Mitochondrial fatty acid oxidation (beta-oxidation), the primary pathway of energy production from fatty acids

Activated long-chain fatty acyl-coenzyme A (CoA) is transported from the cytoplasm across the mitochondrial membrane by the carnitine shuttle. It then enters the cycle of beta-oxidation with a dehydrogenation reaction by very long-chain acyl-CoA dehydrogenase (VLCAD), producing reduced flavin adenine dinucleotide (FADH₂). Trifunctional protein (TFP) catalyzes the remainder of the cycle for long-chain substrates, reducing the fatty acyl-CoA by two carbons, and producing reduced nicotinamide adenine dinucleotide (NADH) and acetyl-CoA. Acetyl-CoA is metabolized by the tricarboxylic acid cycle for energy generation. NADH and FADH₂ donate the high-energy electrons captured by the dehydrogenase reaction to the electron transport chain (ETC).

TFP is embedded in the inner mitochondrial membrane and catalyzes the final three of the four steps of long-chain fatty acid beta-oxidation. It comprises four alpha and four beta subunits, encoded by HADHA and HADHB, respectively. The alpha subunit catalyzes long-chain enoyl-CoA hydratase (LCEH) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) activities. The beta subunit catalyzes long-chain 3-ketoacyl-CoA thiolase (LCKAT) activity. LCHAD deficiency is associated with specific HADHA pathogenic variants where only dehydrogenase activity is impaired, whereas TFP deficiency is associated with either HADHA or HADHB pathogenic variants leading to instability of the entire complex and thus impairment of all three enzymatic activities.