2,4-dienoyl-CoA reductase (DCR) FMN-binding domain. DCR in E. coli is an iron-sulfur flavoenzyme which contains FMN, FAD, and a 4Fe-4S cluster. It is also a monomer, unlike that of its eukaryotic counterparts which form homotetramers and lack the flavin and iron-sulfur cofactors. Metabolism of unsaturated fatty acids requires auxiliary enzymes in addition to those used in b-oxidation. After a given number of cycles through the b-oxidation pathway, those unsaturated fatty acyl-CoAs with double bonds at even-numbered carbon positions contain 2-trans, 4-cis double bonds that can not be modified by enoyl-CoA hydratase. DCR utilizes NADPH to remove the C4-C5 double bond. DCR can catalyze the reduction of both natural fatty acids with cis double bonds, as well as substrates containing trans double bonds. The reaction is initiated by hybrid transfer from NADPH to FAD, which in turn transfers electrons, one at a time, to FMN via the 4Fe-4S cluster. The fully reduced FMN provides a hydrid ion to the C5 atom of substrate, and Tyr and His are proposed to form a catalytic dyad that protonates the C4 atom of the substrate and completes the reaction.
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