Which of the following molecules could decrease the activity of one or more Krebs cycle enzymes?

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11ecba2d_0e6a_12f1_a3bf_0130f28cd833_MD0008_00 The Krebs cycle, also known as the citric acid cycle, is tightly regulated to produce ATP at the optimal rate for the needs of the cell. The cycle includes three irreversible steps, catalyzed by the enzymes citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. The activity of each of these enzymes can be increased or decreased when specific molecules bind to the enzymes and alter their shape (allostery). Each of these molecules serves as an indicator of the energy state of the cell. Molecules that are associated with depleted energy (eg, ADP) and muscle contractions (eg, calcium) indicate a need for additional ATP production. These molecules allosterically activate Krebs cycle enzymes to meet the demand for energy. Conversely, molecules that indicate that the Krebs cycle is producing sufficient energy allosterically inhibit the enzymes. These molecules include several products of the Krebs cycle and the electron transport chain such as ATP, citrate, succinyl-CoA, and NADH. NADH decreases the activity of all three of the regulated Krebs cycle enzymes. (Choice A) Glucagon is a hormone that upregulates glucose production in the liver through glycogen breakdown (glycogenolysis) and gluconeogenesis. The enzymes of the citric acid cycle are not under hormonal control. (Choice C) Acetyl-CoA reacts with oxaloacetate to form citrate at the beginning of the Krebs cycle. Its presence increases the overall rate of the cycle by increasing substrate concentration. (Choice D) ADP is produced when ATP is hydrolyzed, and it signals to the cell that energy has been depleted. It allosterically activates the Krebs cycle enzymes citrate synthase and isocitrate dehydrogenase, thereby increasing their activity. Educational objective: The Krebs cycle is allosterically regulated at the irreversible steps catalyzed by citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. The enzymes are inhibited by NADH, ATP, citrate, and succinyl-CoA, and are activated by ADP and calcium. The Krebs cycle enzymes are not under hormonal control.

Which of the Following Molecules Could Decrease the Activity of One