The essential role of 2,4-dienoyl-CoA reductase for the degradation of complex fatty acid mixtures

Fatty acids (FAs) can be used as a carbon and energy source by most bacteria. FAs are diverse and show variations in aliphatic chain length, degree and kind of branching, and number of double bonds. After activation by a thioester link to coenzyme A (CoA), FAs are degraded by the β-oxidation machinery. The core β-oxidation enzymes can degrade most FAs, except those that bear an unsaturation at even-numbered carbons. Such FAs include the essential arachidonic and linoleic FAs of the mammalian diet. We studied the role of the 2,4-dienoyl-CoA reductase FadH in <i>E. coli</i> FA metabolism. We showed that <i>fadH</i> is essential for growth on linoleic acid and that cysteine residues connecting the FadH-bound [Fe-S] cluster are essential for activity <i>in vivo</i>. Eukaryotes also use 2,4-dienoyl-CoA reductases for β-oxidation in mitochondria, but these enzymes belong to a different family than FadH, with different co-factors and mechanism. Yet, we showed that eukaryotic 2,4-dienoyl-CoA reductases DECR can complement the <i>E. coli fadH</i> mutant for growth on linoleic acid. Moreover, we found that when mixed with other FAs, linoleic acid prevents the growth of the <i>fadH</i> mutant. This was shown to result from the jamming of the β-oxidation by unprocessed linoleic acid, and this blocking was relieved by producing FadH or eukaryotic DECRs. These studies demonstrate the key role of prokaryotic and eukaryotic 2,4-dienoyl-CoA reductases in complex environments containing mixtures of saturated and unsaturated FAs, and might pave the way to search for chemicals targeting DECR activity.IMPORTANCEBacteria and eukaryotes can harness energy from fatty acids (FAs) through the process of β-oxidation. However, information on the β-oxidation in bacteria stems from studies in which degradation of only a limited set of saturated or monounsaturated FAs was investigated, far from reflecting the wide chemical diversity of FAs found in nature. Here, we evidenced the physiological importance of dienoyl-CoA reductase enzymes required for the degradation of specific unsaturated FAs in complex mixtures of FAs, and how their absence leads to the congestion of the β-oxidation machinery. These results will permit a better understanding of the impact of FA degradation in enterobacteria, living in the complex gut environment where FAs are available from the diet or from host lipids. Furthermore, we showed that eukaryotic enzymes can replace the prokaryotic ones, opening the possibility of biomedical application in structure/function studies of the eukaryotic dienoyl-CoA reductases.