2,4-Dienoyl-CoA Reductase from Escherichia coli Is a Novel Iron–Sulfur Flavoprotein That Functions in Fatty Acid β-Oxidation

2,4-Dienoyl-CoA reductase is an enzyme that is required for the β-oxidation of unsaturated fatty acids with even-numbered double bonds. The 2,4-dienoyl-CoA reductase from Escherichia coli was studied to explore the catalytic and structural properties that distinguish this enzyme from the corresponding eukaryotic reductases. The E. coli reductase was found to contain 1 mol of flavin mononucleotide and 4 mol each of acid-labile iron and sulfur in addition to 1 mol of flavin adenine dinucleotide per mole of protein. Redox titrations revealed a requirement for 5 mol of electrons to completely reduce 1 mol of enzyme and provided evidence for the formation of a red semiquinone intermediate. The reductase caused a significant polarization of the substrate carbonyl group as indicated by an enzyme-induced red shift of 38 nm in the spectrum of 5-phenyl-2,4-pentadienoyl-CoA. However, suspected cis → trans isomerase and Δ3,Δ2-enoyl-CoA isomerase activities were not detected in this enzyme. It is concluded that the 2,4-dienoyl-CoA reductases from E. coli and eukaryotic organisms are structurally and mechanistically unrelated enzymes that catalyze the same type of reaction with similar efficiencies.