Which enzyme catalyzes the rate-limiting step of cholesterol synthesis and is the primary target of statin drugs?

Explanation

## Rate-Limiting Enzyme of Cholesterol Synthesis **Key Point:** HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate, the committed and rate-limiting step of the entire cholesterol biosynthetic pathway. **High-Yield:** This enzyme is the primary target of statins (atorvastatin, simvastatin, rosuvastatin), which competitively inhibit HMG-CoA reductase and reduce cholesterol synthesis by up to 50%. **Mnemonic:** **HMG-CoA Reductase = Rate-Limiting Enzyme = Statin Target** — Remember: HMG = 3-hydroxy-3-methylglutaryl; this is the bottleneck of cholesterol production. ## Regulation of HMG-CoA Reductase The enzyme is regulated by: - **Allosteric inhibition:** by cholesterol and its metabolites (negative feedback) - **Transcriptional regulation:** SREBP (Sterol Regulatory Element Binding Protein) senses cellular cholesterol and suppresses HMG-CoA reductase gene expression when cholesterol is abundant - **Covalent modification:** phosphorylation by AMP-activated protein kinase (AMPK) inactivates the enzyme during energy stress - **Protein degradation:** high cholesterol triggers ubiquitination and proteasomal degradation of HMG-CoA reductase ## Comparison of Early Cholesterol Synthesis Enzymes | Enzyme | Substrate | Product | Role | Regulation | | --- | --- | --- | --- | --- | | HMG-CoA synthase | Acetyl-CoA + Acetoacetyl-CoA | HMG-CoA | Condensation step | Mild feedback inhibition | | **HMG-CoA reductase** | **HMG-CoA** | **Mevalonate** | **Rate-limiting** | **Strong feedback inhibition + SREBP control** | | Mevalonate kinase | Mevalonate | Mevalonate-5-phosphate | Early committed step | Mild regulation | | Squalene synthase | Farnesyl-PP | Squalene | Mid-pathway | Feedback inhibition | **Clinical Pearl:** Statins are among the most widely prescribed drugs globally because they safely and effectively lower LDL cholesterol by blocking this single rate-limiting enzyme.