Catalyzes the reduction of a carbon-carbon double bond in an enoyl moiety that is covalently linked to an acyl carrier protein (ACP). Involved in the elongation cycle of fatty acid which are used in the lipid metabolism and in the biotin biosynthesis (By similarity). a 2,3-saturated acyl-[ACP] + NAD(+) = a (2E)-enoyl-[ACP] + H(+) + NADH Lipid metabolism; fatty acid biosynthesis. Cofactor biosynthesis; biotin biosynthesis. Homotetramer. The antibiotic diazaborine interferes with the activity by binding to the protein. Belongs to the short-chain dehydrogenases/reductases (SDR) family. FabI subfamily.