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β-Oxidation of
Fatty Acids

Understand how cells break down fatty acids into acetyl-CoA through the spiral pathway of β-oxidation, and learn how this process is regulated to meet metabolic demands.

Biochemistry

Why Fatty Acid Oxidation Matters

Fatty acids are the body's most energy-dense fuel. A single molecule of palmitate (\(\text{C}_{16}\)) generates far more ATP than one molecule of glucose. During fasting, exercise, and prolonged energy demand, fatty acid oxidation becomes the primary source of ATP for the heart, skeletal muscle, and liver.

Activation: Fatty Acyl-CoA Formation

Before β-oxidation can begin, free fatty acids must be activated in the cytoplasm. The enzyme acyl-CoA synthetase catalyzes the thioesterification of the fatty acid with Coenzyme A, consuming the equivalent of \(2\) ATP:

The Carnitine Shuttle

Long-chain fatty acyl-CoA cannot cross the inner mitochondrial membrane directly. The carnitine shuttle system transports it in three steps:

1
CPT-I (outer membrane): converts fatty acyl-CoA to fatty acylcarnitine by transferring the acyl group to carnitine.
2
Translocase: shuttles acylcarnitine across the inner membrane in exchange for free carnitine.
3
CPT-II (inner membrane): regenerates fatty acyl-CoA inside the mitochondrial matrix.

Carnitine Shuttle Diagram

CYTOPLASM INTERMEMBRANE SPACE MATRIX Outer Membrane Inner Membrane Fatty acyl-CoA CPT-I (rate-limiting) Acylcarnitine Translocase (antiport) CPT-II Fatty acyl-CoA Malonyl-CoA ⊣

The β-Oxidation Spiral

Once inside the matrix, fatty acyl-CoA undergoes a repeating cycle of four reactions that shorten the chain by \(2\) carbons each round, producing one acetyl-CoA, one \(\text{FADH}_2\), and one \(\text{NADH}\) per cycle.

1
Oxidation (acyl-CoA dehydrogenase): introduces a trans-\(\Delta^2\) double bond; produces \(\text{FADH}_2\).
2
Hydration (enoyl-CoA hydratase): adds \(\text{H}_2\text{O}\) across the double bond to form L-β-hydroxyacyl-CoA.
3
Oxidation (β-hydroxyacyl-CoA dehydrogenase): oxidizes the hydroxyl to a ketone; produces \(\text{NADH}\).
4
Thiolysis (thiolase): cleaves the β-ketoacyl-CoA with CoA to release acetyl-CoA + shortened acyl-CoA.

The Four-Step Cycle

Acyl-CoA
\((\text{C}_{16} \rightarrow \text{C}_{14} \ldots)\)
\( \text{FAD} \rightarrow \text{FADH}_2 \)
1. Oxidation trans-Δ²-Enoyl-CoA
\( + \text{H}_2\text{O} \)
2. Hydration L-β-Hydroxyacyl-CoA
\( \text{NAD}^+ \rightarrow \text{NADH} \)
3. Oxidation β-Ketoacyl-CoA
\( + \text{CoA-SH} \)
4. Thiolysis Acetyl-CoA

Energy Yield Per Cycle

Each round of β-oxidation produces three high-energy molecules that feed into the electron transport chain and the citric acid cycle:

\(\text{FADH}_2\)
From Step 1 (acyl-CoA dehydrogenase). Yields \(\sim 1.5\) ATP via ETC.
\(\text{NADH}\)
From Step 3 (β-hydroxyacyl-CoA DH). Yields \(\sim 2.5\) ATP via ETC.
Acetyl-CoA
Enters the TCA cycle, generating \(\sim 10\) ATP per acetyl-CoA.

Worked Example: Palmitate (\(\text{C}_{16}\))

Palmitate has \(16\) carbons. To fully cleave it into \(2\)-carbon acetyl-CoA units, we need \(\frac{16}{2} - 1 = 7\) cycles of β-oxidation.

1
Cycles: \(7\) rounds produce \(7 \;\text{FADH}_2\) and \(7 \;\text{NADH}\).
2
Acetyl-CoA: \(8\) molecules of acetyl-CoA enter the TCA cycle.
3
ETC from β-oxidation: \(7 \times 1.5 = 10.5\) ATP from \(\text{FADH}_2\); \(7 \times 2.5 = 17.5\) ATP from NADH.
4
TCA cycle: \(8 \times 10 = 80\) ATP from 8 acetyl-CoA.
5
Grand total: \(10.5 + 17.5 + 80 = 108\) ATP, minus \(2\) ATP for activation = \(106\) net ATP.

General ATP Yield Formula

For a saturated fatty acid with \(n\) carbons (where \(n\) is even):

Special Cases: Odd-Chain and Unsaturated

Odd-Chain Fatty Acids
The final thiolysis produces propionyl-CoA (\(\text{C}_3\)) instead of acetyl-CoA. Propionyl-CoA is converted to succinyl-CoA (a TCA intermediate) via a \(\text{B}_{12}\)-dependent pathway.
Unsaturated Fatty Acids
Require extra enzymes: enoyl-CoA isomerase (for cis-\(\Delta^3\) bonds) and 2,4-dienoyl-CoA reductase (for polyunsaturated). Each pre-existing double bond skips one FADH₂ (= \(1.5\) fewer ATP).

Regulation of β-Oxidation

β-Oxidation is tightly regulated to match the cell's energy state. The primary control point is CPT-I, which gates fatty acid entry into the mitochondrion:

Malonyl-CoA
Inhibits CPT-I. Malonyl-CoA is the first committed intermediate of fatty acid synthesis. When synthesis is active, oxidation is suppressed.
AMPK
Activates β-oxidation. AMPK phosphorylates and inhibits acetyl-CoA carboxylase (ACC), reducing malonyl-CoA, thereby relieving CPT-I inhibition.
NADH/NAD⁺ Ratio
High \(\text{NADH}/\text{NAD}^+\) slows Step 3 (β-hydroxyacyl-CoA DH). High \(\text{acetyl-CoA}/\text{CoA}\) inhibits thiolase (Step 4).

Malonyl-CoA as the Master Switch

Fed vs. Fasted Regulation Acetyl-CoA ACC Malonyl-CoA CPT-I (blocked) ⊣ β-Oxidation OFF AMPK (fasting, exercise) ⊣ CPT-I active β-Oxidation ON FED STATE Insulin ↑

How to Calculate ATP Yield

1
Count carbons (\(n\)) in the fatty acid. Is it even or odd?
2
Calculate cycles: \(\frac{n}{2} - 1\) for even-chain fatty acids.
3
Count reduced coenzymes: each cycle yields \(1\;\text{FADH}_2\) (\(\times 1.5\) ATP) and \(1\;\text{NADH}\) (\(\times 2.5\) ATP).
4
Count acetyl-CoA: \(\frac{n}{2}\) molecules, each worth \(\sim 10\) ATP via TCA/ETC.
5
Subtract activation cost: \(- 2\) ATP (for the initial acyl-CoA synthetase step).
6
Adjust for unsaturation: subtract \(1.5\) ATP per pre-existing double bond (skipped \(\text{FADH}_2\)).

Worked Example: Laurate (\(\text{C}_{12}\))

Lauric acid has \(12\) carbons, fully saturated. Apply the step-by-step method:

1
\(n = 12\), even-chain.
2
Cycles: \(\frac{12}{2} - 1 = 5\).
3
\(5\;\text{FADH}_2 \rightarrow 5 \times 1.5 = 7.5\) ATP; \(5\;\text{NADH} \rightarrow 5 \times 2.5 = 12.5\) ATP.
4
\(6\) acetyl-CoA \(\rightarrow 6 \times 10 = 60\) ATP.
5
Total: \(7.5 + 12.5 + 60 - 2 = \) \(78\) net ATP.

Common Mistakes

✕ Off-by-One Cycles
Using \(\frac{n}{2}\) instead of \(\frac{n}{2} - 1\) for the number of cycles. The last cycle produces the final two acetyl-CoAs, so there is one fewer cycle than acetyl-CoA molecules.
✕ Forgetting Activation Cost
The initial ATP → AMP + PPᵢ reaction costs the equivalent of \(2\) ATP, not 1. Always subtract 2 from the gross total.
✕ Ignoring Unsaturation
Each pre-existing double bond skips one \(\text{FADH}_2\) production, costing \(1.5\) ATP per double bond. Students often forget this for unsaturated fatty acids.

Synthesis vs. Oxidation

β-Oxidation Fatty Acid Synthesis
Location Mitochondrial matrix Cytoplasm
Acyl carrier Coenzyme A (CoA) Acyl carrier protein (ACP)
2-Carbon unit Acetyl-CoA released Malonyl-CoA added
Electron carriers \(\text{FAD}\) and \(\text{NAD}^+\) are reduced \(\text{NADPH}\) is oxidized
β-Hydroxy intermediate \(\textit{L}\)-isomer \(\textit{D}\)-isomer
Regulatory enzyme CPT-I Acetyl-CoA carboxylase (ACC)
Key inhibitor Malonyl-CoA Palmitoyl-CoA

Question 1 of 5

What is the rate-limiting step of fatty acid β-oxidation?

Question 2 of 5

How many cycles of β-oxidation are required to completely oxidize a \(\text{C}_{18}\) saturated fatty acid?

Question 3 of 5

Which molecule inhibits CPT-I and thus prevents fatty acids from entering the mitochondrion?

Question 4 of 5

An unsaturated fatty acid with one cis double bond and \(18\) carbons yields how many fewer ATP compared to the equivalent saturated \(\text{C}_{18}\) fatty acid?

Question 5 of 5

During fasting, which kinase activates β-oxidation by phosphorylating and inhibiting acetyl-CoA carboxylase (ACC)?