Passage
ATP synthase, also known as Complex V, produces the majority of the ATP in eukaryotic cells. It is a transmembrane protein encoded by both nuclear and mitochondrial genes in humans, and is found in the inner mitochondrial membrane. Complex V consists of over 20 subunits, including three sets of heterodimers arranged in a circle in the mitochondrial matrix. Each dimer contains an α subunit and a β subunit, and each β subunit exists in one of three conformations: one that binds ATP, one that binds ADP and inorganic phosphate, and one that binds neither.As protons flow from the intermembrane space through Complex V and into the matrix, they cause the transmembrane γ subunit to rotate and interact with each β subunit in turn. Four protons provide enough energy for rotation from one β subunit to the next, and each rotation causes a β subunit to release a new ATP molecule as it changes from the ATP-binding to the nonbinding conformation (Figure 1) . Thus 12 protons produce 3 ATP molecules each time the γ subunit makes a complete revolution. Every NADH molecule that enters the electron transport chain (ETC) pumps 10 protons into the intermembrane space for use by Complex V whereas only 6 protons are pumped per FADH₂ molecule.
Figure 1 Depictions of ATP synthase viewed from the side (left) and the bottom (right) Researchers visualized the rotation of the γ subunit by attaching it to a fluorescently labeled actin filament. The observed rotation over time is shown in Figure 2.
Figure 2 Counterclockwise rotation of the γ subunit of ATP synthase (bottom view) over time in seconds as visualized by a fluorescent labelAn L156P mutation in subunit A of Complex V is linked to neuropathy, ataxia, and retinal degeneration. Although the exact molecular bases of these pathological states are unknown, the following hypotheses are currently under study:Hypothesis 1L156P Complex V functions normally but is less abundant than wild-type Complex V.Hypothesis 2The mutation partially decouples proton flow from ATP synthesis.Hypothesis 3The mutation leads to increased oxidative stress from the ETC, resulting in apoptosis.
Adapted from Kucharczyk R, Zick M, Bietenhader M, et al. Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches. Biochim Biophys Acta. 2009;1793(1) :186-99.
-If the pathological effects observed in patients with the L156P mutation are caused by decreased Complex V activity, the effects could be best counteracted by a drug that increases which of the following?

Question

Passage
ATP synthase, also known as Complex V, produces the majority of the ATP in eukaryotic cells. It is a transmembrane protein encoded by both nuclear and mitochondrial genes in humans, and is found in the inner mitochondrial membrane. Complex V consists of over 20 subunits, including three sets of heterodimers arranged in a circle in the mitochondrial matrix. Each dimer contains an α subunit and a β subunit, and each β subunit exists in one of three conformations: one that binds ATP, one that binds ADP and inorganic phosphate, and one that binds neither.As protons flow from the intermembrane space through Complex V and into the matrix, they cause the transmembrane γ subunit to rotate and interact with each β subunit in turn. Four protons provide enough energy for rotation from one β subunit to the next, and each rotation causes a β subunit to release a new ATP molecule as it changes from the ATP-binding to the nonbinding conformation (Figure 1) . Thus 12 protons produce 3 ATP molecules each time the γ subunit makes a complete revolution. Every NADH molecule that enters the electron transport chain (ETC) pumps 10 protons into the intermembrane space for use by Complex V whereas only 6 protons are pumped per FADH₂ molecule.

Figure 1 Depictions of ATP synthase viewed from the side (left) and the bottom (right) Researchers visualized the rotation of the γ subunit by attaching it to a fluorescently labeled actin filament. The observed rotation over time is shown in Figure 2.

Figure 2 Counterclockwise rotation of the γ subunit of ATP synthase (bottom view) over time in seconds as visualized by a fluorescent labelAn L156P mutation in subunit A of Complex V is linked to neuropathy, ataxia, and retinal degeneration. Although the exact molecular bases of these pathological states are unknown, the following hypotheses are currently under study:Hypothesis 1L156P Complex V functions normally but is less abundant than wild-type Complex V.Hypothesis 2The mutation partially decouples proton flow from ATP synthesis.Hypothesis 3The mutation leads to increased oxidative stress from the ETC, resulting in apoptosis.
Adapted from Kucharczyk R, Zick M, Bietenhader M, et al. Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches. Biochim Biophys Acta. 2009;1793(1) :186-99.
-If the pathological effects observed in patients with the L156P mutation are caused by decreased Complex V activity, the effects could be best counteracted by a drug that increases which of the following?

Quizplus · Accepted Answer

11ecba2d_0e3e_95e7_a3bf_bd3612b958dd_MD0008_00 The activity of Complex V depends on the number of protons available to flow through it. A decrease in activity could be counteracted by an increase in proton concentration in the intermembrane space, which is dependent upon the number of NADH and FADH₂ molecules that enter the ETC. NADH pumps more protons into the intermembrane space than FADH₂ does, so an increase in the number of NADH molecules entering the ETC would yield the greatest increase in Complex V activity. Glycolysis produces two cytosolic NADH molecules per glucose consumed, but these are unable to enter the mitochondrial matrix directly. Instead, the cytosolic NADH must transfer its electrons to the matrix through an intermediate. This can be achieved most efficiently by reducing oxaloacetate to malate, which can enter the mitochondrial matrix (malate-aspartate shuttle). In the matrix, malate can be oxidized back to oxaloacetate as part of the citric acid cycle, generating mitochondrial NADH in the process. This process effectively transfers cytosolic NADH to the mitochondria, increasing the total proton output by the ETC. Therefore, increased malate-aspartate shuttle activity would stimulate Complex V. (Choice B) The energy released when protons flow through Complex V is partially due to the difference in charge across the inner membrane. Decreasing the charge difference by adding potassium to the matrix would cause a reduction in ATP synthesis. (Choice C) ATP synthase is driven by protons flowing from a high concentration in the intermembrane space toward a low concentration in the matrix. Equalizing the pH of the intermembrane space and the mitochondria by raising the pH of the intermembrane space would destroy the proton gradient and stop ATP synthesis. (Choice D) The glycerol 3-phosphate shuttle effectively converts cytosolic NADH to mitochondrial FADH₂ by oxidizing glycerol 3-phosphate to DHAP, not by reducing it. FADH₂ pumps fewer protons than NADH does, so this system is less efficient than malate translocation at stimulating Complex V activity. Educational objective: Complex V activity depends on proton availability. Proton concentration in the intermembrane space can be increased by increasing the number of NADH molecules that enter the electron transport chain. NADH from glycolysis can indirectly enter the mitochondrial matrix by first passing its electrons to oxaloacetate, forming malate.

Passage ATP Synthase, Also Known as Complex V, Produces the Majority