Impact of the intramitochondrial enzyme organization on fatty acid oxidation

The enzymes of mitochondrial β-oxidation are thought to be organized in at least two functional complexes, a membrane-bound, long-chain-specific β-oxidation system and a matrix system consisting of soluble enzymes with preferences for medium-chain and short-chain substrates. This hypothesis is supported by the observation that the inactivation of long-chain 3-ketoacyl-CoA thiolase by 4-bromotiglic acid (4-bromo-2-methylbut-2-enoic acid) causes the complete inhibition of palmitate β-oxidation even though 3-ketoacyl-CoA thiolase, which acts on 3-ketopalmitoyl-CoA, remains partly active. The observed substrate specificities of long-chain acyl-CoA dehydrogenase (LCAD) and very-long-chain acyl-CoA dehydrogenase prompt the suggestion that LCAD is a functional component of the long-chain-specific β-oxidation system. Altogether, a view is emerging of the organization of β-oxidation enzymes in mitochondria that supports the idea of intermediate channelling and explains the apparent absence of true intermediates of β-oxidation from mitochondria.