Rate limiting step in cholesterol synthesis –
**Core Concept**
The rate-limiting step in cholesterol synthesis is a critical regulatory point in the mevalonate pathway, which is responsible for producing cholesterol from acetyl-CoA. This step is crucial for maintaining cholesterol homeostasis and is a target for several drugs used to lower cholesterol levels.
**Why the Correct Answer is Right**
The rate-limiting step in cholesterol synthesis is catalyzed by the enzyme HMG-CoA reductase. This enzyme catalyzes the conversion of HMG-CoA to mevalonate, which is the first committed step in the mevalonate pathway. HMG-CoA reductase is a key regulatory enzyme, and its activity is tightly controlled by feedback inhibition from cholesterol and other intermediates in the pathway. The activity of HMG-CoA reductase is also influenced by various hormones, including insulin and glucagon, which play important roles in regulating cholesterol synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while HMG-CoA synthase is an important enzyme in the mevalonate pathway, it is not the rate-limiting step in cholesterol synthesis.
**Option B:** This option is incorrect because HMG-CoA lyase is an enzyme involved in the breakdown of HMG-CoA, rather than its synthesis.
**Option C:** This option is incorrect because mevalonate kinase is an enzyme involved in the conversion of mevalonate to phosphomevalonate, which is a later step in the mevalonate pathway.
**Option D:** This option is incorrect because acetyl-CoA carboxylase is an enzyme involved in the synthesis of fatty acids, rather than cholesterol.
**Clinical Pearl / High-Yield Fact**
The mevalonate pathway is a critical target for several drugs used to lower cholesterol levels, including statins, which inhibit HMG-CoA reductase and reduce cholesterol synthesis.
**Correct Answer:** C. Mevalonate kinase is not the rate limiting step in cholesterol synthesis rather mevalonate kinase is involved in the later step of the mevalonate pathway.