Enoate reductase with broad substrate specificity for different alpha,beta-unsaturated carbonyl compounds. Prefers NADPH over NADH as cofactor. butanoate + NAD(+) = (2E)-2-butenoate + H(+) + NADH Homodimer or heterodimer. Belongs to the NADH:flavin oxidoreductase/NADH oxidase family.