29.3 Catabolism of Triacylglycerols: β-Oxidation

Catabolism of Triacylglycerols: β-Oxidation

Fatty acids are one of the body's most energy-dense fuel sources, and β-oxidation is the pathway that unlocks that energy. This process takes place in the mitochondrial matrix, where a repeating cycle of four reactions clips two carbons off a fatty acid chain at a time, generating acetyl-CoA for the citric acid cycle along with reduced coenzymes (FADH₂ and NADH) that feed the electron transport chain.

Understanding β-oxidation means tracking two things simultaneously: the organic chemistry of each step (what bonds break and form) and the bookkeeping of how many acetyl-CoA, FADH₂, and NADH molecules you get from a given fatty acid.

Activation and Transport

Before β-oxidation can begin, a free fatty acid must be activated and moved into the mitochondrion.

1. Activation. Acyl-CoA synthetase, located on the outer mitochondrial membrane, links the fatty acid to coenzyme A (CoA) via a thioester bond. This costs the equivalent of 2 ATP (ATP → AMP + 2 PPᵢ), producing fatty acyl-CoA.
2. Transport via the carnitine shuttle. The inner mitochondrial membrane is impermeable to long-chain acyl-CoA. To cross it, the acyl group is temporarily transferred from CoA to carnitine by carnitine palmitoyltransferase I (CPT-I), shuttled across, then transferred back to a mitochondrial CoA by CPT-II. The fatty acyl-CoA is now in the matrix, ready for oxidation.

Sequence of β-Oxidation Reactions

Each turn of the β-oxidation spiral consists of four steps. All four target the bond between C-2 (the α-carbon) and C-3 (the β-carbon), which is where the pathway gets its name.

1. Oxidation (dehydrogenation). Acyl-CoA dehydrogenase removes two hydrogens from C-2 and C-3, creating a trans-$\Delta^2$-enoyl-CoA (a trans double bond between C-2 and C-3). The coenzyme FAD is reduced to FADH₂.

2. Hydration. Enoyl-CoA hydratase adds water across the trans double bond in a Markovnikov-like fashion, placing the hydroxyl group on C-3. The product is L-β-hydroxyacyl-CoA. Note the stereochemistry: the enzyme is specific for the L-isomer.

3. Oxidation. L-β-hydroxyacyl-CoA dehydrogenase oxidizes the C-3 hydroxyl to a ketone, forming β-ketoacyl-CoA. NAD⁺ is reduced to NADH.

4. Thiolysis (cleavage). β-Ketothiolase cleaves the bond between C-2 and C-3 using the thiol group of a free CoA molecule. This releases one acetyl-CoA (the two-carbon fragment) and a fatty acyl-CoA that is two carbons shorter than the one you started with.

The shortened acyl-CoA then re-enters step 1, and the cycle repeats until the entire chain has been converted to acetyl-CoA units.

β-Oxidation of Even- vs. Odd-Chain Fatty Acids

Even-chain fatty acids (e.g., palmitic acid C16, stearic acid C18) are fully converted to acetyl-CoA. The final cycle cleaves a four-carbon unit into two acetyl-CoA molecules, so no special handling is needed.

Odd-chain fatty acids (e.g., pentadecanoic acid C15, heptadecanoic acid C17) proceed through β-oxidation normally until a three-carbon fragment remains: propionyl-CoA. Because propionyl-CoA can't be cleaved into acetyl-CoA, it's routed into the citric acid cycle through a three-step conversion:

1. Carboxylation. Propionyl-CoA carboxylase adds a carboxyl group (using biotin and ATP) to form D-methylmalonyl-CoA.
2. Epimerization. Methylmalonyl-CoA epimerase converts the D-isomer to L-methylmalonyl-CoA.
3. Rearrangement. Methylmalonyl-CoA mutase rearranges the carbon skeleton to produce succinyl-CoA, a citric acid cycle intermediate. This enzyme requires vitamin B₁₂ as a cofactor.

Acetyl-CoA Yield from Fatty Acids

For an even-chain fatty acid with $n$ carbons:

• Acetyl-CoA produced = $\frac{n}{2}$
• Number of β-oxidation cycles = $\frac{n}{2} - 1$ (the last cycle produces two acetyl-CoA at once)

For an odd-chain fatty acid with $n$ carbons:

• Acetyl-CoA produced = $\frac{n - 3}{2}$
• Propionyl-CoA produced = 1

Energy Yield

Each cycle of β-oxidation produces:

• 1 FADH₂ (worth ~1.5 ATP via the electron transport chain)
• 1 NADH (worth ~2.5 ATP)
• 1 acetyl-CoA (worth ~10 ATP when fully oxidized in the citric acid cycle)

For palmitic acid (C16): 7 cycles yield 7 FADH₂ + 7 NADH + 8 acetyl-CoA. After subtracting the 2 ATP equivalent spent on activation, the net yield is 106 ATP. This is why fats are such a calorie-dense fuel compared to carbohydrates.

One more connection worth knowing: when glucose is scarce (fasting, prolonged exercise), the liver generates acetyl-CoA from β-oxidation faster than the citric acid cycle can consume it. The excess acetyl-CoA is diverted into ketone body synthesis (ketogenesis). Ketone bodies then serve as an alternative fuel for tissues like the brain, which normally relies on glucose.