The mitochondrial trifunctional protein: centre of a beta-oxidation metabolon?

The trifunctional enzyme comprises three consecutive steps in the mitochondrial beta-oxidation of long-chain acyl-CoA esters: 2-enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase and 3-ketoacyl-CoA thiolase. Deficiencies in either 3-hydroxyacyl-CoA dehydrogenase activity, or all three activities, are important causes of human disease. The dehydrogenase and thiolase have a requirement for NAD+ and CoA respectively, whose levels are conserved within the mitochondrion and thus provide possible means for control and regulation of beta-oxidation. Using analysis of the intact CoA ester intermediates produced by the complex, we have examined the sensitivity of the complex to NAD+/NADH and acetyl-CoA. We consider the evidence for channelling within the trifunctional protein and propose a model for a beta-oxidation 'metabolon'.