The Citric Acid Cycle: Study with Video Lessons, Practice Problems & Examples

The citric acid cycle, a crucial metabolic pathway, consists of three phases: citrate formation, succinyl CoA formation, and oxaloacetate regeneration. Initially, Acetyl CoA combines with oxaloacetate to form citrate. This is followed by isomerization and oxidation to produce succinyl CoA, generating NADH and CO₂. Finally, succinyl CoA is converted back to oxaloacetate, yielding NADH, FADH₂, and ATP. The NADH and FADH₂ produced are essential for the electron transport chain, facilitating further ATP production.

The Citric Acid Cycle Concept 1

Now, the Citric Acid Cycle is a sequence of 8 biochemical reactions. For phase a, we have citrate formation. Here we're going to say, phase a consists of the first reaction of the pathway. Here, we have Oxaloacetate, which was created in phase c. It then reacts or interacts with Acetyl CoA that comes into the Krebs Cycle or Citric Acid Cycle in order to form our Citrate molecule.

So again, we're creating this as a product in the form of Oxaloacetate. It then becomes interactive with Acetyl CoA for the first reaction in order to form our citrate molecule. Alright. So let's click on the next video and see what exactly does this entail in the formation of citrate.

The Citric Acid Cycle Concept 2

So here in our discussion of the mechanism of the citric acid cycle, we take a look at reaction 1, which is citrate formation. Here, an acetyl group from acetyl CoA combines with carbon 2 of oxaloacetate to give our second metabolite, citrate. What happens first is we have a basic residue, which will come and deprotonate this alpha hydrogen. The bond breaks, carbon holds onto the electrons, moves it here to make a double bond, and kicks the pi bond up to the oxygen, giving us an enolate. So, here we have enolization at the very beginning.

From here, this Acetyl CoA Enolate would react with Acetyl CoA. So this oxygen brings down its lone pair to make a pi bond, causing this pi bond to break and forming a connection with this carbon here. This carbon can't make 5 bonds, so its pi bond would break. Doing this, we would use this pi bond to grab an H+, so that we create an OH group. So here we have basically aldol addition.

And what we have here initially is our intermediate. Now, through a thioester hydrolysis, we're able to cleave this S CoA, giving us a carboxylate group here at the end, forming our citrate or our second metabolite. Alright. So this will represent the mechanism of reaction 1 of the citric acid cycle.

The Citric Acid Cycle Example 1

How many carbon atoms are added to oxaloacetate to produce citrate? Remember, we are undergoing hydrolysis of our Acetyl CoA, and it is this portion, this acetyl group that gets added to oxaloacetate in order to form citrate. This acetyl CoA, which I've just put within this black box has how many carbons? It has 1, 2 carbons. So, 2 carbon atoms are added to oxaloacetate in order to produce citrate.

That means that option b would be our correct answer.

The Citric Acid Cycle Concept 3

In this video, we're going to take an overview of Phase B of the Citric Acid Cycle, which deals with succinyl CoA formation. Here we're going to say Phase B, which is highlighted in this yellow portion. It consists of pathways reactions 2, 3, and 4. In reaction 2, we have Citrate becoming Isocitrate. In step 3, or reaction 3, we're going from Isocitrate to Alpha-Ketoglutarate.

And to do that, we also have the release of NADH, our electronic carrier, as well as carbon dioxide. And then for reaction 4, we're going from alpha-ketoglutarate to succinyl CoA. This also has the releasing of another mole of NADH and carbon dioxide. So we can see that within Phase B, we have 2 moles of NADH and CO₂ being produced. 1 of each happening in our reaction 3, and again in reaction 4.

Now that we've looked at the overview of phase B, let's start going one by one for each one of these reactions and delve a little bit deeper into what causes the transformation of the starting material to the next step, the next reaction.

The Citric Acid Cycle Concept 4

In reaction 2 of the citric acid cycle, we say that the tertiary alcohol in citrate isomerizes to a secondary alcohol for oxidation in the following step. Remember, tertiary alcohols cannot be oxidized, but secondary alcohols can. What we'll have here is a basic residue comes and deprotonates this hydrogen here, while this carbon here takes its hydrogen. The bond breaks here to make a double bond. We'd say now that this bond here would break and that OH^- that's released would combine with this H⁺ here.

And what we're having occur here is basically an E2 dehydration. We have a pi bond here now; this will represent our intermediate. And what we have here is an anti-Markovnikov hydration. So remember in anti-Markovnikov hydration, OH would go here, and then H would come here. This is how we'd go from citrate to isocitrate in reaction 2 of the citric acid cycle.

The Citric Acid Cycle Concept 5

Reaction 3, we have alcohol oxidation and decarboxylation. So here this is our first decarboxylation step that occurs within the citric acid cycle. And what it really is is just a C2 oxidation through our coenzyme of NAD⁺. So in this process, our 1 NAD⁺ is reduced to 1 NADH, and one carbon atom is lost as CO₂. Remember, decarboxylation means we're going to lose carbon dioxide.

So here, this is the secondary alcohol that we have in the red box that's going to be oxidized, and this is isocitrate. This alcohol is oxidized with our coenzyme NAD⁺. In the process of oxidizing isocitrate, NAD⁺ is reduced to NADH. We also have this H⁺. Now what we've made here is our oxidized intermediate.

Notice that we have a magnesium ion there, which has just helped to stabilize the overabundance of negative charges. What's going to happen now is this oxygen that's negatively charged will take one of its lone pairs here to make a double bond. So that carbon is fulfilling its tetravalent nature where it's making 4 bonds by breaking this bond here. Because if we didn't break that bond, we'd be making 5 bonds, which is too many. That double bond is formed there, which causes this Pi bond to come here.

In the process, we'd have the magnesium leaving as well. And this is how we lose CO₂. So right now what we've just created is an enolate, and through tautomerization, we can make our alpha-ketoglutarate. Now here to do that, this oxygen that's negatively charged would form a Pi bond yet again, causing this Pi bond to break and grab an H⁺. And that's how this Carbon now has 2 Hydrogens and that's how this Carbon here gains a Carbonyl group.

Alright? So this will represent the mechanism for reaction 3 of the Citric Acid Cycle.

The Citric Acid Cycle Concept 6

For reaction 4, the mechanism itself is composed of various steps. What's most important here is to know that it represents an oxidation and a decarboxylation reaction. This would represent the second decarboxylation step within the citric acid cycle. What it is, is a C1 oxidation plus decarboxylation. In this process, our 1 NAD⁺ coenzyme is reduced to NADH, and one carbon atom is lost as CO₂ because again, it is a decarboxylation, so carbon dioxide is lost.

Here we have alpha-ketobutyrate. This here is lost as CO₂. We have coenzyme A. We have NAD⁺ which will be reduced to NADH. In the process, we've gone from alpha-ketoglutarate to succinyl CoA.

This represents an oxidation where we're adding, attaching the more electronegative sulfur to this Carbon 1. Right? This is why it represents a Carbon 1 oxidation. We're adding a more electronegative element directly to Carbon 1. It is also a decarboxylation since we lose CO₂.

The Citric Acid Cycle Example 2

For each of the following reactions described below, identify a corresponding step of the Citric Acid Cycle. So the first one says the oxidation of Alpha-Ketoglutarate produces succinyl CoA. Now, from what we've talked about earlier, we know that this is reaction 4 that happens in phase b of the citric acid cycle. Next, we have oxaloacetate is converted to citrate. Remember, this happens in reaction 1, phase a of the citric acid cycle because we're going to say oxaloacetate interacts with Acetyl CoA in order to make this citrate.

Then we're going to say an oxidation reaction is catalyzed by isocitrate dehydrogenase. So, isocitrate dehydrogenase is the enzyme used on isocitrate to change it into Alpha-Ketoglutarate. This happens in reaction 3 of phase b of the Citric Acid Cycle. And then finally, we're using this enzyme that catalyzes the isomerization of citrate to isocitrate. We're going to say this changes a tertiary alcohol in citrate to a secondary alcohol in isocitrate.

Basically prepping that secondary alcohol to be oxidized later on. So here, this would happen in reaction 2 of phase b of the citric acid cycle. So these are the reactions or steps in which they would occur for the citric acid cycle.

Which step oxidizes α-ketoglutarate of the Citric Acid Cycle?

• A. 3
• B. 1
• C. 4
• D. 5

Which two steps of the citric acid cycle produce CO₂?

• A. 1 and 3
• B. 3 and 4
• C. 2 and 4
• D. 2 and 3

The Citric Acid Cycle Concept 7

In this video, we take a look at phase C of the citric acid cycle. At the end of phase C, we have oxaloacetate regeneration. We're going to say phase C consists of the pathways of reactions 5 to 8. So remember, at the end of Phase B, we create succinyl CoA. It then goes into reaction 5.

By losing its S CoA portion, we create succinate for reaction 5. From there, we go to reaction 6 where FAD is used to oxidize succinate to change it into fumarate, our flat molecule. From there, we do hydration which is step reaction 7. Here, we hydrate fumarate to malate, giving us an alcohol. And then that is oxidized by NAD⁺ coenzyme to give us oxaloacetate.

If this is the first run through of the citric acid cycle, we can go through it yet again. Because remember, we can do two runs of the citric acid cycle. Alright, so, remember, Phase C represents reactions 5 to 8. At the end of reaction 8, we have the regeneration of oxaloacetate.

The Citric Acid Cycle Concept 8

Now in reaction 5, we have a carbon 1 nucleophilic acyl substitution reaction, so the reaction of succinyl CoA followed by the transfer of a phosphate group to GDP. We are going to say here that we have a basic residue which would deprotonate, remove this hydrogen here, this bond would break, go to the oxygen. At the same time, this oxygen that's negatively charged would come in and attack this carbon, this carbonyl carbon, causing the pi bond to break and move up to the oxygen. But here, treat this as a carboxylic acid derivative, which again does not do addition, but instead substitution. That means that the pi bond would reform, so we would show that this lone pair comes back down to reform our pi bond and jettison this S CoA group.

This is how we get this structure right now where we have a phosphate group connected to that carbonyl carbon. From here, we would say that this phosphate group could come in and attack this phosphorus causing this pi bond to move up to oxygen. But again, we are dealing with a nucleophilic acyl substitution here, so this oxygen decides to reform its double bond, and then it is going to cause this bond to break to go to the oxygen. And that is how we get our succinate. We've reformed our dicarboxyl group, we've created succinate, and we've added an additional phosphate group to this structure giving us ATP.

So, we can see that in this step, we've created some energy in the ATP molecule which is a high-energy molecule.

The Citric Acid Cycle Concept 9

In reaction 6, we have a dehydrogenation, which means we're going to lose hydrogen atoms. Here, this is a C2-C3 dehydrogenation with FAD, which converts succinate to fumarate. Now, here we'd say 1, 2, 3, 4, or the other way since it's symmetrical. We say here that 1 FAD is reduced to 1 FADH₂. The way it works is we'd have a basic residue here, which would deprotonate, so we remove this H, causing a pi bond to form here, and then causing a hydride ion to be jettisoned and attached to FAD.

In the process, we create FADH₂, so FAD is oxidizing succinate. As a result of this, we create a pi bond right here; we've created fumarate. We're going to say FAD converts single bonds to double bonds. So this represents the mechanism of reaction 6 of the citric acid cycle.

The Citric Acid Cycle Example 3

Here it says, "Which of one of the following statements is incorrect about the citric acid cycle?" Here, reaction 5 of the cycle converts succinyl CoA to succinate. Well, that's true. We do know that through hydrolysis we're able to convert succinyl CoA to succinate. So this is true.

Oxidation of succinate in reaction 6 produces fumarate. Yes. Through the use of our coenzyme FAD, we're able to oxidize succinate to fumarate. Phase C of the citric acid cycle does not contain any oxidation reactions. So this statement is incorrect.

We know that we're using NAD+ and FAD at different reactions in phase C of the citric acid cycle. Those will oxidize our substrate, so oxidation reactions do occur in phase C. Next, hydrolysis of succinyl CoA to succinate creates a high energy molecule. Yes. We create ATP in this process which is a high energy molecule, so this is true.

The only statement here that is incorrect is option C.

The Citric Acid Cycle Concept 10

In reaction 7, we have hydration. Now, this is important. This is a C₂C₃ conjugate addition of water that converts fumarate to malate. So remember, we learned about conjugate addition reactions earlier within this course, now we're applying that concept to the citric acid cycle. So here we're going to have a basic residue, and it's going to deprotonate water here.

So it comes and removes a hydrogen here, and then here, this bond breaks, we use it to attack this carbon here, which causes this pi bond to resonate here, and this pi bond to resonate to the oxygen. That's going to give us a negatively charged oxygen here now, and we've attached this OH. This oxygen can resonate back down, and we've also had a pi bond here. This oxygen resonates its lone pair back down to make a pi bond, and that's going to cause this pi bond, one of these pi bonds here to break, and grab an H⁺, and that's how, at the end, we're going to have an H here, and still our OH over here, through protonation to create malate at the end. Alright.

So this represents the mechanism for reaction 7 of the Citric Acid Cycle.

The Citric Acid Cycle Concept 11

Reaction 8 represents an oxidation reaction. Here it is a C₂ oxidation with the coenzyme NAD⁺ and that helps to change malate to oxaloacetate. Now, the 1 NAD⁺ coenzyme is reduced to NADH. And the way this works is we have a basic residue which is going to deprotonate, so we take this H here, which causes this bond to break to form a pi bond here. This H is kicked out or jettisoned as a hydride ion which attaches to NAD⁺ to give us NADH.

So in this oxidation, that's how we go from Malate to Oxaloacetate at the end. Right? So this is our mechanism for reaction 8 of the citric acid cycle.

The Citric Acid Cycle Example 4

In this example, it says, for each of the following reactions described below, identify a corresponding step of the citric acid cycle. Alright. So first, we have the oxidation of malate to oxaloacetate. Now remember, oxaloacetate is the first metabolite that we're going to deal with in the citric acid cycle, and it's regenerated at the very last reaction of the citric acid cycle.

So, this would be reaction 8. Next, succinate loses 2 hydrogen atoms to yield fumarate. So, here, this happens in reaction 6. Now, as we're going through this, realize that there are a lot of steps within the citric acid cycle. In this example, we're going off of what we've learned thus far to determine the reaction, but later on, we'll employ memory tools to help us remember the order in which some of this is made.

So just keep that in mind. Next, Succinyl CoA undergoes hydrolysis to produce succinate. Alright. So here, this hydrolysis reaction happens in reaction 5. And then finally, Malate is produced from the hydration of fumarate.

Well, we create fumarate in reaction 6, so that would mean that we go from fumarate to malate in reaction 7. So it'd be 8, 6, 5, and 7 in terms of the reactions based on the descriptions provided.

How many final high-energy molecules are produced in phase C of the citric acid cycle?

• A. 2
• B. 4
• C. 1
• D. 3

The Citric Acid Cycle Concept 12

In this video, we're going to talk about memory tools that we can use to help us remember the types of mechanisms that occur in each reaction step of the citric acid cycle. So our memory tool here to help us remember reactions 1 to 8, in terms of the mechanisms in each part, is "never ignore ordinary habits, daily habits, outperform." Alright. So here in reaction one, we go from oxaloacetate to citrate, NEVA. "Na" represents a nucleophilic addition. Reaction 2, we go from citrate to isocitrate.

So, this is "ignore" because it's an isomerization; we're going from citrate to isocitrate, they are isomers of each other, look at the names, very similar. In reaction 3, "ordinary," we go from isocitrate to alpha-ketoglutarate, so this is an oxidation plus decarboxylation. Remember, this is the first decarboxylation that happens, and it happens yet again in the very next reaction. So "ordinary" here is for reactions 3 and 4 because alpha-ketoglutarate to succinyl-CoA is also an oxidation and decarboxylation. "Habits" is hydrolysis, where we go from succinyl-CoA to succinate.

"Daily" is dehydration, where we go from succinate to fumarate in reaction 6. In reaction 7, "habits," we hydrate, so hydration of fumarate to malate, and then finally, "outperform" to end it all. We go oxidation where we oxidize malate to oxaloacetate. So, just remember your memory tool, "never ignore ordinary habits twice, daily habits outperform." This will help us remember the types of mechanisms from reactions 1 to 8 of the citric acid cycle.

The Citric Acid Cycle Example 5

Here in this example, it asks which of the phone reaction steps could be classified as an electrophilic addition reaction. Right. So remember, what types of molecules prefer this type of reaction? Those with pi bonds, typically alkenes and alkynes. If we think back to the metabolites that we create in the citric acid cycle, we know that fumarate is a flat molecule and that's because we have pi bonds between its two internal carbons.

And we know that we can go from fumarate to malate through hydration, where we're adding water to that alkene to make it an alcohol. That represents an electrophilic addition reaction. Now, all we have to think about is what reaction step is that? Well, we made fumarate in reaction 6. Right?

We oxidized succinate to fumarate in reaction 6. And in reaction 7, is where we go from fumarate to malate through hydration. So here, we'd say the answer is reaction 7. In reaction 7, I hydrate my fumarate to malate by using water. This represents an electrophilic addition.

So, option c will be our final answer.

The Citric Acid Cycle Concept 13

Hey everyone. So let's take a look at the Citric acid summary. Here we're going to say the Citric acid cycle degrades Acetyl groups to produce carbon dioxide and high energy molecules. Now, when we say high energy molecules, we're referring to NADH, FADH₂, and ATP. Now, here we have some memory tools to help us remember how many of these will be produced, where they're produced.

So let's take a look at memory tool number 1. Here it says, Krebs Cycle is a big crab. Remember when it comes to the Krebs Cycle or the Citric Acid Cycle, we have phases a, b, and c. Next, we have memory tool 2. Here we say there are 4 owls and 1 sea hawk in a circus ring.

So here, what does exactly mean? Well, for owls, there are 4 oxidation reactions that occur within the citric acid cycle. And these oxidation reactions involve NADH and FADH₂. Here we say 1 c Hawk. 1 c, c here is referring to the letter c, which is phase c of the citric acid cycle.

In phase c, this is where we have a hydrolysis reaction occurring. Hydrolysis helps to create ATP. And when we talk about a circus ring we're talking about the cyclic nature of the citric acid cycle. Now, here we're going to say that there are 4 oxidation reactions total between phases b and c each one has 2. So we have 2 oxidation reactions each in phases b and c, and then we're gonna say oxidation reactions yield, NADH and FADH₂.

Next, we're going to say hydrolysis reactions. Again, we said earlier, yield ATP. So, either just a couple of memory tools that'll help us remember what's going on in terms of the Citric Acid Cycle or Krebs Cycle. We have phases a, b, and c. We have 4 oxidation reactions.

2 of them happening in phase b. 2 of them happening in phase c. We have one hydrolysis reaction happening within phase c of the citric acid cycle. Now, remember oxidation reactions help to yield NADH and FADH₂, and hydrolysis helps to yield ATP.

The Citric Acid Cycle Concept 14

So in our continued discussion of the summary of the citric acid cycle, let's take a look at the numbers involved with some of these high energy molecules. Now remember, one glucose will help us to have 2 Acetyl CoA's being involved in terms of this process. Now, we're going to say here that reactions 3 and 4 of phase b, remember this is phase b here, help us to make NADH. So reaction 3 makes 1 NADH. Reaction 4 makes 1 NADH.

But remember, we have 2 runs of this cycle. So, yes, reactions 3 and 4 give us 2 NADH's, but again, we can do one more turn of the citric acid cycle. That gives us a total of 4 NADH's that are produced. Then we're gonna say here, that when it comes to phase c, that is reactions 5 to 8.

We're gonna say here that one turn of the citric acid cycle produces 1 ATP, or we do 2 turns. Right? So that's 2 ATPs that we'd make. We're gonna say here for FADH₂, each turn makes 1 FADH₂ through reaction 6 where we go from succinate to fumarate, but it happens twice so that's 2. Then remember, NAD⁺ oxidizes malate to oxaloacetate in reaction 8, but it happens twice, so this is 2 as well.

Alright. Now, here, if we add up everything, again, we're starting out with 2 Acetyl CoA's. In terms of ATP, we make 2 ATPs through the 2 turns of the citric acid cycle. We make 2 FADH₂s from the 2 turns of the citric acid cycle, and then we're gonna say, phase b makes 4 NADH's total, and then phase c makes 2 NADHs total. So adding those together, that's a total of 6 NADHs that are produced.

And again, our end molecule is Oxaloacetate. Right? So, these are the numbers you need to remember in terms of our high energy molecules in respect to ATP, FADH₂, and NADH.

The Citric Acid Cycle Concept 15

Now, memory tool 3 will help us remember the high energy molecules that are created in the citric acid cycles and what reactions produce them. Now, the memory tool 3 here is under trees in a forest, there lived 5 ants and 6 flies. So here we're going to say, translating this, under, so "nd" still, "trees" or "threes" in a forest, "forest", 4, 8, there lived 5 ants and 6 flies. So let's translate this. "N d" equals NADH.

And, "trees" in a forest, that means NADH is produced in reactions 3, 4, and 8. Next, "a" stands for ATP. We're going to say, it happens, it's created in reaction 5, which remember is the hydrolysis reaction where we go from succinyl CoA to just succinate. And then "f" for flies, that stands for FADH₂. And that is created in reaction 6.

Remember, in reaction 6 is where we go from succinate to fumarate. So here we have our 3 high energy molecules of NADH, ATP, and FADH₂, and now we have the reactions in which they're produced. Right? So just remember, memory tool 3 to help us remember the 3 high energy molecules and the reactions that produce them.

The Citric Acid Cycle Example 6

Under this example question, it asks how many reactions in the citric acid cycle produce high-energy molecules. Remember, when we say high-energy molecules, we're referring to NADH, FADH₂, and ATP. And what we have to do here is remember our memory tool. And remember, it is "under threes in a forest, so 48, there lived 5 ants, and 6 flies".

So here we're talking about NADH, and it's in steps 3, 4, and 8, where we have the production of NADH.

“There lived 5 ants,” where 'a' here stands for ATP, which occurs in step 5, because remember, we have the GTP being used to do a substrate level phosphorylation of ADP to change it into ATP. And then step 6, 'f' for flies, that is FADH₂. So, we can see that steps 3, 4, 8, 5, and 6 would produce some of these high-energy molecules. So that means there are 5 total reactions that produce these high-energy molecules within the citric acid cycle.

Which reaction of the citric acid cycle produces NADH from the choices provided?

• A. Reaction 3
• B. Reaction 5
• C. Reaction 2
• D. Reaction 6
• E. Reaction 1
• F. None of these