Transport of fatty acids across the membrane

Degradation of fatty acids proceeds via an inducible set of enzymes that catalyze the pathway of P-oxidation [18]. P-Oxidation occurs via repeated cycles of reactions that are essentially the reverse of the reactions of fatty acid synthesis (Fig. 8). However, three major differences distinguish the two pathways. First, P-oxidation utilizes acyl-CoA thioesters and not acyl-ACPs. Second, the P-hydroxy intermediates have the opposite stereochemistry (L in P-oxidation and d in synthesis). Finally, the enzymes of P-oxidation share no homology with those of synthesis. 

Unsaturated fatty acids can also be degraded by the 3-oxidation pathway. The FadB protein possesses cw-P-enoyl-CoA isomerase activity, which converts cis-3 double bonds to trans-2 (Fig. 8). A 2,4-dienoyl-CoA reductase encoded by fadH is also required for the metabolism of polyunsaturated fatty acids (Fig. 8). This protein is a 73-kDa monomeric, NADP -dependent, 4Fe-4S flavoprotein. The FadH protein can utilize compounds with either cis or trans double bonds at the 4-position. An epimerase activity of FadB allows for the utilization of D-hydroxy fatty acids. The epimerase is actually a combination of a Z)-P-hydroxyacyl-CoA dehydratase and the crotonase (hydratase) activities, resulting in the conversion of the d to the L enantiomer (Fig. 8). 

Finally, there is also an anaerobic pathway for fatty acid degradation in E. coli that is shared by other gram-negative bacteria [19]. This system allows the utilization of fatty acids in the anaerobic intestinal environment. 

---