Which of the following enzymes is common to the synthesis of cholesterol and ketone bodies:
**Core Concept**
The question tests the understanding of the biochemical pathways involved in cholesterol synthesis and ketone body production. Both processes involve the breakdown of acetyl-CoA, a key intermediate in metabolism. The correct enzyme is a key regulator of this pathway, catalyzing a crucial step in both processes.
**Why the Correct Answer is Right**
HMG-CoA synthase is the correct answer. It catalyzes the condensation of acetyl-CoA and acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), a common intermediate in both cholesterol synthesis and ketone body production. In cholesterol synthesis, HMG-CoA is further processed to form mevalonate, which is then converted into cholesterol. In ketone body production, HMG-CoA is reduced to form acetoacetate, which is then converted into ketone bodies. This enzyme is a key regulator of both pathways, and its activity is tightly controlled by feedback inhibition from cholesterol and other products of the pathway.
**Why Each Wrong Option is Incorrect**
* **Option A:** HMG-CoA reductase is a key enzyme in cholesterol synthesis, but it is not common to the synthesis of ketone bodies. It catalyzes the conversion of HMG-CoA to mevalonate, which is then converted into cholesterol.
* **Option B:** Pyruvate dehydrogenase is an enzyme that catalyzes the conversion of pyruvate to acetyl-CoA, a key intermediate in both cholesterol synthesis and ketone body production. However, it is not a common enzyme to both pathways.
* **Option D:** Acetyl-CoA carboxylase is an enzyme that catalyzes the carboxylation of acetyl-CoA to form malonyl-CoA, which is then converted into fatty acids. It is not directly involved in cholesterol synthesis or ketone body production.
**Clinical Pearl / High-Yield Fact**
The regulation of HMG-CoA synthase is a key point of control in both cholesterol synthesis and ketone body production. Feedback inhibition from cholesterol and other products of the pathway tightly regulates its activity, ensuring that both pathways are coordinated to maintain energy homeostasis.
**Correct Answer: C. HMG-CoA synthase**