Citric Acid Cycle - Biochemistry
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What vitamin does pyruvate dehydrogenase need in order to make pyruvate into acetyl-CoA for the citric acid cycle?
What vitamin does pyruvate dehydrogenase need in order to make pyruvate into acetyl-CoA for the citric acid cycle?
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Thiamine (B1) acts as a cofactor to enable pyruvate dehydrogenase to convert pyruvate from glycolysis into acetyl-CoA so it can enter the citric acid cycle.
Thiamine (B1) acts as a cofactor to enable pyruvate dehydrogenase to convert pyruvate from glycolysis into acetyl-CoA so it can enter the citric acid cycle.
Pyruvate enters the citric acid cycle after being converted to a molecule with how many carbons?
Pyruvate enters the citric acid cycle after being converted to a molecule with how many carbons?
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The three-carbon molecule pyruvate produced from glycolysis is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one 
. Acetyl-CoA has one less carbon than pyruvate; this third carbon from pyruvate was lost as carbon dioxide during its conversion to acetyl-CoA via the pyruvate dehydrogenase complex.
The three-carbon molecule pyruvate produced from glycolysis is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one
Which molecule is not a citric acid cycle intermediate?
Which molecule is not a citric acid cycle intermediate?
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Phosphoenolpyruvate (PEP) is an intermediate in glycolysis, not the citric acid cycle. PEP is the product of the ninth reaction in glycolysis, which involves the enolase-catalyzed conversion of 2-phosphoglycerate into PEP. All other molecules are indeed intermediates in the citric acid cycle.
Phosphoenolpyruvate (PEP) is an intermediate in glycolysis, not the citric acid cycle. PEP is the product of the ninth reaction in glycolysis, which involves the enolase-catalyzed conversion of 2-phosphoglycerate into PEP. All other molecules are indeed intermediates in the citric acid cycle.
What is the intermediate between citrate and isocitrate?
What is the intermediate between citrate and isocitrate?
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The citric acid cycle begins when a four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) to produce the six-carbon molecule citrate. The enzyme citrate synthase carries out this reaction. Citrate then becomes the six-carbon molecule cis-aconitate via catalysis by aconitase. The same enzyme then converts cis-aconitate to isocitrate, which is an isomer of citrate.
The citric acid cycle begins when a four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) to produce the six-carbon molecule citrate. The enzyme citrate synthase carries out this reaction. Citrate then becomes the six-carbon molecule cis-aconitate via catalysis by aconitase. The same enzyme then converts cis-aconitate to isocitrate, which is an isomer of citrate.
The citric acid cycle begins when the two-carbon acetyl group from acetyl-CoA combines with the four-carbon molecule to form the six-carbon molecule citrate.
The citric acid cycle begins when the two-carbon acetyl group from acetyl-CoA combines with the four-carbon molecule to form the six-carbon molecule citrate.
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Oxaloacetate combines with acetyl-CoA to form citrate. This is the first stage of the citric acid cycle. Eventually, citrate will lose two molecules of 
to regenerate the four-carbon molecule oxaloacetate.
Oxaloacetate combines with acetyl-CoA to form citrate. This is the first stage of the citric acid cycle. Eventually, citrate will lose two molecules of
The enzyme aconitase is responsible for catalyzing which of the following reactions?
The enzyme aconitase is responsible for catalyzing which of the following reactions?
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The first step of the citric acid cycle involves the combination of acetyl-CoA with oxaloacetate, producing citrate. Next, aconitase catalyzes the isomerization of citrate to isocitrate, via the intermediate known as cis-aconitate. The conversion of isocitrate to alpha-ketoglutarate is catalyzed by isocitrate dehydrogenase. The citric acid cycle involves the conversion of fumarate to malate, not the reverse, although some bacteria do the reverse citric acid cycle to produce complex organic compounds. Succinyl-CoA is converted to succinate via succinyl-CoA synthetase (also known as succinate thiokinase).
The first step of the citric acid cycle involves the combination of acetyl-CoA with oxaloacetate, producing citrate. Next, aconitase catalyzes the isomerization of citrate to isocitrate, via the intermediate known as cis-aconitate. The conversion of isocitrate to alpha-ketoglutarate is catalyzed by isocitrate dehydrogenase. The citric acid cycle involves the conversion of fumarate to malate, not the reverse, although some bacteria do the reverse citric acid cycle to produce complex organic compounds. Succinyl-CoA is converted to succinate via succinyl-CoA synthetase (also known as succinate thiokinase).
There are at least four types of glucose transporter in the body. GLUT1 and GLUT3 are located in most tissues including the brain and the red blood cells. These glucose transporters rapidly take up glucose from the blood but have the lowest 
value. GLUT2 is commonly found in the liver and the pancreas. GLUT2 has a lower affinity for glucose but has the highest 
value. GLUT4 is common in skeletal tissues and in adipose tissues. This transporter is normally not active for uptake unless stimulated by insulin or during exercise.
During strenuous exercise, GLUT4 will be highly active. Which of the following intermediates will also increase?
I. Pyruvate
II. 
III. ADP
There are at least four types of glucose transporter in the body. GLUT1 and GLUT3 are located in most tissues including the brain and the red blood cells. These glucose transporters rapidly take up glucose from the blood but have the lowest
During strenuous exercise, GLUT4 will be highly active. Which of the following intermediates will also increase?
I. Pyruvate
II.
III. ADP
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During strenuous exercise, GLUT4 will be active to bring glucose into the cell. Glucose is then pushed through glycolysis to generate pyruvate. Pyruvate is then pushed through pyruvate dehydrogenase complex, where it is converted into acetyl-CoA, which feeds into the Krebs cycle (assuming aerobic conditions) to generate ATP, NADH, 
, and carbon dioxide.
During strenuous exercise, GLUT4 will be active to bring glucose into the cell. Glucose is then pushed through glycolysis to generate pyruvate. Pyruvate is then pushed through pyruvate dehydrogenase complex, where it is converted into acetyl-CoA, which feeds into the Krebs cycle (assuming aerobic conditions) to generate ATP, NADH,
Which of the following Krebs cycle intermediate molecules can be used directly in another pathway to make fatty acids?
Which of the following Krebs cycle intermediate molecules can be used directly in another pathway to make fatty acids?
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Most of the intermediate molecules in the Krebs cycle can, rather than continuing through the cycle itself, go through other pathways to form macromolecules. Citrate can be used to create fatty acids and sterols. Alph-ketoglutarate can be used to make some of the amino acids. Succinyl-CoA can be used to make porphyrins, heme, and chlorophyll. Aspartate can be used to make some of the amino acids. Oxaloacetate can be used in gluconeogenesis to create glucose.
Most of the intermediate molecules in the Krebs cycle can, rather than continuing through the cycle itself, go through other pathways to form macromolecules. Citrate can be used to create fatty acids and sterols. Alph-ketoglutarate can be used to make some of the amino acids. Succinyl-CoA can be used to make porphyrins, heme, and chlorophyll. Aspartate can be used to make some of the amino acids. Oxaloacetate can be used in gluconeogenesis to create glucose.
How many atoms of carbon are present in the citric acid cycle intermediate, fumarate?
How many atoms of carbon are present in the citric acid cycle intermediate, fumarate?
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The citric acid cycle intermediate, fumarate, contains four atoms of carbon.
As a frame of reference, one molecule of glucose, the starting material for glycolysis, contains six atoms of carbon. The carbohydrate products of glycolysis are two molecules of pyruvate, with one molecule of pyruvate containing three atoms of carbon.
In preparation for entering the citric acid cycle, pyruvate loses one molecule of carbon dioxide, and therefore one molecule of carbon, to form acetyl-CoA, which contains two atoms of carbon. Acetyl-CoA is then combined with a molecule of oxaloacetate, which contains four atoms of carbon, to produce a molecule of citrate, which contains six atoms of carbon, and is the starting point for the citric acid cycle.
Citrate undergoes a number of a reactions, via the citric acid cycle, most notably two reactions in which a single molecule of carbon dioxide, and therefore carbon, is lost, thereby decreasing the total number of carbons to four atoms. The two reactions that remove carbons are the conversion of isocitrate to alpha-ketoglutarate and the conversion of alpha-ketoglutarate to succinyl-CoA. No additional carbons are removed prior to the production of fumarate, and therefore, fumarate contains four atoms of carbon.
The citric acid cycle intermediate, fumarate, contains four atoms of carbon.
As a frame of reference, one molecule of glucose, the starting material for glycolysis, contains six atoms of carbon. The carbohydrate products of glycolysis are two molecules of pyruvate, with one molecule of pyruvate containing three atoms of carbon.
In preparation for entering the citric acid cycle, pyruvate loses one molecule of carbon dioxide, and therefore one molecule of carbon, to form acetyl-CoA, which contains two atoms of carbon. Acetyl-CoA is then combined with a molecule of oxaloacetate, which contains four atoms of carbon, to produce a molecule of citrate, which contains six atoms of carbon, and is the starting point for the citric acid cycle.
Citrate undergoes a number of a reactions, via the citric acid cycle, most notably two reactions in which a single molecule of carbon dioxide, and therefore carbon, is lost, thereby decreasing the total number of carbons to four atoms. The two reactions that remove carbons are the conversion of isocitrate to alpha-ketoglutarate and the conversion of alpha-ketoglutarate to succinyl-CoA. No additional carbons are removed prior to the production of fumarate, and therefore, fumarate contains four atoms of carbon.
How many molecules of carbon are present in the citric acid cycle intermediate, malate?
How many molecules of carbon are present in the citric acid cycle intermediate, malate?
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The citric acid cycle intermediate, malate, contains four atoms of carbon.
A single glucose molecule, which is the starting material for glycolysis, contains six carbon atoms. Glycolysis produces two pyruvate molecules, and one pyruvate molecule contains three carbon atoms.
Prior to entering the citric acid cycle, pyruvate loses one carbon dioxide molecule (e.g. one carbon atom), forming acetyl-CoA, which contains two carbon atoms. Acetyl-CoA then combines with one oxaloacetate molecule, a **four-**carbon molecule, to produce a molecule of citrate, which contains six carbon atoms, and is the starting material for the citric acid cycle.
Citrate undergoes a number of a reactions in the citric acid cycle, including two reactions where one atom of carbon dioxide (e.g. carbon) is lost, which decreases the total number of carbons to four atoms. The two reactions that remove carbons are the conversion of isocitrate to alpha-ketoglutarate and the conversion of alpha-ketoglutarate to succinyl-CoA. No additional carbons are removed prior to the production of malate. Therefore, malate contains four atoms of carbon.
The citric acid cycle intermediate, malate, contains four atoms of carbon.
A single glucose molecule, which is the starting material for glycolysis, contains six carbon atoms. Glycolysis produces two pyruvate molecules, and one pyruvate molecule contains three carbon atoms.
Prior to entering the citric acid cycle, pyruvate loses one carbon dioxide molecule (e.g. one carbon atom), forming acetyl-CoA, which contains two carbon atoms. Acetyl-CoA then combines with one oxaloacetate molecule, a **four-**carbon molecule, to produce a molecule of citrate, which contains six carbon atoms, and is the starting material for the citric acid cycle.
Citrate undergoes a number of a reactions in the citric acid cycle, including two reactions where one atom of carbon dioxide (e.g. carbon) is lost, which decreases the total number of carbons to four atoms. The two reactions that remove carbons are the conversion of isocitrate to alpha-ketoglutarate and the conversion of alpha-ketoglutarate to succinyl-CoA. No additional carbons are removed prior to the production of malate. Therefore, malate contains four atoms of carbon.
Which enzyme of the citric acid cycle is membrane-bound?
Which enzyme of the citric acid cycle is membrane-bound?
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Succinate dehydrogenase (also known as succinate-coenzyme Q reductase or complex II) is bound to the inner mitochondrial membrane. It participates in both the citric acid cycle and the electron transport chain. Aconitase is an enzyme involved in glycolysis, not the citric acid cycle (Krebs cycle).
Succinate dehydrogenase (also known as succinate-coenzyme Q reductase or complex II) is bound to the inner mitochondrial membrane. It participates in both the citric acid cycle and the electron transport chain. Aconitase is an enzyme involved in glycolysis, not the citric acid cycle (Krebs cycle).
Which of the following steps in the citric acid cycle do not have a largely negative 
?
Which of the following steps in the citric acid cycle do not have a largely negative
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Even though an 
is generated when malate is dehydrogenated to oxaloacetate, this oxidation is very unfavorable because of the addition of a reactive ketone in place of an alcohol on the 2nd carbon. In fact, the only way this reaction can proceed is if oxaloacetate concentration is very low. All of the other reactions have large negative 
values.
Even though an
Which of the following statements about the citric acid cycle is true?
Which of the following statements about the citric acid cycle is true?
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Acetyl-CoA is not part of the cycle but is oxidized by it. There are two decarboxylations in the cycle, from isocitrate to alpha-ketoglutarate, and from alpha-ketoglutarate to succinyl-CoA. In total, three equivalents of 
are produced in the cycle. Isocitrate is a compound in the cycle, produced from citrate.
Acetyl-CoA is not part of the cycle but is oxidized by it. There are two decarboxylations in the cycle, from isocitrate to alpha-ketoglutarate, and from alpha-ketoglutarate to succinyl-CoA. In total, three equivalents of
Which reaction of the citric acid cycle makes the entire process unidirectional (i.e. irreversible)?
Which reaction of the citric acid cycle makes the entire process unidirectional (i.e. irreversible)?
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The formation of alpha-ketoglutarate from isocitrate using the enzyme alpha-ketoglutarate dehydrogenase is an irreversible reaction due to its largely negative 
value.
The formation of alpha-ketoglutarate from isocitrate using the enzyme alpha-ketoglutarate dehydrogenase is an irreversible reaction due to its largely negative
Suppose that in a certain neuron, an action potential has caused 
ions to enter the cell. In order to restore the resting membrane potential, the sodium-potassium pump uses 1 molecule of ATP to push 
ions out of the cell and to bring 
ions into the cell. How many molecules of acetyl-CoA must pass through the citric acid cycle in order to provide enough energy for this process to occur?
Suppose that in a certain neuron, an action potential has caused
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This question is providing us with a scenario in which 
ions enter a cell. We're further told that it will take a single molecule of ATP to move three of these ions out of the cell. Finally, we are being asked to determine the total number of acetyl-CoA molecules that must pass through the Krebs cycle in order to provide the energy necessary for the export of these 
ions.
First, we'll need to determine the total number of ATP molecules generated from the passage of a single molecule of acetyl-CoA through the Krebs cycle. It's important to remember that the passage of acetyl-CoA through the Krebs cycle generates one molecule of ATP directly by substrate-level phosphorylation, but it also produces other intermediate energy carriers in the form of 
and 
.
For each acetyl-CoA ran through the cycle, one molecule of 
and three molecules of 
are produced. Furthermore, each molecule of 
will go on to donate its electrons to the electron transport chain to generate 
molecules of ATP per molecule of 
oxidized. Likewise, each 
will also produce ATP via oxidative phosphorylation, but at a rate of 
molecules of ATP per molecule of 
oxidized.
Adding these up, we obtain:
ATP via substrate-level phosphorylation
Adding these values up, we have a total of 
molecules of ATP produced for every molecule of acetyl-CoA oxidized. Now that we know how much ATP is produced from one acetyl-CoA, we can calculate the number needed to move the 
ions out of the cell.
This question is providing us with a scenario in which
First, we'll need to determine the total number of ATP molecules generated from the passage of a single molecule of acetyl-CoA through the Krebs cycle. It's important to remember that the passage of acetyl-CoA through the Krebs cycle generates one molecule of ATP directly by substrate-level phosphorylation, but it also produces other intermediate energy carriers in the form of
For each acetyl-CoA ran through the cycle, one molecule of
Adding these up, we obtain:
Adding these values up, we have a total of
Which of the following enzymes would be inhibited by the addition of NADH?
Which of the following enzymes would be inhibited by the addition of NADH?
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All enzymes that have NADH as a product would be inhibited by the addition of NADH. The correct answers are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. Pyruvate dehydrogenase, which synthesizes acetyl-CoA from pyruvate, is also inhibited by NADH.
All enzymes that have NADH as a product would be inhibited by the addition of NADH. The correct answers are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. Pyruvate dehydrogenase, which synthesizes acetyl-CoA from pyruvate, is also inhibited by NADH.
Which of the following steps within the citric acid cycle directly produces ATP (or GTP) as a side product?
Which of the following steps within the citric acid cycle directly produces ATP (or GTP) as a side product?
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The only step of the citric acid cycle (also known as the Krebs cycle, or the TCA cycle) that directly produces ATP or GTP is the conversion of succinyl-CoA to succinate.
In this reaction, succinyl-CoA is converted to succinate with the assistance of the enzyme, succinyl-CoA synthetase. During this reaction, ADP + Pi (or GDP + Pi) is also converted to ATP (or GTP) using the energy from the breaking of the bond between CoA and succinate. Thus, the overall reaction appears as:
While side products of some of the other reactions listed produce intermediaries that may be used to produce ATP in the future, these reactions do not directly produce ATP.
The only step of the citric acid cycle (also known as the Krebs cycle, or the TCA cycle) that directly produces ATP or GTP is the conversion of succinyl-CoA to succinate.
In this reaction, succinyl-CoA is converted to succinate with the assistance of the enzyme, succinyl-CoA synthetase. During this reaction, ADP + Pi (or GDP + Pi) is also converted to ATP (or GTP) using the energy from the breaking of the bond between CoA and succinate. Thus, the overall reaction appears as:
While side products of some of the other reactions listed produce intermediaries that may be used to produce ATP in the future, these reactions do not directly produce ATP.
Which of the following steps within the citric acid cycle does not produce 
as a side product?
Which of the following steps within the citric acid cycle does not produce
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The only citric acid cycle (also known as the Krebs cycle or TCA cycle) step listed that does not result in the production of 
as a side product is the conversion of fumarate to malate.
In the conversion of fumarate to malate, fumarate is chemically combined with water in the presence of the enzyme fumarase to produce malate. In this conversion, there is no concomitant production of 
.
In each of the other reactions listed, 
is converted to 
and 
as side products.
The only citric acid cycle (also known as the Krebs cycle or TCA cycle) step listed that does not result in the production of
In the conversion of fumarate to malate, fumarate is chemically combined with water in the presence of the enzyme fumarase to produce malate. In this conversion, there is no concomitant production of
In each of the other reactions listed,
Which of the following steps of the citric acid cycle results in the production of 
as a side product?
Which of the following steps of the citric acid cycle results in the production of
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The correct answer is that none of the citric acid cycle steps listed result in the production of 
. The only step of the citric acid cycle that results in the production of 
is the conversion of succinate to fumarate (catalyzed by succinate dehydrogenase). In this reaction, 
is concomitantly converted to 
using the hydrogen molecules removed from succinate by succinate dehydrogenase. This reaction was not listed in the answer choices though, and therefore none of the reactions listed produced 
.
Each of the reactions listed did produce other side products. The conversions of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate all result in the production of 
as a side product, but not 
. The conversion of succinyl-CoA to succinate results in the production of ATP or GTP and CoA-SH as side products, but not 
.
The correct answer is that none of the citric acid cycle steps listed result in the production of
Each of the reactions listed did produce other side products. The conversions of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate all result in the production of
Which enzyme catalyzes the conversion of citrate to isocitrate?
Which enzyme catalyzes the conversion of citrate to isocitrate?
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Aconitase is the enzyme that catalyzes the conversion of citrate to isocitrate. This essential enzyme is vital in energy production, as it acts like an iron regulatory protein. The conversion of citrate to isocitrate is important since it is needed to react with isocitrate dehydrogenase.
Aconitase is the enzyme that catalyzes the conversion of citrate to isocitrate. This essential enzyme is vital in energy production, as it acts like an iron regulatory protein. The conversion of citrate to isocitrate is important since it is needed to react with isocitrate dehydrogenase.