Preparation: Activation and Transport
Before entering chemical reactions, fatty acids (FAs) inside the cell must be activated. This process requires energy and is catalyzed by acyl-CoA synthetase. The resulting product is acyl-CoA, the active form of the fatty acid linked to coenzyme A.
Further oxidation takes place inside the mitochondria, but the inner mitochondrial membrane is completely impermeable to acyl-CoA. To deliver fuel into the matrix, a specialized transport system is used — the carnitine shuttle:
- At the outer mitochondrial membrane, the regulatory enzyme carnitine acyltransferase I transfers the acyl group to carnitine.
- A specific translocase protein transports the resulting acylcarnitine across the membrane.
- On the inner side, carnitine acyltransferase II transfers the acyl group back to mitochondrial coenzyme A. The fuel is now ready for oxidation.
The $\beta$-Oxidation Cycle: 4 Reactions
The pathway is named "$\beta$-oxidation" because bond cleavage always occurs at the $\beta$-carbon atom of the fatty acid radical. In each cycle, the fatty acid chain is shortened by two carbon atoms. One cycle consists of four sequential steps:
- First dehydrogenation. The enzyme acyl-CoA dehydrogenase removes hydrogen, transferring it to FAD. A double bond is formed.
- Hydration. The enzyme enoyl-CoA hydratase adds a water molecule across the double bond.
- Second dehydrogenation. Oxidation continues with the participation of $\beta$-hydroxyacyl-CoA dehydrogenase and $NAD^+$.
- Cleavage (thiolase reaction). With the participation of coenzyme A, a two-carbon fragment — acetyl-CoA — is cleaved off. The remaining shortened acyl-CoA enters the next round.
Cycles repeat until the entire fatty acid chain is completely degraded into acetyl-CoA molecules.
Energy Balance (ATP Formula)
The amount of energy produced depends on the number of carbon atoms ($n$) in the fatty acid.
- The fatty acid yields $\frac{n}{2}$ molecules of acetyl-CoA. Each molecule entering the citric acid cycle (TCA cycle) generates 12 ATP.
- The number of $\beta$-oxidation cycles is always one less than half the carbons: $(\frac{n}{2} - 1)$. The final cycle splits a four-carbon fragment directly into two acetyl-CoA molecules. Each $\beta$-oxidation cycle generates 5 ATP (via the reduction of FAD and $NAD^+$ and their subsequent oxidation in the electron transport chain).
- From the final sum, 2 ATP equivalents consumed during the initial activation step must be subtracted.
Example: Complete oxidation of stearic acid (18 carbon atoms) yields 120 ATP, making fats the most energy-dense cellular fuel.
Important: The process requires oxygen. Under anaerobic conditions, the electron transport chain halts, coenzymes cannot be regenerated, and oxidation stops.
Regulation: Fed State vs. Starvation
The key regulatory enzyme of the pathway is carnitine acyltransferase I, and its most potent inhibitor is malonyl-CoA.
- Fed state (insulin): Fatty acid biosynthesis is activated in the body. One of the intermediates of this synthesis is malonyl-CoA, which inhibits the carnitine shuttle. Fatty acids cannot enter the mitochondria, halting their oxidation (synthesis and degradation do not occur simultaneously).
- Starvation or exercise (glucagon, epinephrine): The concentration of malonyl-CoA drops sharply. Inhibition of the transporter is lifted, $\beta$-oxidation accelerates, and fatty acids become the primary energy source for muscles, heart, and liver.
Brain Metabolism and Ketone Bodies
Despite the high energy density of fats, the brain cannot utilize them directly. Fatty acids are hydrophobic and unable to cross the blood-brain barrier (BBB).
During prolonged starvation, the liver comes to the rescue by diverting about 50% of incoming fatty acids into an alternative pathway — ketogenesis. The synthesized ketone bodies readily cross the blood-brain barrier and serve as a reliable backup energy source for the brain, preventing systemic failure during carbohydrate depletion.