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.
-OCTN2 facilitates transport of L-carnitine by which mechanism?

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.
-OCTN2 facilitates transport of L-carnitine by which mechanism?

Quizplus · Accepted Answer

11ecba2d_0ef5_89e2_a3bf_bba31aeb946c_MD0008_00 Cell membranes contain multiple types of transport proteins that facilitate the controlled movement of molecules in and out of the cell. The concentration of molecules within the cell is usually different than the concentration of molecules outside the cell, and molecules may travel down their concentration gradient (from high to low concentration) or against their concentration gradient (from low to high concentration). Transport across a membrane down a molecule's concentration gradient is referred to as passive transport and requires no energy input. In contrast, transport against a molecule's gradient requires energy input and is called active transport. The required energy can be provided by ATP hydrolysis or by the simultaneous transport of another molecule down its concentration gradient. Transport of a single molecule is called uniport. When two molecules are transported in the same direction, it is known as symport, and when two molecules are transported in opposite directions, it is called antiport. The passage states that L-carnitine moves into the cell through OCTN2 against its concentration gradient, so OCTN2 facilitates active transport of L-carnitine. The passage further states that the energy needed for L-carnitine transport is provided by OCTN2 simultaneously transporting Na⁺ into the cell. Because both Na⁺ and L-carnitine are transported in the same direction (into the cell), OCTN2 facilitates symport. Therefore, OCTN2 transports L-carnitine into the cell by active symport. (Choice B) Antiport occurs when two molecules are transported through a channel in opposite directions (ie, one into the cell and one out of the cell). Na⁺ and L-carnitine both go into the cell through OCTN2, so OCTN2 facilitates symport. (Choices C and D) Passive transport is the transport of molecules down their concentration gradient. L-carnitine is transported against its concentration gradient, so it requires active transport. Educational objective: Many molecules enter and exit cells through transport proteins. Molecules may travel down their concentration gradient (passive transport, no energy required) or against their concentration gradient (active transport, energy required). Transport proteins facilitate uniport if they transport one molecule, symport if they transport two molecules in the same direction, and antiport if they transport two molecules in opposite directions.

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