Fatty acids are a major energy source and a key component of cellular membranes. Their metabolism involves two opposing pathways: beta-oxidation, which breaks them down for energy, and lipogenesis, which builds them for storage.
Fatty Acid Oxidation (Beta-Oxidation)
Activation
Fatty acids are activated in the cytoplasm by attachment to coenzyme A, forming fatty acyl-CoA. This reaction consumes ATP. The fatty acyl-CoA is then transported into the mitochondria via the carnitine shuttle.
The Beta-Oxidation Cycle
Inside the mitochondrial matrix, fatty acyl-CoA undergoes repeated cycles of four reactions: oxidation by acyl-CoA dehydrogenase (producing FADH2), hydration by enoyl-CoA hydratase, oxidation by beta-hydroxyacyl-CoA dehydrogenase (producing NADH), and thiolysis by thiolase (producing acetyl-CoA and a shortened fatty acyl-CoA).
Energy Yield
Each cycle removes two carbon atoms as acetyl-CoA. A 16-carbon palmitate molecule undergoes seven cycles, producing 8 acetyl-CoA, 7 FADH2, and 7 NADH. The acetyl-CoA then enters the citric acid cycle for further energy production.
Fatty Acid Synthesis (Lipogenesis)
Citrate Shuttle
When energy is abundant, excess acetyl-CoA in the mitochondria is exported to the cytoplasm via the citrate shuttle. Citrate is cleaved by ATP-citrate lyase to produce acetyl-CoA and oxaloacetate.
Malonyl-CoA Formation
Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA, the committed step of fatty acid synthesis. This enzyme is activated by insulin and citrate and inhibited by palmitoyl-CoA.
Elongation Cycle
Fatty acid synthase, a large multifunctional enzyme, carries out a repeated cycle of condensation, reduction, dehydration, and reduction. Each cycle adds two carbon atoms. The process requires NADPH and produces palmitate (16:0) as the primary product.
Regulation
Fatty acid oxidation and synthesis are reciprocally regulated. When energy is low, AMPK activates oxidation and inhibits synthesis. When energy is abundant, insulin activates synthesis and inhibits oxidation.
Practical Fatty Acid Oxidation Assay
Isolate mitochondria from fresh liver or heart tissue by differential centrifugation — homogenize 200 mg of tissue in ice-cold isolation buffer (250 mM sucrose, 10 mM Tris-HCl pH 7.4, 1 mM EGTA), centrifuge at 600 × g for 10 minutes to remove nuclei and debris, then centrifuge the supernatant at 10,000 × g for 10 minutes to pellet mitochondria. Resuspend the mitochondrial pellet in assay buffer (100 mM KCl, 10 mM Tris-HCl pH 7.4, 5 mM MgCl2, 1 mM ATP, 0.1% BSA). Measure fatty acid oxidation by monitoring oxygen consumption using a Clark-type oxygen electrode or a Seahorse XF Analyzer. Add 0.5–1 mg of mitochondrial protein to 0.5 mL of assay buffer containing 50 µM palmitoyl-CoA, 2 mM L-carnitine, and 1 mM malate. Record oxygen consumption rate (OCR) for 10 minutes. Basal OCR reflects endogenous substrate oxidation. Specific fatty acid oxidation is confirmed by adding 50 µM etomoxir, an irreversible inhibitor of carnitine palmitoyltransferase I (CPT1) — a significant decrease in OCR (>50%) confirms that the measured respiration is driven by fatty acid oxidation. For cellular fatty acid oxidation, incubate intact cells with [9,10-³H]palmitic acid (0.5 µCi/mL) for 2 hours, then measure ³H2O released into the medium by passing it through an ion-exchange column (3H2O flows through while palmitate is retained). Count the flow-through by liquid scintillation.
Real-World Application
In a mouse model of non-alcoholic fatty liver disease (NAFLD), hepatic fatty acid oxidation is measured in isolated mitochondria using palmitoyl-CoA as substrate. OCR is reduced by 30% compared to control mice, indicating impaired β-oxidation. Treatment with a PPARα agonist (fenofibrate, 100 mg/kg/day for 4 weeks) restores oxidation rates to control levels, reducing hepatic steatosis. These measurements directly link mitochondrial dysfunction to fat accumulation in the liver.
