Biochemistry : Citric Acid Cycle

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Citric Acid Cycle Carbohydrate Intermediates

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

Possible Answers:

II and III

I only

I and II

III only

II only

Correct answer:

I and II

Explanation:

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.  

Example Question #3 : Citric Acid Cycle Carbohydrate Intermediates

Which of the following Krebs cycle intermediate molecules can be used directly in another pathway to make fatty acids?

Possible Answers:

Aspartate

Succinyl-CoA

Alpha-ketoglutarate

Citrate

Oxaloacetate

Correct answer:

Citrate

Explanation:

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.

Example Question #941 : Biochemistry

How many atoms of carbon are present in the citric acid cycle intermediate, fumarate?

Possible Answers:

2

4

6

1

3

Correct answer:

4

Explanation:

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. 

Example Question #942 : Biochemistry

How many molecules of carbon are present in the citric acid cycle intermediate, malate?

Possible Answers:

1

4

3

6

2

Correct answer:

4

Explanation:

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. 

Example Question #943 : Biochemistry

Which of the following statements about the citric acid cycle is true?

Possible Answers:

Isocitrate is one of the compounds in the cycle

Acetyl-CoA is one of the compounds in the cycle

Two equivalents of  are produced in the cycle

There is only one decarboxylation in the cycle

None of the other answers are true

Correct answer:

Isocitrate is one of the compounds in the cycle

Explanation:

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.

Example Question #942 : Biochemistry

Which of the following steps in the citric acid cycle do not have a largely negative 

Possible Answers:

None of these reactions have largely negative  values

Correct answer:

Explanation:

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.

Example Question #944 : Biochemistry

Which reaction of the citric acid cycle makes the entire process unidirectional (i.e. irreversible)?

Possible Answers:

Alpha-ketoglutarate  succinyl-CoA

Succinyl-CoA  malate

Succinate  fumarate

Citrate  isocitrate

Isocitrate  alpha-ketoglutarate

Correct answer:

Isocitrate  alpha-ketoglutarate

Explanation:

The formation of alpha-ketoglutarate from isocitrate using the enzyme alpha-ketoglutarate dehydrogenase is an irreversible reaction due to its largely negative  value.

Example Question #945 : Biochemistry

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?

Possible Answers:

Correct answer:

Explanation:

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.

Example Question #946 : Biochemistry

Which of the following steps within the citric acid cycle directly produces ATP (or GTP) as a side product? 

Possible Answers:

The conversion of fumarate to malate

The conversion of malate to oxaloacetate

The conversion of succinyl-CoA to succinate 

The conversion of alpha-ketoglutarate to succinyl-CoA

The conversion of citrate to cis-aconitate

Correct answer:

The conversion of succinyl-CoA to succinate 

Explanation:

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.

Example Question #947 : Biochemistry

Which of the following steps within the citric acid cycle does not produce  as a side product?

Possible Answers:

The conversion of isocitrate to alpha-ketoglutarate

All of these steps produce 

The conversion of fumarate to malate

The conversion of malate to oxaloacetate

The conversion of alpha-ketoglutarate to succinyl-CoA

Correct answer:

The conversion of fumarate to malate

Explanation:

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.  

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