The Citric Acid Cycle - Video Tutorials & Practice Problems
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1
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Phase A - Citrate Formation Concept 1
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Now, the citric acid cycle is a sequence of eight biochemical reactions. Now, for phase A, we have citrate formation here we're gonna say f um phase A consists of the first reaction of the pathway. Here we have oxy acetate 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. Right. So again, we're creating this as a product in the form of oxy acetate. It then becomes interactive, it becomes interactive with acetyl coa for the first reaction in order to form our citrate molecule. Right. So let's click on the next video and see what exactly does this entail in the formation of citrate.
2
concept
Phase A - Citrate Formation Concept 2
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So in step one, we have citrate formation here, the acetyl group from the acetyl coa combines with oxy acetate in order to produce citrate. Remember we're gonna say here that acetyl coa is basically this structure here and it is this portion of acetyl coa that gets added here because the rest of the structure was this part right here. So we can see here that we have oxyl acetate being changed into citrate. And then in order to do this, we have water being involved, which means that um we're gonna have some type of hydrolysis taking place. We have acetyl coa just becoming our co a enzyme with our thy group for fully formed because the acetyl group has been transferred over in order to make citrate. All of this is accomplished with our enzyme citrate synthese. Now remember here that syntax basically catalyze the synthesis of new compounds within this cell. And we're gonna say here, the energy that drives this reaction comes from the hydrolysis of this acetyl coa the carbon sulfur bond is high energy. So we're using the water in order to cut or cleave that bond that energy that we release is being used to drive the formation of citrate from oxy acetate. So keep this in mind, we're talking about step one of citrate formation.
3
example
Phase A - Citrate Formation Example 1
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How many carbon atoms are added to oxy acetate to produce citrate? Remember we are undergoing hydrolysis of our acetyl coa and it is this portion, this acetyl group that gets added to my A cloaca in order to form citrate. This acetyl coa which I have just put within this black box has how many carbons? It has 12 carbons. So two carbon atoms are added to oxy acetate in order to produce citrate. That means that option B would be our correct answer.
4
concept
Phase B - Succinyl CoA Formation Concept 3
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In this video, we're gonna take a overview of phase B of the citric acid cycle which deals with succinylate formation. Here, we're gonna say phase B which is highlighted in this yellow portion. It consists of pathways reactions 23 and four. In reaction two, we have citrate becoming isocitrate in step three or reaction three. We're going from isocitrate to alpha ketoglutarate. And to do that, we also have the, the releasing of N A DH our electronic carrier as well as carbon dioxide. And then for reaction four, we're going from alpha ketoglutarate to succinate. This also has a releasing of another mole of N A DH and carbon dioxide. So we can see that within phase B, we have two moles of N A DH and CO2 being produced one of each happening in our reaction three and again, in reaction four. 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
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Phase B - Succinyl CoA Formation Concept 4
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So in this video, we're gonna take a look at reaction two of phase B of the citric acid cycle. So here reaction two deals with isomerization. We're gonna say that the tertiary alcohol in citrate must be is summarized to a secondary alcohol so that it can be oxidized. So this is for oxidation. If we take a look here, we have citrate. Here is our carbon with the oh that carbon is connected to two of uh well to three other carbons. And that's why it's a tertiary alcohol. Remember, tertiary alcohols cannot be oxidized. That's why we have to use our enzyme acona in order to change citrate into isocitrate. So the enzyme here a conta I summarize the citrate to iso citrate. Now all that happens is we had the oh here in the H here and they're gonna basically switch places. So now our oh is here and our H is over here. And if we're looking at where the oh is connected, we say that this carbon which is our alcohol carbon, it's only connected to two carbons. So it is now a secondary alcohol, secondary alcohols can be oxidized. So reaction two is just changing our alcohol into a new alcohol that can undergo oxidation within the next reaction, right? So keep that in mind for reaction two of phase B of the citric acid cycle.
6
concept
Phase B - Succinyl CoA Formation Concept 5
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Now, in reaction three, we have isocitrate which represents a secondary alcohol which can be oxidized. So here we have oxidation and then decarboxylate. Remember, decarboxylate just means the loss of CO2. Now, here we're going to say the enzyme that's going to be used is going to be our isocitrate dehydrogenase and it oxidizes isocitrate to alpha keto uh Gluta. Right. Now for all oxidation reactions, we're going to be using a dehydrogenase enzyme. And it's easy to remember the name because it's all it is is the substrate followed by dehydrogenase. The substrate here is isocitrate. So then add isocitrate dehydrogenase as the enzyme required to change it from isocitrate to alpha ketoglutarate. Now, here in this process, we have N ad positive, which is our oxidized form of our coenzyme. It's gonna be reduced to one N A DH. So here it's become an A DH and then also one carbon atom is lost as CO2. Remember decarboxylate means that we have the loss of CO2 and this is the first location where we're gonna have the creation of one mole of N A DH and one mole of CO2, right? So if we take a look here isocitrate is a secondary alcohol. And because this is an oxidation that secondary alcohol is transformed into a carbonel group. Remember the carbon oh group here is gonna be oxidized and it changes into a carbon group which is c double bond O. This gives us our alpha ketoglutarate.
7
concept
Phase B - Succinyl CoA Formation Concept 6
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Now, here we have reaction for a phase B of the citric acid cycle. Here we also undergo oxidation and decarboxylate. This is gonna be the second location where we lose yet another mole of N A DH and also CO2 and by lose that means we're gonna release it or produce it. Now, here we're going to say that our alpha ketoglutarate. What's the enzyme we use here? Remember for all these oxidation reactions, it's just gonna be the substrate followed by dehydrogenase. So this would be alpha keto lute dehydrogenase and it oxidizes our alpha ketoglutarate to succinyl coa. So if we take a look here, we have our succinyl uh our alpha ketoglutarate. And here we have our carboxyl group. We're gonna say in this process, we use our alpha ketoglutarate dehydrogenase enzyme. And what this is going to do, it's going to oxidize this and help us connect and form the highly energetic carbon sulfur bond. Remember this highly energetic bond, if we can hydrolyze and break it, we can have the releasing of a TP. So we know that A TP will be formed at some point. If we can successfully break this carbon sulfur bond. Now, in this process, we say that the oxidized form of our coenzyme N ad positive is reduced to one mole of N A DH. And then also we're gonna say one carbon atom is lost as CO2. Remember decarboxylate means the loss of CO2. So here we'd have our N ad positive become N A DH. Then here we have our coenzyme A with its thy group and we have the loss of CO2 as a result. So we lose the CO2, but we have the coenzyme A being connected to the structure in order to make our succinyl coa. So this would represent reaction for a phase B of the citric acid cycle.
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example
Phase B - Succinyl CoA Formation Example 2
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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 four that happens in phase B of the citric acid cycle. Next, we have oxo acetate is converted to citrate. Remember this happens in reaction one phase a of the citric acid cycle because we're gonna say oxo a interacts with acetylate in order to make this citrate. Then we're gonna 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 three 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 gonna say this changes a tertiary alcohol in citrate to a secondary alcohol to isocitrate, basically prepping that secondary alcohol to be oxidized later on. So here this would happen in reaction to a phase B of the citric acid cycle. So these are the reactions or steps in which they would occur for the citric acid cycle.
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Problem
Problem
Which enzyme oxidizes α-ketoglutarate in step 4 of the citric acid cycle?
A
isocitrate dehydrogenase
B
α-ketoglutarate oxidase
C
succinyl CoA synthase
D
α-ketoglutarate dehydrogenase
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Problem
Problem
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
11
concept
Phase C - Oxaloacetate Regeneration Concept 7
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So in this video, we're gonna take an overview of our citric acid cycle and pay closer attention to phase C. In phase C, we have oxy acetate regeneration. Remember we can recreate oxy acetate and then it can combine with our acetyl away to make citrate yet again to go through the whole cycle again. Now, here we're gonna say phase C consists of the pathway reactions of reactions five to eight. And here we've already talked about steps or reactions 1 to 45 to 8. Here we just continuing from SUO away, we'd have the production of GTP which is guanine triphosphate. All it is there. It's used to generate a TP through substrate level phosphorylation. So here ad P can become a TP. So we have the generation of energy and then doing this changes Suon Kuwait to succinate, succinate can then undergo a change to fate. And in the process release fa DH two yet another electron uh carrier. Now, here we can have the introduction of water which changes ferra into mate and then finally going from mali to oxy AA which we want to regenerate. We have the releasing of another N A DH. Right. So here we're gonna go in greater depth when it comes to reactions. 5 to 8. Here we're just looking at a general overview of phase c of the citric acid cycle.
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concept
Phase C - Oxaloacetate Regeneration Concept 8
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Now, in step five or reaction five, we have hydrolysis. Here, the enzyme succinylate synthese hydrolyzes succinyl coa to succinate. Now here, remember hydrolysis means that we're gonna cut or or break or cleave a highly energetic bond. And as a result, energy is released. So energy will be released from the hydrolysis reaction and that produces GTP. Now GTP, we're gonna do hydrolysis of that in order to free up an inorganic phosphate group which provides energy to produce a TP. Now, here we have our succinyl coa and our high energy bond is this carbon sulfur bond through hydrolysis. We're going to recreate our coenzyme A with its file group that's reattached. And we're also going to produce GTP, which remember that's going to undergo hydrolysis as well, freeing up an inorganic phosphate, giving a phosphate group to AD P to create a TP, creating energy. This helps drive the reaction forward and that creates succinate here as our product. And remember what's being used to do. This is substrate followed by synthese. So, since the substrate is succinyl coa, the enzyme utilized is succinyl coa synthese
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concept
Phase C - Oxaloacetate Regeneration Concept 9
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In step six or reaction six, we have oxidation. The enzyme that we're gonna use is substrate name followed by dehydrogenase. Remember these oxidation reactions that we're going to cover. Utilize the enzyme dehydrogenase. Here, the enzyme succinate dehydrogenase oxidizes our succinate to fate. Now, as a result of this one, F ad is reduced to one fa DH two. So here we have our succinate molecule F ad is reduced to fa DH two as a result through the utilization of our enzyme succinate dehydrogenase. This creates our fumarate here. Now, remember, carbon must make four bonds. So what's happening here to make four bonds? These two carbons have to double bond. So we can see here that dehydrogenase utilize F ad to convert carbon, single bond, carbon bonds to double bonded carbon bonds. So that's what we do in terms of this particular step.
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example
Phase C - Oxaloacetate Regeneration Example 3
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Which of the following statements is incorrect. About the citric acid cycle. A reaction. Five of the cycle converts succinyl coa to succinate. Yes, that is true. It's done through hydrolysis and the utilization of the enzyme succinyl coa synth phase. So this is true oxidation of succinate reactions. Six produces fumarate. Yes, that is also true. We utilize F ad in order to do this phase C of the citric acid cycle does not result in the loss of any carbon atoms. Right. So this is also true. Now, we may have instances where carbons that are single bonded to each other become double bonded where that's a loss of hydrogens, but we don't have the loss of carbon atoms energy released from the hydrolysis of. So, Sinoa comes from the GTP. No, we hydrolyze our energetic carbon sulfur bond. This released energy helps us to create GTPGTP. Later on is hydrolyzed and its energetic phosphate is given to AD P to make a TP. So the energy doesn't come from GTP. The energy is used to create GTP. So this is what's incorrect. So here our answer is option D
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concept
Phase C - Oxaloacetate Regeneration Concept 10
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Now step seven or reaction seven, the enzyme fumarate, hydra tase also called pumera converts fumarate to mate by adding water. Remember, hydration means the addition of water in fumarate, we have our carbon double minded carbon. Remember this pi bond can be broken so that things can be added to each one of these carbons. Here we use frase. In order to add the water molecule, the water molecule will add an oh here and each year here we've added the water to our fate and this transforms it into mala. Oh So step step seven or reaction seven deals with hydration. The addition of water to our pi bo of form in order to create mate.
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concept
Phase C - Oxaloacetate Regeneration Concept 11
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40s
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Reaction eight or step eight is yet another oxidation reaction. Here. The enzyme malade dehydrogenase oxidizes malade to oxalacetate. Here, we're gonna say that one N ad positive is reduced to one N A DH. If we take a look, here is our ma and what we have here is a secondary alcohol. Remember, secondary alcohols can be oxidized as it's being oxidized and be positive becomes reduced. And this is all facilitated through the use of the enzyme malate dehydrogenase. Here. Our oh group becomes double bond O. So we've gone from an alcohol to carbon.
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example
Phase C - Oxaloacetate Regeneration Example 4
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For each of the following reactions described below. Identify corresponding step of the citric acid cycle. All right. So mala dehydrogenase catalyzes the oxidation of matte to oxy A. Remember the regeneration of oxy A happens in step or reaction eight. So that the whole cycle can go one more time if necessary. Next succinate loses two hydrogen atoms to yield fumarate in this process. One f ad becomes one fa DH two. This happens in step six of the citric acid cycle. Succinyl coa undergoes hydrolysis to produce uh succinate. So this here is hydrolysis of succinyl koe. This happens in step five and then malad is produced for migration of fumarate. This happens in step seven. So these will be the steps for each of the citric acid cycle reactions.
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Problem
Problem
Which one of the following enzymes catalyzes the addition of water to the C=C bond in fumarate?
A
Fumarate reductase
B
Malate synthase
C
Fumarase (Fumarate hydratase)
D
Malate dehydrogenase
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Problem
Problem
How many final high-energy molecules are produced in phase C of the citric acid cycle?
A
2
B
4
C
1
D
3
20
concept
Citric Acid Cycle Summary Concept 12
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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 referring to N A DH fa DH two and A TP. 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 one. Here it says crep cycle is a big crab. Remember when it comes to the crep cycle or the citric acid cycle, we have phases A B and C. Next, we have memory tool two. Here we say there are four owls and one C Hawk in a circus ring. So here what does exactly mean? Well, four owls, there are four oxidation reactions that occur within the citric acid cycle. And these oxidation reactions involve N A DH and fa DH too. Here we say one C Hawk, one CC. 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 a TP. 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 four oxidation reactions total between phases B and C. Each one has two. So we have two oxidation reactions each in phases B and C. And then we're gonna say oxidation reactions yield N A DH and fa DH two. Next, we're going to say hydrolysis reactions again, we said earlier yield A TP. So these are just a couple of memory tools that help us remember what's going on. In terms of the citric acid cycle or creb cycle. We have phases A B and C, we have four oxidation reactions. Two of them happening in phase B, two 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 N A DH and FA DH two. And hydrolysis helps to yield a TP.
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concept
Citric Acid Cycle Summary Concept 13
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So remember that one glucose molecules helps to create two acetyl Coates. So that means we can go through the citric acid cycle twice. Now, here, as we're going from the citric acid cycle, we're going from acetyl coa and with, in conjunction with oxy acetate, we create citrate. Now going from citrate to succinyl coa, we have reactions three and four, each one makes one N A DH and carbon dioxide. So through one cycle, we're making one N A DH in reaction three and one A N A DH in reaction four. Since we're going over this twice, that'd be two times two. So we have four N A DH is being created. Then we're gonna say here going through the citric acid cycle once we're gonna make 1 CO2 in reaction three and 1 CO2 in reaction four. So one cycle creates two co twos. But since we're doing this twice, it'd be two times two. So we also make four co twos over here. A TP, going through the citric acid cycle once, gives us one A TP. Going through it again, gives us our second a TP. So we make a total of two A TP fa DH two, we make one fa DH two in reaction six. And since we're going through the, the citric acid cycle twice, we make our second F AD two and then N A DH, we create one N A DH in reaction eight. And we're gonna say here, since we're doing this twice, we're gonna make another N A DH. So that's two over here. So coming over here, our starting material again is two A K A. How many carbon dioxides do we make through the two rounds of the citric acid cycle? Four. How many A TP do we make in the two rounds? Two fa DH also too. And then finally, we make a total of four N A DH S here plus another two over here. So that would be six N A DH S that are created as we go through the citric acid cycle twice. And then our N molecule from reaction eight would be oxy acetate, right? So this is how we look at a summary when it comes to the citric acid cycle.
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concept
Citric Acid Cycle Summary Concept 14
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Now remember we classify our high energy molecules as being N A DH fa DH two natp a way for us to remember where they, where they made within the citric acid cycle is with memory tool three. And it says that under trees in a forest there lived five ants and six flies. So under this stands for N A DH because NND we're gonna say trees, we say threes in a forest. 448, there lived five ants and six flies. All right. So Nand is N A DH N A DH is created in reactions 34 and eight of the citric acid cycle. And we're going to say here that gives us what in one round of TC A that's three and a DH s but we go through it twice. So that's how we come up with six and a DH s there lived five ants. So it is in reaction five that we make a TP. And then here f for flies is fa DH two which we make in reaction six. So this is how we're able to remember what are the high energy molecules created in the electron transport or the citric acid cycle and then which reactions are they made? So N A DH is reaction 34 and eight A TP is reaction five and FA DH two is reaction six. Now, what is memory tool four? Well, besides these three, we also have the generation of CO2 carbon dioxide. Well, 1 CO2 is made in reactions three and four. So C 10234. So just remember 1234. So 1 CO2 is created in reactions three and four, giving us a total of two co twos per round of the citric acid cycle, which is a total of four co twos when we go through the citric acid cycle twice, right? So just keep these memory tools on hand, they'll help you remember what the high energy molecules are and which reaction creates them.
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example
Citric Acid Cycle Summary Example 5
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Under this example question, it says how many reactions in a citric acid cycle produce high energy molecules. Remember when we say high energy molecules, we're referring to N A DH fa DH two NATP. 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 five ants and six flights. So remember here we're talking about N A DH and it's in steps three, four and eight where we have the production of N A DH. There lived five ants. A here stands for a TP which happens in step five because remember we have the GTP being used to do a substrate level phosphorylation of AD P to change it into a TP. And then step six F for flies. That is fa DH two. So we can see that that is steps 3485 and six would produce some of these high energy molecules. So that means there are five total reactions that will produce these high energy molecules within the citric acid cycle.
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Problem
Problem
Complete the following net equation for one turn of the citric acid cycle.
Acetyl CoA + __H2O + 3 NAD+ + FAD + GDP + ____ ⭢ CoA + ___NADH + ___H+ + FADH2 + GTP + ___CO2
A
2 H2O; ATP ; 3 NADH; 3 H+; 1 CO2
B
3 H2O; Pi ; 3 NADH; 3 H+; 1 CO2
C
2 H2O; ATP ; 3 NADH; 3 H+; 2 CO2
D
1 H2O; Pi ; 3 NADH; 3 H+; 2 CO2
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Problem
Problem
Which reaction of the citric acid cycle produces ATP directly?
A
Reaction 3
B
Reaction 5
C
Reaction 4
D
Reaction 6
E
Reaction 8
F
None of these
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concept
Remembering Citric Acid Cycle Concept 15
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Now, when it comes to remembering the citric acid cycle, just recall that each reaction of the citric acid cycle can be remembered by memorizing the intermediate metabolite. Next here, a memory tool is Oxford City ice crates kegs and such contain succulent for me milk. All right. So how does this work? OK. So Oxford, so we're talking about oxy acetate here. Remember that's produced in reaction eight, we're gonna say see theo is coming in and together they're going to help to make our citrate citrate city. Then we're gonna go to reaction two where citrate is going to become iso citrate ice crates. Then going from iso citrate that um reaction three, we're going to kegs which is alpha ketoglutarate. They were gonna say going to four alpha ketoglutarate is going to become succinum. K A. So such succulent, oh no, such contain so succinyl coa then in reaction five suc ka becomes succinate, so succulent, succinate. Then we're gonna say that it's going to undergo an oxidation where succinate is going to become fumarate foamy. Then we're going to say here in reaction seven, we're gonna say that fumarate becomes mate milk and then finally reaction eight is where we go from mate to oxy acetate. So remembering this memory tool is this helping us remember the order of each one of these compounds and the reactions for the citric acid cycle.
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concept
Remembering Citric Acid Cycle Concept 16
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Now, when it comes to the citric acid cycle, the name of the enzyme can be predicted by knowing the substrate and the type of reaction. So here if we take a look at hit one hit, one talks about reaction, one, we have citrate formation is catalyzed by citrate. Remember when we're synthesizing a new compound, we use a synthese. So this would be citrate, synthese reaction to citrate is summarizes or undergoes is summarization and is catalyzed by a connotates. Now, next, in three reactions, 346 and eight, we're gonna say all of these are oxidation reactions. And when it comes to oxidation reactions, we say that the enzymes we're using are dehydrogenase and four reaction five is a hydrolysis where we can use that energy harness that energy in order to create um to help push the reaction forward. So reaction five is a hydrolysis reaction. Seven is a hydration or adding water. Now, here we have an exception. Reaction five is catalyzed by a synthese though instead of a hydrolase, right. So these are just hints and shortcuts to help us remember the types of enzymes that are involved within the different steps of the citric acid cycle.
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example
Remembering Citric Acid Cycle Example 6
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Write the name for the enzyme that catalyzes the first reaction of the citric acid cycle. Remember in the first reaction or step one, we have acetyl coa interacting with oxalacetate together, they help to make citrate. Now, here we are synthesizing citrate and because we're synthesizing a new compound, that means we're gonna use the enzyme synthese. It is citrate that we're synthesizing. So we'd have to use citrate synthese as our enzyme. This would mean that option D would be our answer.
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Problem
Problem
Write the name for the substrate, enzyme, and product of reaction 6 of the citric acid cycle.
A
Succinyl CoA, succinyl CoA synthase, succinate
B
Succinate, succinate dehydrogenase, and fumarate
C
Succinyl CoA, succinate dehydrogenase, and fumarate
D
Fumarate, fumarase, and malate
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Problem
Problem
Identify the two reactions that involve oxidation and decarboxylation.
A
Reactions 1 and 5
B
Reactions 3 and 6
C
Reactions 6 and 8
D
Reactions 2 and 7
E
Reactions 3 and 4
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Problem
Problem
Write the name of the substrate and the enzyme of reaction 8 of the citric acid cycle.
A
Oxalate, oxalate oxidase
B
Malate, malate dehydrogenase
C
Succinate, succinate dehydrogenase
D
Fumarate, fumarase
32
Problem
Problem
What is the name of the enzyme for the reaction that produces a GTP molecule in the citric acid cycle?
A
Succinyl CoA synthase
B
Succinate dehydrogenase
C
α-ketoglutarate dehydrogenase
D
Fumarate hydratase
33
Problem
Problem
Which reactions of the citric acid cycle produce NADH?
A
Reactions 1, 2, and 8
B
Reactions 2, 4, and 6
C
Reactions 2, 5, and 7
D
Reactions 3, 4, and 8
E
Reactions 3, 4, and 6
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