Insights into the stability and substrate specificity of the E. coli aerobic beta-oxidation trifunctional enzyme complex.

Degradation of fatty acids by the beta-oxidation pathway results in the formation of acetyl-CoA which enters the TCA cycle for the production of ATP. In E. coli, the last three steps of the beta-oxidation are catalyzed by two heterotetrameric alpha2beta2 enzymes namely the aerobic trifunctional enzyme (EcTFE) and the anaerobic TFE (anEcTFE). The alpha-subunit of TFE has 2E-enoyl-CoA hydratase (ECH) and 3S-hydroxyacyl-CoA dehydrogenase (HAD) activities whereas the beta-subunit is a thiolase with 3-ketoacyl-CoA thiolase (KAT) activity. Recently, it has been shown that the two TFEs have complementary substrate specificities allowing for the complete degradation of long chain fatty acyl-CoAs into acetyl-CoA under aerobic conditions. Also, it has been shown that the tetrameric EcTFE and anEcTFE assemblies are similar to the TFEs of Pseudomans fragi and human, respectively. Here the properties of the EcTFE subunits are further characterized. Strikingly, it is observed that when expressed separately, EcTFE-alpha is a catalytically active monomer whereas EcTFE-beta is inactive. However, when mixed together active EcTFE tetramer is reconstituted. The crystal structure of the EcTFE-alpha chain is also reported, complexed with ATP, bound in its HAD active site. Structural comparisons show that the EcTFE hydratase active site has a relatively small fatty acyl tail binding pocket when compared to other TFEs in good agreement with its preferred specificity for short chain 2E-enoyl-CoA substrates. Furthermore, it is observed that millimolar concentrations of ATP destabilize the EcTFE complex, and this may have implications for the ATP-mediated regulation of beta-oxidation in E. coli.