Involved in the degradation of long-chain fatty acids. a (3S)-3-hydroxyacyl-CoA = a (2E)-enoyl-CoA + H2O a 4-saturated-(3S)-3-hydroxyacyl-CoA = a (3E)-enoyl-CoA + H2O Lipid metabolism; fatty acid beta-oxidation. Repressed by FadR in the absence of LCFAs (fatty acids of 14-20 carbon atoms). When LCFAs are present in the medium, they are converted to long-chain acyl-CoAs, which antagonize FadR as to its binding to fadR boxes on target DNA and thus derepress transcription. Belongs to the enoyl-CoA hydratase/isomerase family.