Passage
Whole grains have been shown to reduce the risk of cardiovascular disorders. A team of scientists discovered that the compound 5-aminovaleric acid betaine (5-AVAB) was present at high levels in cardiomyocytes of mice given a diet rich in whole grains and decided to investigate its role in metabolism. Preliminary evidence indicated that 5-AVAB did not alter the citric acid cycle in these cardiomyocytes, but that it did alter fatty acid metabolism.The plasma membrane transport protein organic cation/carnitine transporter 2 (OCTN2) transports extracellular L-carnitine into the cell against its concentration gradient using the energy provided by the simultaneous transport of Na⁺ into the cell. Cellular L-carnitine is required for the transport of fatty acids from the cytosol into the mitochondria. Based on the structures of 5-AVAB and L-carnitine (Figure 1) , scientists hypothesized that 5-AVAB may interfere with OCTN2-mediated transport of L-carnitine.
Figure 1 Structures of L-carnitine and 5-AVABTo test their hypothesis, scientists incubated mouse cardiomyocytes with various concentrations of 5-AVAB for 24 hours. Cells were then provided with the fatty acid palmitate for energy and were deprived of all other energy sources. The cardiomyocytes were also given 50 μM L-carnitine (endogenous extracellular concentration) or 2 mM L-carnitine, and the rate of oxygen consumption was monitored (Figure 2) .
Figure 2 Oxygen consumption rate of cardiomyocytes at various concentrations of 5-AVAB and L-carnitineIn a follow-up experiment, the scientists used siRNA to silence expression of OCTN2 and found that intracellular L-carnitine levels were similarly depleted in siRNA-treated cells as in cells treated with 100 μM 5-AVAB.
Adapted from Kärkkäinen O, Tuomainen T, Koistinen V, et al. Whole grain intake associated molecule 5-aminovaleric acid betaine decreases ß-oxidation of fatty acids in mouse cardiomyocytes. Sci Rep. 2018;8(1) :13036.
-Under which of the conditions tested in Figure 2 would cardiomyocytes have the highest ADP/ATP ratio?

Question

Passage
Whole grains have been shown to reduce the risk of cardiovascular disorders. A team of scientists discovered that the compound 5-aminovaleric acid betaine (5-AVAB) was present at high levels in cardiomyocytes of mice given a diet rich in whole grains and decided to investigate its role in metabolism. Preliminary evidence indicated that 5-AVAB did not alter the citric acid cycle in these cardiomyocytes, but that it did alter fatty acid metabolism.The plasma membrane transport protein organic cation/carnitine transporter 2 (OCTN2) transports extracellular L-carnitine into the cell against its concentration gradient using the energy provided by the simultaneous transport of Na⁺ into the cell. Cellular L-carnitine is required for the transport of fatty acids from the cytosol into the mitochondria. Based on the structures of 5-AVAB and L-carnitine (Figure 1) , scientists hypothesized that 5-AVAB may interfere with OCTN2-mediated transport of L-carnitine.

Figure 1 Structures of L-carnitine and 5-AVABTo test their hypothesis, scientists incubated mouse cardiomyocytes with various concentrations of 5-AVAB for 24 hours. Cells were then provided with the fatty acid palmitate for energy and were deprived of all other energy sources. The cardiomyocytes were also given 50 μM L-carnitine (endogenous extracellular concentration) or 2 mM L-carnitine, and the rate of oxygen consumption was monitored (Figure 2) .

Figure 2 Oxygen consumption rate of cardiomyocytes at various concentrations of 5-AVAB and L-carnitineIn a follow-up experiment, the scientists used siRNA to silence expression of OCTN2 and found that intracellular L-carnitine levels were similarly depleted in siRNA-treated cells as in cells treated with 100 μM 5-AVAB.
Adapted from Kärkkäinen O, Tuomainen T, Koistinen V, et al. Whole grain intake associated molecule 5-aminovaleric acid betaine decreases ß-oxidation of fatty acids in mouse cardiomyocytes. Sci Rep. 2018;8(1) :13036.
-Under which of the conditions tested in Figure 2 would cardiomyocytes have the highest ADP/ATP ratio?

Quizplus · Accepted Answer

11ecba2d_0ef9_8189_a3bf_b380964e09e1_MD0008_00 Adenosine triphosphate (ATP) is the main source of energy for cellular processes. In aerobic organisms, it is primarily produced in the mitochondria using energy from the electron transport chain (ETC) that drives ATP synthesis from ADP and inorganic phosphate (P_i). Therefore, increased ETC activity corresponds to a higher level of ATP and a lower level of ADP whereas decreased ETC activity corresponds to the opposite. The final step in the ETC is reduction of oxygen to form water, so the rate of oxygen consumption serves as a measure of ETC activity. The graph in Figure 2 shows that oxygen consumption is lowest in the presence of 100 μM 5-AVAB and 50 μM L-carnitine. Under these conditions ATP will be synthesized at a slower rate than under any other conditions tested, and ADP will be consumed at a slower rate. Therefore, the ADP/ATP ratio will be highest in cells at 100 μM 5-AVAB and 50 μM L-carnitine. (Choices A and B) At 0 μM or 15 μM 5-AVAB, in the presence of 50 μM L-carnitine, the rate of oxygen consumption is higher than the rate at 100 μM 5-AVAB. Accordingly, the ADP/ATP ratio will be lower at 0 or 15 μM 5-AVAB than at 100 μM 5-AVAB when L-carnitine is held at 50 μM. (Choice D) When L-carnitine is present at 2 mM, oxygen consumption occurs at a higher rate than at any other condition tested. ATP will be produced and ADP will be consumed at the fastest rate under these conditions, yielding the highest ATP/ADP ratio but the lowest ADP/ATP ratio. Educational objective: Aerobic organisms primarily generate ATP using energy provided by the electron transport chain (ETC). The final step in the chain is reduction of oxygen to water, and the rate of oxygen consumption can be used as a measure of ETC activity. Higher ETC activity corresponds to a higher ATP/ADP ratio or a lower ADP/ATP ratio.

Passage Whole Grains Have Been Shown to Reduce the Risk of of Cardiovascular