Involved in the fatty acid synthesis (FAS II). Catalyzes the reduction of the carbon-carbon double bond of crotonyl-CoA to yield butyryl-CoA. In vitro it can also use hexenoyl-CoA and dodecenoyl-CoA as substrates (PubMed:22906002). a 2,3-saturated acyl-CoA + NAD(+) = a (2E)-enoyl-CoA + H(+) + NADH Inhibited by lauroyl-CoA. kcat is 385.9 sec(-1) for reductase activity (at pH 6.2 and 25 degrees Celsius) (PubMed:23050861). kcat is 112 sec(-1) for reductase activity with hexenoyl-CoA as substrate (PubMed:22906002). kcat is 91 sec(-1) for reductase activity with crotonyl-CoA as substrate (PubMed:22906002). kcat is 90 sec(-1) for reductase activity with dodecenoyl-CoA as substrate (PubMed:22906002). Lipid metabolism; fatty acid biosynthesis. Monomer. Used in the biosynthesis of medium-chain volatile alcohols as biofuels engineered by microorganisms. The switch from the native flavin-dependent enoyl-CoA reductase used in the production of n-butanol, a key second-generation biofuel, to a flavin-independent trans-enoyl-CoA reductase from T.denticola leads to an order of magnitude increase in product yield in engineered E.coli. Possesses high activity for the reduction reaction, but no activity for the reverse oxidation reaction. Belongs to the TER reductase family.