Involved in the fatty acid synthesis (FAS II). Catalyzes the reduction of the carbon-carbon double bond of crotonyl-CoA to yield butyryl-CoA. a 2,3-saturated acyl-CoA + NAD(+) = a (2E)-enoyl-CoA + H(+) + NADH kcat is 28.2 sec(-1) for reductase activity (at pH 6.2 and 25 degrees Celsius). 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 C.acetobutylicum 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.