there’s energy released in that in the form of those electrons that are attached to the NAD+. This is a pretty unstable arrangement, and so the hydrogen that was released (or another nearby hydrogen) enters the system and releases carbon dioxide, which then diffuses away and is therefore an irreversible part of this reaction. The product of all of this is α-ketoglutarate.
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Succinyl Coenzyme A Is Formed by the Oxidative Decarboxylation of Alpha-Ketoglutarate
α-Ketoglutarate dehydrogenase complex catalyzes the synthesis of succinyl CoA from α-ketoglutarate, generating NADH.
The enzyme and the reactions are structurally and mechanistically similar to the pyruvate dehydrogenase complex.
Created by Brett Barbaro
The next step is a further oxidative decarboxylation and the corresponding harnessing of a couple of electrons. And that is catalyzed by the α-ketoglutarate dehydrogenase complex. And if you remember the pyruvate dehydrogenase complex, it did basically the same thing. It removed two electrons from pyruvate, released a carbon dioxide, and attached the coenzyme A to what was left. And that's exactly what happens here.
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Pyruvate
Acetyl CoA
Pyruvate dehydrogenase:
α-Ketoglutarate dehydrogenase:
H
H
Comparison of Reactions of Alpha-Ketoglutarate vs. Pyruvate
Created by Brett Barbaro
If you look at the structure of α-ketoglutarate vs. pyruvate, it becomes pretty obvious why the mechanisms are very similar. It’s because the structure is almost exactly the same, except α-ketoglutarate has got an extra couple carbons on it. Nature figured out a good mechanism for transferring this type of structure over to coenzyme A, and so it makes multiple uses of it.
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From: resources.rcsb.org/motm/tiff/153-PyruvateDehydrogenaseComplex_pyruvatedehydrogenase.tif
α-Ketoglutarate dehydrogenase complex
pyruvate dehydrogenase complex
Illustrated comparison of pyruvate dehydrogenase complex to the alpha-ketoglutarate dehydrogenase complex.
Created by Brett Barbaro
Here's a little side by side comparison of the pyruvate dehydrogenase complex to the α-ketoglutarate dehydrogenase complex, and I think you can readily see the similarities between the two.
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Pyruvate dehydrogenase complex (as a reminder)
Created by Brett Barbaro
The mechanism is also very similar, so just to review what the pyruvate dehydrogenase complex mechanism was - it began with the addition of pyruvate and the release of a carbon dioxide. And then the remaining acetyl group was passed along and attached to coenzyme A in step 4. And then the lipoamide group was recharged in step 5. This is very similar to the process that occurs with the α-ketoglutarate dehydrogenase complex.
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Section 19.3 Stage Two Regenerates Oxaloacetate and Harvests Energy-Rich Electrons
A compound with high phosphoryl-transfer potential is generated from succinyl coenzyme A.
Succinyl CoA synthetase catalyzes the cleavage of a thioester linkage and concomitantly forms ATP.
Created by Brett Barbaro
Succinyl CoA synthetase removes the thioester bond and makes ATP. It gets the high energy from the sulfur-carbon bond and harnesses it as ATP. Note that this is another one of those examples where the enzyme was named for the reverse reaction. It is not, in this case, making succinyl CoA, but breaking it down - but as you can probably guess, it is able to catalyze the reversed reaction as well.
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Cleavage of the thioester of succinyl CoA powers the formation of ATP.
The formation of ATP by succinyl coenzyme A synthetase is an example of a substrate-level phosphorylation because succinyl phosphate, a high phosphoryl-transfer potential compound, donates a phosphate to ADP.
Succinyl Coenzyme A Synthetase Transforms Types of Biochemical Energy
Created by Brett Barbaro
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Figure 19.5 The reaction mechanism of succinyl CoA synthetase. The reaction proceeds through a phosphorylated enzyme intermediate. (1) Orthophosphate displaces coenzyme A, which generates another energy-rich compound, succinyl phosphate. (2) A histidine residue removes the phosphoryl group with the concomitant generation of succinate and phosphohistidine. (3) The phosphohistidine residue then swings over to a bound nucleoside diphosphate. (4) The phosphoryl group is transferred to form the nucleoside triphosphate.
The mechanism of this reaction is a little interesting. You've got your succinyl CoA and a phosphate that are bound in the active site, stabilized by the histidine group. And the phosphate actually is what knocks off the coenzyme A from the molecule, creating the succinyl phosphate intermediate. That's an unstable intermediate which is then attacked by the neighboring histidine, releasing succinate, and that attachment of the histidine to the phosphate is also unstable. The high energy involved there is enough to synthesize an ATP molecule. So an ADP drifts into that active site and then picks up that phosphate and drifts away. You can only imagine the complicated structure of this active site, which is able to accommodate succinyl CoA, the phosphate, and also an ADP molecule. It's not very well demonstrated in these diagrams but it's quite amazing.
Oxaloacetate Is Regenerated by the Oxidation of Succinate
DID YOU KNOW?
Apples are a rich source of malic acid, which used to be called “acid of apples.” In fact, the word malic is derived from the Latin malum, meaning “apple.”
Succinate dehydrogenase, fumarase, and malate dehydrogenase catalyze successive reactions to regenerate oxaloacetate.
FADH2 and NADH are generated.
Oxaloacetate can condense with another acetyl CoA to initiate another cycle.
Created by Brett Barbaro
And then finally you regenerate oxaloacetate by the oxidation of succinate. This is an oxidation, so there is some energy is released - that energy is caught by FADH2 and NADH. At this point the cycle is complete, and the oxaloacetate can then combine with another acetyl CoA and start the cycle all over again.
One of the intermediates in this process is malate, which is the acid that you find in apples. So there is actually a very close relationship between that acid in apples and the acid in oranges and lemons which is citric acid.
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The Citric Acid Cycle Produces High-Transfer-Potential Electrons, an ATP, and Carbon Dioxide
The net reaction of the citric acid cycle is
The electrons from NADH will generate 2.5 ATP when used to reduce oxygen in the electron-transport chain.
The electrons from FADH2 will power the synthesis of 1.5 ATP with the reduction of oxygen in the electron-transport chain.
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wrong in the book!
Created by Brett Barbaro
So the overall reaction is, you start off with your acetyl CoA, and you break it down and create three NADH, one FADH2, one ATP, and a couple of other byproducts. And these electrons from the NADH and FADH2 will generate ATP in the electron transport chain. There will be 2.5 ATP that are generated from each NADH, and 1.5 ATP generated from each FADH2.
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Diagram of the Citric Acid Cycle
Created by Brett Barbaro
Figure 19.6 The citric acid cycle. Notice that because succinate is a symmetric molecule, the identity of the carbon atoms from the acetyl unit is lost.
A kind of interesting point is that you have two carbons being added to oxaloacetate to generate citrate up at the top. Those two carbons are marked in green. And those two carbons continue along, along the backbone of this molecule and are not the ones that are turned into carbon dioxide, it’s the two other carbons that are released as carbon dioxide. So after that, at the state of succinyl CoA, you have two carbons that were there before and then the two new carbons together. And once it reaches the state of succinate, though, this molecule is symmetrical. So it's not sure what happens to {those two carbons} after that. It could go either way. So the next two carbons to be removed may be the new ones that were introduced or they may be ones that were already there.
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Section 19.4 The Citric Acid Cycle Is Regulated
Learning objective 5: Describe how the citric acid cycle is regulated.
Learning objective 6: Describe the role of the citric acid cycle in anabolism.
The key control points in the citric acid cycle are the reactions catalyzed by isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
Recall that pyruvate dehydrogenase controls entry of glucose-derived acetyl CoA into the cycle.
Created by Brett Barbaro
Now we're going to talk a little bit about how the citric acid cycle is regulated. There's two key control points, and those are the two places where carbon dioxide is released by the system. The two irreversible steps. And remember that it's usually the irreversible steps that are the best control points. These reactions are catalyzed by isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
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The citric acid cycle is controlled at several points
Remember from Pyruvate Dehydrogenase:
Same for α-Ketoglutarate dehydrogenase!
Created by Brett Barbaro
Figure 19.7 Control of the citric acid cycle. The citric acid cycle is regulated primarily by the concentrations of ATP and NADH. The key control points are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.
As you can see in this diagram, those two enzymes catalyze the change of isocitrate to α-ketoglutarate, and α-ketoglutarate to succinyl CoA. So those enzymes are actually regulated by ATP, NADH, ADP, and succinyl CoA levels. If you have a lot of ATP and NADH, then it will start to interfere with the processing of isocitrate to α-ketoglutarate, whereas if you have a great deal of ADP then it will promote that reaction. Similarly, ATP, succinyl CoA, and NADH all inhibit the enzyme that changes α-ketoglutarate to succinyl CoA. This is just a way of making sure that if you have enough ATP already, if you have enough energy, then you don't need to make any more.
Now notice the ways of controlling the α-ketoglutarate dehydrogenase complex are pretty much identical to the ways that you control the pyruvate dehydrogenase complex, with ATP, the CoA factor, and NADH. I imagine that the mechanism is also the same, because these complexes are so similar - probably it's a phosphorylation of α-ketoglutarate dehydrogenase that turns it off and then a phosphatase can turn it back on, like you see in the diagram on the right that was taken from chapter 18. So luckily you can reuse all of that information that you learned in chapter 18 to understand this reaction.
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Biosynthetic Roles of the Citric Acid Cycle
Many of the components of the citric acid cycle are precursors for biosynthesis of key biomolecules.
Created by Brett Barbaro
Figure 19.8 Biosynthetic roles of the citric acid cycle. Intermediates are drawn off for biosyntheses (shown by red arrows) when the energy needs of the cell are met. Intermediates are replenished by the formation of oxaloacetate from pyruvate (green arrow).
So what do you do if you have enough energy and you still have all of these molecules that are part of the citric acid cycle floating around? Well it turns out you can still use them for many different purposes. Citrate can be turned into fatty acids for long term storage and also sterols which are important for lipids and steroids and stuff. If you've got a lot of α-ketoglutarate sticking around, then that can be turned into glutamate which is, of course, an amino acid and also a very important neurotransmitter, a very important signaling molecule, and also the umami flavor that you get from monosodium glutamate. That’s the same glutamate. Glutamate is an amino acid that can be also be turned into many other amino acids, and in fact purines as well. Purines are the elements of RNA/DNA, part of the rings, the bases of DNA and RNA. Those ones that have two rings, those are the purines. Succinyl CoA can be turned into porphyrins, heme, and chlorophyll, all of which are very important molecules for transferring energy, like electrons and oxygen, and I think they all have iron in them as one of the key elements; it may be some other ion {Mg in chlorophyll}. But you know the heme is important, because you need heme to carry the oxygen in your blood; chlorophyll, obviously one of the most fundamental molecules around. And then you've got oxaloacetate up at the top, can be turned into aspartate, which you know is also an amino acid, and that can also be further processed to create other amino acids, and also purines and pyrimidines which go into your RNA and DNA. ATP is a purine. So purines are extremely important all around.
So, basically, the citric acid cycle is one of the central elements of our metabolism. Not only does it harvest a great deal of energy, but it also creates a great deal of molecules that can be used in other ways as well. This is one of the reasons that the taking in of the mitochondria turned out to be so advantageous for the cell, because it was able to produce all of these various features without having to invest a whole lot more of its own energy into it.
You can see up at the top, also, oxaloacetate can also be turned into glucose. That would be in the process of gluconeogenesis, and very important for the brain, because the brain needs glucose to run. It doesn't run on any other of these molecules.
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The Citric Acid Cycle Must Be Capable of Being Rapidly Replenished
Because the citric acid cycle provides precursors for biosynthesis, reactions to replenish the cycle components are required if the energy status of the cells changes.
One such reaction is catalyzed by pyruvate carboxylase, which synthesizes oxaloacetate by the carboxylation of pyruvate. Recall that this reaction is also used in gluconeogenesis and is dependent on the presence of acetyl CoA.
Created by Brett Barbaro
So there's a lot of ways that elements of the citric acid cycle can be diverted into other pathways. So it becomes necessary sometimes to replenish these elementary elements of the citric acid cycle. For example, you need to start the citric acid cycle with some oxaloacetate. So if you've been using your oxaloacetate to make other things, then you need to make some more oxaloacetate. And that can be done by taking pyruvate and adding a carbon dioxide group to it, with the breakdown of an ATP molecule, and this is a reaction that is catalyzed by pyruvate carboxylase. So they can get your oxaloacetate back straight from some pyruvate.
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Diagram of How Pyruvate Carboxylase Replenishes the Citric Acid Cycle
Created by Brett Barbaro
Figure 19.9 Pyruvate carboxylase replenishes the citric acid cycle. The rate of the citric acid cycle increases during exercise, requiring the replenishment of oxaloacetate and acetyl CoA. Oxaloacetate is replenished by its formation from pyruvate. Acetyl CoA can be produced from the metabolism of both pyruvate and fatty acids.
And this is just a simple diagram showing that pathway. So interestingly, yes, pyruvate can turn into oxaloacetate, which can then be combined with more pyruvate in the form of acetyl CoA to make citrate and start the whole process off. So you can see, with some creative help from a couple of enzymes, pyruvate actually becomes the source of the entire citric acid cycle. And of course pyruvate comes from glucose so the citric acid cycle is, you know, directly descended from glucose, you might say. But remember, glycolysis is an anaerobic process - so if a cell doesn't have mitochondria, then that's all it can do is take it down to pyruvate, and then turn it into alcohol or lactic acid. They need the mitochondria to get all of these other benefits out of glycolysis.
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At this point, the Citric Acid Cycle has transferred the energy stored in pyruvate into ATP and high-energy electrons being carried by NADH and FADH2.
This energy now becomes a “battery” using the electron transport chain.
CONCLUSION
Created by Brett Barbaro
So that's your basic glycolysis, the breakdown of the molecules that is needed to get the energy out of them. The energy that was originally stored in them by the Sun. And now that energy is in the form of some ATP, and also this NADH and FADH2. And that's what we're going to get into next, as these high energy electrons that are in the NADH and FADH2 now kind of charge up the cell, to create a sort of battery. And that's using the electron transport chain, and that's what we're talking about next.
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