The amino acid methionine can be converted to succinyl-CoA to enter the citric acid cycle. How many NADH molecules can be produced by each methionine molecule that enters the citric acid cycle by this pathway?

Question

The amino acid methionine can be converted to succinyl-CoA to enter the citric acid cycle.  How many NADH molecules can be produced by each methionine molecule that enters the citric acid cycle by this pathway?

Quizplus · Accepted Answer

11ecba2d_0eed_9a97_a3bf_a5c880281dd2_MD0008_00 The citric acid cycle is a metabolic process that oxidizes a series of molecules to produce the reduced electron carriers NADH and FADH₂ as well as the high-energy molecule GTP. The reduced electron carriers then enter the electron transport chain and contribute the electrons necessary for ATP production. Various molecules can enter the citric acid cycle at different points, and each produces a distinct number of reduced electron carriers, depending on the number of oxidative steps that remain in the cycle at the point of entry. According to the question, methionine enters the citric acid cycle by conversion to succinyl-CoA. At this point in the cycle, two oxidative steps remain. The first step is the conversion of succinate to fumarate, which produces one FADH₂ molecule. The second step is conversion of malate to oxaloacetate, which produces one NADH molecule. All other oxidative steps in the cycle require precursors to succinyl-CoA, which do not form in the metabolism of methionine. Based on this, entry of methionine into the citric acid cycle will yield one NADH molecule. (Choice B) To produce 2 NADH molecules, methionine would have to be converted to α-ketoglutarate and enter the citric acid cycle. At this point in the cycle, three oxidative steps (two of which produce NADH) would remain. However, the carbon atoms in methionine enter the cycle as succinyl-CoA. (Choice C) To produce 3 NADH molecules, the point of entry into the citric acid cycle would need to be a precursor to α-ketoglutarate, such as citrate, isocitrate, or acetyl-CoA. (Choice D) Complete oxidation of pyruvate in the citric acid cycle yields four NADH molecules, one when pyruvate is converted to acetyl-CoA and three more in the citric acid cycle. Accordingly, methionine would have to be converted to pyruvate to produce 4 NADH molecules. Educational objective: Different molecules can enter the citric acid cycle at different points. The cycle oxidizes these molecules to produce the reduced electron carriers NADH and FADH₂. The number of electron carriers produced depends on the number of oxidative steps remaining in the cycle at the point of entry.

The Amino Acid Methionine Can Be Converted to Succinyl-CoA to Enter