Which among the following controls is an allosteric inhibitor of TCA cyclea) Pyruvate dehydrogenaseb) Ketoglutarate dehydrogenasec) Isocitrate dehydrogenased) Malate dehydrogenase
**Core Concept**
The tricarboxylic acid (TCA) cycle is a crucial metabolic pathway that generates energy and biosynthetic intermediates. The TCA cycle is tightly regulated by various allosteric modulators, including inhibitors, to ensure proper metabolic homeostasis.
**Why the Correct Answer is Right**
Ketoglutarate dehydrogenase is a key enzyme in the TCA cycle that catalyzes the conversion of ketoglutarate to succinyl-CoA. The enzyme is inhibited by ATP and NADH, which are products of the TCA cycle. This negative feedback regulation ensures that the TCA cycle is not over-activated when energy levels are high. In addition, ketoglutarate dehydrogenase is also inhibited by pyruvate, which is an allosteric inhibitor. This inhibition is mediated by the pyruvate dehydrogenase complex, which is a separate enzyme complex that converts pyruvate to acetyl-CoA, a substrate for the TCA cycle.
**Why Each Wrong Option is Incorrect**
**Option A:** Pyruvate dehydrogenase is not an allosteric inhibitor of the TCA cycle; rather, it is an enzyme complex that catalyzes the conversion of pyruvate to acetyl-CoA, which is a substrate for the TCA cycle.
**Option C:** Isocitrate dehydrogenase is not inhibited by allosteric modulators; rather, it is a key enzyme in the TCA cycle that catalyzes the conversion of isocitrate to α-ketoglutarate.
**Option D:** Malate dehydrogenase is not inhibited by allosteric modulators; rather, it is a key enzyme in the TCA cycle that catalyzes the conversion of malate to oxaloacetate.
**Clinical Pearl / High-Yield Fact**
The regulation of the TCA cycle is crucial for maintaining proper metabolic homeostasis. Understanding the allosteric inhibitors of the TCA cycle, such as pyruvate, is essential for appreciating the complex interplay between various metabolic pathways.
**Correct Answer:** B. Ketoglutarate dehydrogenase is an allosteric inhibitor of the TCA cycle.