Catalyzes the reduction of a carbon-carbon double bond in an enoyl moiety that is covalently linked to an acyl carrier protein (ACP). It has a preference for a long chain (C12) substrate compared to the shorter (C4) acyl group. Involved in the elongation cycle of fatty acid which are used in the lipid metabolism (By similarity). a 2,3-saturated acyl-[ACP] + NADP(+) = a (2E)-enoyl-[ACP] + H(+) + NADPH Inhibited by 1,4-disubstituted imidazoles, 1,4-benzodiazepine, naphthyridinone derivatives, triclosan and its diphenyl ether analgues. Lipid metabolism; fatty acid biosynthesis. Homotetramer. Follows a sequential ordered bi-bi catalytic mechanism. Belongs to the short-chain dehydrogenases/reductases (SDR) family. FabI subfamily.