Catalyzes the NADPH-dependent reduction of trans-2-enoyl thioesters in mitochondrial fatty acid synthesis (fatty acid synthesis type II). Fatty acid chain elongation in mitochondria uses acyl carrier protein (ACP) as an acyl group carrier, but the enzyme accepts both ACP and CoA thioesters as substrates in vitro. Displays a preference for medium-chain over short- and long-chain substrates (By similarity). May provide the octanoyl chain used for lipoic acid biosynthesis, regulating protein lipoylation and mitochondrial respiratory activity particularly in Purkinje cells (PubMed:30266742). a 2,3-saturated acyl-[ACP] + NADP(+) = a (2E)-enoyl-[ACP] + H(+) + NADPH (2E)-butenoyl-[ACP] + H(+) + NADPH = butanoyl-[ACP] + NADP(+) (2E)-hexenoyl-[ACP] + H(+) + NADPH = hexanoyl-[ACP] + NADP(+) (2E)-octenoyl-[ACP] + H(+) + NADPH = NADP(+) + octanoyl-[ACP] (2E)-decenoyl-[ACP] + H(+) + NADPH = decanoyl-[ACP] + NADP(+) (2E)-dodecenoyl-[ACP] + H(+) + NADPH = dodecanoyl-[ACP] + NADP(+) (2E)-tetradecenoyl-[ACP] + H(+) + NADPH = NADP(+) + tetradecanoyl-[ACP] (2E)-hexadecenoyl-[ACP] + H(+) + NADPH = hexadecanoyl-[ACP] + NADP(+) Homodimer. Expressed in Purkinje cells (at protein level). Homozygous deficiency causes placental insufficiency and embryonic lethality between 9.5 and 11.5 dpc. Belongs to the zinc-containing alcohol dehydrogenase family. Quinone oxidoreductase subfamily.