Biochemistry : Other Citric Acid Cycle Concepts

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Other Citric Acid Cycle Concepts

The citric acid cycle is __________.

Possible Answers:

anabolic

neither anabolic, nor catabolic

linear

catabolic

both anabolic and catabolic

Correct answer:

both anabolic and catabolic

Explanation:

The citric acid cycle is amphibolic—that is, both anabolic and catabolic. Anabolism occurs when the citric acid cycle generates reduced factors, such as NADH and FADH2. Catabolism occurs when the citric acid cycle oxidizes the two carbon atoms of acetyl CoA to carbon dioxide (CO2).  

Example Question #2 : Other Citric Acid Cycle Concepts

What is the end product of glycolysis?

Possible Answers:

Citrate

Pyruvate

Acetyl-CoA

Glucose

Correct answer:

Pyruvate

Explanation:

Glycolysis involves the conversion of glucose into pyruvate. Recall that glucose is a six-carbon molecule, while pyruvate is a three-carbon molecule. Thus for each molecule of glucose that undergoes glycolysis, two molecules of pyruvate are yielded. Next, pyruvate is converted into acetyl-CoA via the pyruvate dehydrogenase complex. Finally, acetyl-CoA enters the citric acid cycle, combining with oxaloacetate as the first step.

Example Question #53 : Citric Acid Cycle

Which enzyme is not found in the citric acid cycle?

Possible Answers:

Aconitase

Enolase

Fumarase

Succinate dehydrogenase

Correct answer:

Enolase

Explanation:

Enolase is the enzyme responsible for catalyzing the conversion of 2-phosphoglycerate into phosphoenolpyruvate. This reaction takes place during glycolysis. All other enzymes are involved in the sequence of reactions known as the citric acid cycle.

Example Question #3 : Other Citric Acid Cycle Concepts

What is the correct sequence of intermediates in the citrate acid cycle?

Possible Answers:

Citrate  cis-aconitate  isocitrate  alpha-ketoglutarate  succinyl-CoA  succinate  fumarate  malate  oxaloacetate

Citrate  cis-aconitate  isocitrate  alpha-ketoglutarate  succinyl-CoA  succinate  malate  fumarate  oxaloacetate

Citrate  cis-aconitate  isocitrate  succinyl-CoA  alpha-ketoglutarate  succinate  fumarate  malate  oxaloacetate

Citrate  cis-aconitate  isocitrate  alpha-ketoglutarate  succinate  succinyl-CoA  fumarate  malate  oxaloacetate

Correct answer:

Citrate  cis-aconitate  isocitrate  alpha-ketoglutarate  succinyl-CoA  succinate  fumarate  malate  oxaloacetate

Explanation:

This is the correct sequence of intermediates in the citric acid cycle. Note that both citrate and cis-aconitase are substrates for the same enzyme, aconitase. The net yield of one turn of the citric acid cycle is: , and . The electron carriers then participate in the electron transport system along the inner mitochondrial membrane.

Example Question #4 : Other Citric Acid Cycle Concepts

Which gas is produced during the citric acid cycle?

Possible Answers:

Nitric oxide

Nitrous oxide

Carbon dioxide

Methane

Correct answer:

Carbon dioxide

Explanation:

The citric acid cycle starts with the combination of a four-carbon molecule (oxaloacetate) and a two-carbon molecule (acetyl-CoA) to form a six-carbon molecule (citrate). Since the citric acid cycle is indeed a cycle, oxaloacetate must be regenerated. Thus, two molecules of carbon dioxide are produced throughout the citric acid cycle. The first molecule of carbon dioxide is produced during the conversion of isocitrate into alpha-ketoglutarate. This reaction is catalyzed by isocitrate dehydrogenase. Alpha-ketoglutarate, a five-carbon molecule, is then converted into the four-carbon molecule succinyl-CoA via alpha-ketoglutarate dehydrogenase, yielding another molecule of carbon dioxide. The remaining steps of the citric acid cycle do not involve any more production of carbon dioxide since both succinyl-CoA and oxaloacetate are both four-carbon molecules.

Example Question #5 : Other Citric Acid Cycle Concepts

In the Krebs cycle, alpha-ketoglutarate is converted to succinyl-CoA. During this same step, one molecule of __________ is produced.

Possible Answers:

Correct answer:

Explanation:

The enzyme alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate (5 carbons) to succinyl-CoA (4 carbons). During this step, one carbon is lost as carbon dioxide and one molecule of  is produced. This step is the second, and last step in the Krebs cycle in which carbon dioxide is formed. Recall that the starting material, oxaloacetate, is also 4 carbons long.

Example Question #6 : Other Citric Acid Cycle Concepts

How many molecules of  and , respectively, are produced during the conversion of citrate to oxaloacetate?

Possible Answers:

 . . . 

 . . . 

 . . . 

 . . . 

Correct answer:

 . . . 

Explanation:

During the step in which one of the carbons of isocitrate is lost as carbon dioxide, one molecule  is also produced. This reaction is catalyzed by isocitrate dehydrogenase. This leaves a five-carbon molecule known as alpha-ketoglutarate. In the next step, alpha ketoglutarate dehydrogenase acts upon alpha-ketoglutarate and a carbon is lost as carbon dioxide and another molecule of  is produced. Later in the cycle, succinate dehydrogenase catalyzes the conversion of succinate to fumarate. This reaction produces one molecule of . The enzyme fumarase then converts fumarate to malate. The final step in the citric acid cycle is the regeneration of oxaloacetate from malate. Malate dehydrogenase catalyzes this reaction, which produces the third molecule of . Note that this is for one turn of the citric acid cycle i.e., for one molecule of acetyl-CoA. Each molecule of glucose yields two molecules of acetyl-CoA via glycolysis and pyruvate dehydrogenase complex.

Example Question #7 : Other Citric Acid Cycle Concepts

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

Possible Answers:

In eukaryotes, the cleavage of citrate to form acetyl-CoA takes place in the cytosol

Anaplerotic reactions remove metabolites from the citric acid cycle 

Although some citric acid cycle intermediates are precursors to other biosynthetic pathways, no other biosynthetic pathways produce citric acid cycle intermediates

All organisms except archaea have a complete citric acid cycle

Correct answer:

In eukaryotes, the cleavage of citrate to form acetyl-CoA takes place in the cytosol

Explanation:

Some catabolic pathways do indeed make citric acid cycle intermediates; for example, plants and bacteria use phospoenolpyruvate carboxylase to create oxaloacetate from phosphoenolypyruvate. Anaplerotic reactions refill the citric acid cycle with intermediates, rather than remove them. Some archaea have a complete citric acid cycle; it is the bacteria that mostly do not have a complete cycle. In eukaryotes, citrate cleavage does indeed take place in the cytosol; that citrate is transported to the cytosol from mitochondria, and the acetyl-CoA can be used for fatty acid synthesis.

Example Question #8 : Other Citric Acid Cycle Concepts

Which of the following most clearly states the main purpose of the citric acid cycle?

Possible Answers:

Helps produce energy for prokaryotic or eukaryotic cells via anaerobic fermentation

Takes in ammonia, a byproduct of amino acid catabolism, and converts it into harmless urea

Helps produce energy for eukaryotic cells when oxygen is abundant

Contains hydrolytic enzymes to degrade and recycle organic compounds

Breaks down fatty acids through a cyclical series of steps that produces acetyl-CoA

Correct answer:

Helps produce energy for eukaryotic cells when oxygen is abundant

Explanation:

The citric acid cycle is a series of reactions that occur within the matrix of the mitochondria. With every turn of the cycle, one acetyl-CoA molecule enters, and a variety of molecules leave. These leaving molecules include , and ATP. The acetyl-CoA that enters the cycle is derived from other cellular pathways, such as beta-oxidation and glycolysis. In this way, the citric acid cycle serves as a conduit by which metabolites from other pathways can be broken down to ultimately provide energy for the cell.

Since the citric acid cycle occurs in mitochondria, only eukaryotes are capable of performing this process. Also, the citric acid cycle is not responsible for the production of urea from ammonia. Rather, it is the urea cycle that performs this role. It is worth noting, however, that the citric acid cycle and the urea cycle are energetically linked by the aspartate-argininosuccinate shunt. This is because these two cycle share a few common intermediates, and the citric acid cycle can help to offset the demanding energy requirements of the urea cycle.

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