Regarding the β-oxidation of fatty acids in mitochondria, all of the following statements are correct EXCEPT:

## Understanding β-Oxidation Mechanism **Key Point:** The first oxidation step in β-oxidation uses FAD (not NAD⁺) as the electron acceptor, producing FADH₂. This is a critical mechanistic detail frequently tested in NEET PG. ### Correct Statements (Options 0, 1, 2): | Statement | Details | |-----------|----------| | **Fatty acid activation** | Free fatty acids are activated to acyl-CoA by fatty acyl-CoA synthetase in the cytoplasm; this is the first committed step | | **CPT-I is rate-limiting** | CPT-I catalyzes transfer of the acyl group onto carnitine, allowing transport across the inner mitochondrial membrane; this is the major control point | | **Each β-oxidation cycle** | Produces 1 FADH₂ (from acyl-CoA dehydrogenase), 1 NADH (from 3-hydroxyacyl-CoA dehydrogenase), and 1 acetyl-CoA | ### Why Option 3 is WRONG: **High-Yield:** The **first oxidation step** is catalyzed by **acyl-CoA dehydrogenase** and uses **FAD** (not NAD⁺) as the electron acceptor: $$\text{Acyl-CoA} + \text{FAD} \xrightarrow{\text{acyl-CoA dehydrogenase}} \text{Enoyl-CoA} + \text{FADH}_2$$ The **second oxidation step** (hydration followed by oxidation) uses NAD⁺: $$\text{3-Hydroxyacyl-CoA} + \text{NAD}^+ \xrightarrow{\text{3-hydroxyacyl-CoA DH}} \text{3-Ketoacyl-CoA} + \text{NADH}$$ **Mnemonic:** **FAD First, NAD Next** — the first dehydrogenation uses FAD; the second uses NAD⁺. ### The Four Enzymes of β-Oxidation: 1. **Acyl-CoA dehydrogenase** → FAD → FADH₂ 2. **Enoyl-CoA hydratase** → adds water 3. **3-Hydroxyacyl-CoA dehydrogenase** → NAD⁺ → NADH 4. **Thiophorase (3-ketoacyl-CoA thiolase)** → releases acetyl-CoA [cite:Lehninger Principles of Biochemistry Ch 21]