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Example Questions
Example Question #941 : Biochemistry
Which of the following statements about the citric acid cycle is true?
None of the other answers are true
Isocitrate is one of the compounds in the cycle
There is only one decarboxylation in the cycle
Acetyl-CoA is one of the compounds in the cycle
Two equivalents of are produced in the cycle
Isocitrate is one of the compounds in the cycle
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 #1 : Citric Acid Cycle Energetics
Which of the following steps in the citric acid cycle do not have a largely negative ?
None of these reactions have largely negative values
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 #941 : Biochemistry
Which reaction of the citric acid cycle makes the entire process unidirectional (i.e. irreversible)?
Isocitrate alpha-ketoglutarate
Succinate fumarate
Alpha-ketoglutarate succinyl-CoA
Succinyl-CoA malate
Citrate isocitrate
Isocitrate alpha-ketoglutarate
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 #4 : Citric Acid Cycle Energetics
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?
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 #942 : Biochemistry
Which of the following steps within the citric acid cycle directly produces ATP (or GTP) as a side product?
The conversion of succinyl-CoA to succinate
The conversion of fumarate to malate
The conversion of malate to oxaloacetate
The conversion of citrate to cis-aconitate
The conversion of alpha-ketoglutarate to succinyl-CoA
The conversion of succinyl-CoA to succinate
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 #6 : Citric Acid Cycle Energetics
Which of the following steps within the citric acid cycle does not produce as a side product?
The conversion of malate to oxaloacetate
The conversion of isocitrate to alpha-ketoglutarate
The conversion of fumarate to malate
The conversion of alpha-ketoglutarate to succinyl-CoA
All of these steps produce
The conversion of fumarate to malate
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.
Example Question #31 : Citric Acid Cycle
Which of the following steps of the citric acid cycle results in the production of as a side product?
The conversion of succinyl-CoA to succinate
The conversion of alpha-ketoglutarate to succinyl-CoA
None of the answers listed result in the production of as a side product.
The conversion of isocitrate to alpha-ketoglutarate
The conversion of malate to oxaloacetate
None of the answers listed result in the production of as a side product.
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
.
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