SB-BB

Passage 12

Fatty acid synthase (FASN) is a key cytoplasmic enzyme responsible for the synthesis of long-chain fatty acids. Abnormal FASN expression is associated with tumor cell progression in human cancer cells, and as such, FASN is being investigated as a target for cancer therapy through development of specific inhibitors such as C75, a substrate analog.

In addition to the FASN pathway, a mitochondrial fatty acid synthesis pathway has been identified. The key enzyme of this pathway is the mitochondrial ketoacyl-acyl carrier protein synthase (mtKAS). mtKAS is also involved in the synthesis of lipoic acid (LA), a cofactor for several metabolic enzymes.

The deletion of the gene encoding for mtKAS results in dissipation of the electrochemical gradient in mitochondria due to an increase in membrane permeability. Since mtKAS shares conserved catalytic domains with FASN that are involved in binding to C75, researchers investigated the effects of C75 on mtKAS function.

Experiment 1

293T cells were treated with three different concentrations of C75, and inhibition of mtKAS was analyzed by measuring cell viability and mitochondrial membrane potential (MMP) as a function of time.

Figure 1 Effect of C75 treatment on cell viability (A) and mitochondrial membrane potential (B) at different concentrations and different time points

Experiment 2

Cells were treated with siRNA to knock down FASN, mtKAS, or both, and cell viability and MMP were measured.

Figure 2 Results of siRNAs studies on (A) cell viability and (B) MMP

Questions

91. Which conclusion about the data shown in Figure 1 is valid?

  1. Changes in the MMP likely affect cell viability.
  2. Changes in cell viability likely affect the MMP.
  3. MMP is unrelated to cell viability.
  4. Cell viability can be restored by decreasing MMP.

92. In a follow-up experiment, researchers treated cells with 50 µM of C75 in the presence or absence of the free radical scavenger N-acetyl cysteine (NAC) and then measured cell viability. The most likely rationale for this experiment was to test whether:

  1. mitochondrial respiration plays a role in mediating the effects of C75.
  2. reactive oxygen species play a role in mediating the effects of C75.
  3. transport proteins play a role in mediating the effects of C75.
  4. G protein signaling plays a role in mediating the effects of C75.

93. Which control experiments should be included in the design of Experiment 2 to validate the results shown in Figure 2?

  1. Evaluating the protein levels of FASN and mtKAS following siRNA treatment
  2. Evaluating the siRNAs specificity for FASN and mtKAS by assessing the cellular levels of unrelated proteins
  3. Evaluating the siRNAs specificity by showing that a non-specific siRNA has no effects on the protein levels of FASN and mtKAS
  1. I and II only
  2. I and III only
  3. II and III only
  4. I, II and III

94. Which metabolic reaction is most likely affected by treatment of cells with C75? The reaction that converts:

  1. fructose 6-phosphate to fructose 1,6-bisphosphate
  2. fumarate to malate
  3. pyruvate to acetyl-CoA
  4. oxaloacetate to phosphoenolpyruvate

95. Based on the information from the passage, the deletion of mtKAS most likely results in decreased level of:

  1. protons in the mitochondrial matrix.
  2. protons in the mitochondrial intermembrane space.
  3. electrons in the mitochondrial matrix.
  4. electrons in the mitochondrial intermembrane space.

96. Researchers studied the effect of lipoic acid on mtKAS expression. The results of their investigation are shown. (Note: * indicates p < 0.05 and ** indicates p < 0.01.)

Based on these results, lipoic acid regulates mtKAS expression through:

  1. negative feedback at the level of transcription.
  2. negative feedback at the level of translation.
  3. positive feedback at the level of transcription.
  4. positive feedback at the level of translation.

97. A partial sequence of the reactions in the citric acid cycle is shown.

Which compound is the intermediate X?

  1. Succinate
  2. Fumarate
  3. Malate
  4. Oxaloacetate

98. In the pentose phosphate pathway, which enzyme catalyzes the production of 6-phosphogluconolactone?

  1. Glucose 6-phosphate dehydrogenase
  2. 6-Phosphogluconolactonase
  3. Phosphogluconate dehydrogenase
  4. Glycerol 3-phosphate dehydrogenase

99. A partial DNA sequence of the coding strand of a gene is shown.

5–GACATGGACTCGCTA–3

Which sequence corresponds to the mRNA for this DNA sequence?

  1. 5–GACAUGGACUCGCUA–3
  2. 5–CUGUACCUGAGCGUA–3
  3. 5–UAGCGAGUCCAUGUC–3
  4. 5–AUCGCUCAGGUACAG–3

100. Which technique CANNOT be used to analyze gene expression?

  1. Western blotting
  2. Northern blotting
  3. Southern blotting
  4. Reverse transcription PCR

Answers

Question 91 Solution: The correct answer is A. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is A because the passage changes in MMP are noticed at 2 hours post-treatment, whereas changes in cell viability appear after 12 hours of treatment. This suggests that changes in MMP precede and likely affect cell viability. It is a Data-based and Statistical Reasoning question because you must examine the temporal patterns of changes in cell viability and MMP shown in Figure 1 and determine which conclusion is valid.
Question 92 Solution: The correct answer is B. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is B because free radical scavengers such as NAC mitigate the effects of reactive oxygen species (ROS), thereby indicating if ROS were involved in mediating the effects of C75. It is a Reasoning about the Design and Execution of Research question because you must determine the appropriate rationale for the design of an experiment.
Question 93 Solution: The correct answer is D. This is correct. This Biology question falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is D because in order to connect the effects of siRNA to the observed results in Figure 2, researchers must ascertain that I. siRNA treatment results in knocking down the protein levels of FASN and mtKAS; II. each siRNA is specific to its target protein and does not interfere with expression of other unrelated prpteins; and III. a non-specific scrambled siRNA does not interfere with the expression of FASN or mtKAS. It is a Reasoning about the Design and Execution of Research question because you must identify the proper controls for the validity of the experiment.
Question 94 Solution: The correct answer is C. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is C because the passage states that mtKAS is involved in the synthesis of lipoic acid and Figure 1 shows that mtKAS is inhibited by C75. Therefore, treatment of cells with C75 results in lower cellular level of lipoic acid, a cofactor for the enzyme pyruvate dehydrogenase which catalyzes the conversion of pyruvate to acetyl-CoA. It is a Knowledge of Scientific Concepts and Principles question because you must recall which metabolic enzyme that has lipoic acid as its cofactor catalyzes the conversion of pyruvate to acetyl-CoA.
Question 95 Solution: The correct answer is B. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is B because the passage states that deletion of the gene encoding for mtKAS results in dissipation of the electrochemical gradient generated by the electron transport chain. The electron transport chain uses the free energy from redox reactions to pump protons from mitochondrial matrix to the intermembrane space thereby generating an electrochemical gradient across the inner mitochondrial membrane. Therefore any event that causes the dissipation of the electrochemical gradient across the inner mitochondrial membrane will result in decreased level of protons in mitochondrial intermembrane space. It is a Scientific Reasoning and Problem Solving question because you must work with the scientific model of chemiosmosis to answer the question.
Question 96 Solution: The correct answer is C. This is correct. This Biology question falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is C because increasing the concentration of R-LA, a downstream product of mtKAS, results in an increase in the level of mtKAS mRNA, indicating a positive feedback loop at the level of transcription. It is a Data-based and Statistical Reasoning question because to answer the question you must analyze the data presented in the figure shown.
Question 97 Solution: The correct answer is B. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is B because fumarate is the intermediate compound between succinate and malate in the citric acid cycle. It is a Knowledge of Scientific Concepts and Principles question because you must recall the structures and the sequences of the intermediates in metabolic pathways.
Question 98 Solution: The correct answer is A. This is correct. This Biochemistry question falls under the content category “Principles of bioenergetics and fuel molecule metabolism.” The answer to this question is A because Glucose 6-phosphate dehydrogenase catalyzes the conversion of glucose 6-phosphate to 6-phosphogluconolactone in the pentose phosphate pathway. It is a Knowledge of Scientific Concepts and Principles question because you must recall the substrates and products of metabolic enzymes.
Question 99 Solution: The correct answer is A. This is correct. This Biology question falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is A because for any given gene, the nucleotide sequence of both the coding strand and the mRNA are complementary to the sequence of the template strand. It is a Scientific Reasoning and Problem Solving question because you must identify the correct mRNA sequence based on the model for transcription of genetic materials.
Question 100 Solution: The correct answer is C. This is correct. This Biology question falls under the content category “Transmission of genetic information from the gene to the protein.” The answer to this question is C because Southern blotting is a technique used to detect a particular sequence in a sample of DNA. It is a Reasoning about the Design and Execution of Research question because you must identify the appropriate techniques to conduct research in biological