Irreversible step in citric acid cycle is catalyzed by ?
**Core Concept**
The citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) is a key metabolic pathway that generates energy through the oxidation of acetate derived from carbohydrates, fats, and proteins. It is a crucial step in cellular respiration, producing NADH and FADH2 as byproducts, which are then used in the electron transport chain to produce ATP.
**Why the Correct Answer is Right**
The irreversible step in the citric acid cycle is catalyzed by the enzyme succinyl-CoA synthetase, also known as succinic thiokinase. This enzyme catalyzes the conversion of succinyl-CoA to succinate, generating one molecule of ATP and one molecule of CoA. This reaction is irreversible because the conversion of succinyl-CoA to succinate is highly energetically favorable, and the enzyme does not have the ability to catalyze the reverse reaction.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the enzyme that catalyzes the irreversible step in the citric acid cycle. While isocitrate dehydrogenase is an enzyme involved in the citric acid cycle, it catalyzes a reversible reaction.
**Option B:** This option is incorrect because it is not a specific enzyme, and it does not accurately describe the irreversible step in the citric acid cycle. While the citric acid cycle is a critical pathway for energy production, it is not the enzyme that catalyzes the irreversible step.
**Option D:** This option is incorrect because it is not a specific enzyme, and it does not accurately describe the irreversible step in the citric acid cycle. While the citric acid cycle is a complex pathway, it is not the enzyme that catalyzes the irreversible step.
**Clinical Pearl / High-Yield Fact**
The citric acid cycle is a critical pathway for energy production in the cell, and it is regulated by various mechanisms, including feedback inhibition and allosteric control. Understanding the irreversible step in the citric acid cycle is essential for understanding how the cell regulates energy production and how various diseases, such as cancer, affect metabolic pathways.
**Correct Answer: C. Succinyl-CoA synthetase.**