Regarding the β-oxidation of fatty acids, all of the following statements are TRUE EXCEPT:

Explanation

## Analysis of β-Oxidation Statements ### Correct Statements (Options 0, 1, 2) **Option 0: Activation in cytoplasm** ✓ Fatty acids are indeed activated to acyl-CoA by acyl-CoA synthetase (also called fatty acyl-CoA ligase) in the cytoplasm. This is an ATP-dependent reaction: Fatty acid + CoA + ATP → Acyl-CoA + AMP + PPi [cite:Lehninger Principles of Biochemistry Ch 21] **Option 1: First oxidative step** ✓ Acyl-CoA dehydrogenase catalyzes the first step, introducing a double bond between C-2 (α) and C-3 (β) carbons. This produces an enoyl-CoA intermediate. The reaction is FAD-dependent and generates FADH₂. **Option 2: CPT I regulation** ✓ CPT I is the rate-limiting enzyme for β-oxidation, located on the outer mitochondrial membrane. It catalyzes the transfer of the acyl group from CoA to carnitine. Malonyl-CoA (the first committed intermediate of fatty acid synthesis) is a potent allosteric inhibitor of CPT I, preventing simultaneous fatty acid synthesis and oxidation. ### Incorrect Statement (Option 3) ✗ **Key Point:** Each cycle of β-oxidation produces **ONE molecule of acetyl-CoA and a fatty acyl-CoA shortened by TWO carbons (not four)**. The β-oxidation cycle involves four enzymatic steps: 1. Oxidation (acyl-CoA dehydrogenase) → enoyl-CoA + FADH₂ 2. Hydration (enoyl-CoA hydratase) → 3-hydroxyacyl-CoA 3. Oxidation (3-hydroxyacyl-CoA dehydrogenase) → 3-ketoacyl-CoA + NADH 4. Thiolysis (3-ketoacyl-CoA thiolase) → acetyl-CoA + acyl-CoA (shortened by 2 carbons) Each complete cycle removes only a 2-carbon unit (acetyl-CoA), not a 4-carbon unit. A 16-carbon palmitate requires 7 cycles to yield 8 acetyl-CoA molecules. ### High-Yield Summary Table | Feature | Details | |---------|----------| | **Location** | Mitochondrial matrix (inner membrane for some enzymes) | | **Substrate** | Acyl-CoA (activated fatty acid) | | **Products per cycle** | 1 acetyl-CoA + 1 acyl-CoA (C-2 shorter) + FADH₂ + NADH | | **Rate-limiting enzyme** | CPT I (carnitine palmitoyltransferase I) | | **Regulation** | Malonyl-CoA ↓ CPT I; AMP/ATP ratio ↑ activity | | **Energy yield** | Palmitate (C16) → 8 acetyl-CoA + 7 FADH₂ + 7 NADH | **High-Yield:** The "2-carbon rule" is a frequent exam trap. Students often confuse the 2-carbon removal per cycle with a 4-carbon removal. Remember: each cycle = 1 acetyl-CoA = 2 carbons removed.