Enoyl-ACP reductase of the type II fatty acid syntase (FAS-II) system, which is involved in the biosynthesis of mycolic acids, a major component of mycobacterial cell walls (PubMed:10708367). Catalyzes the NADH-dependent reduction of the double bond of 2-trans-enoyl-[acyl-carrier protein], an essential step in the fatty acid elongation cycle of the FAS-II pathway (PubMed:10869086, PubMed:10708367). Shows preference for long-chain fatty acyl thioester substrates (>C16), and can also use 2-trans-enoyl-CoAs as alternative substrates (By similarity). The mycobacterial FAS-II system utilizes the products of the FAS-I system as primers to extend fatty acyl chain lengths up to C56, forming the meromycolate chain that serves as the precursor for final mycolic acids (PubMed:10869086, PubMed:10708367). Is the primary target of the first-line antitubercular drug isoniazid (INH) and of the second-line drug ethionamide (ETH) (PubMed:12406221, PubMed:10869086). Overexpressed inhA confers INH and ETH resistance to M.smegmatis (PubMed:12406221). The mechanism of isoniazid action against InhA is covalent attachment of the activated form of the drug to the nicotinamide ring of NAD and binding of the INH-NAD adduct to the active site of InhA (By similarity). Similarly, the ETH-NAD adduct binds InhA (By similarity). a 2,3-saturated acyl-[ACP] + NAD(+) = a (2E)-enoyl-[ACP] + H(+) + NADH a 2,3-saturated acyl-CoA + NAD(+) = a (2E)-enoyl-CoA + H(+) + NADH (2E)-octenoyl-CoA + H(+) + NADH = NAD(+) + octanoyl-CoA (2E)-dodecenoyl-CoA + H(+) + NADH = dodecanoyl-CoA + NAD(+) InhA activity is controlled via phosphorylation: phosphorylation on Thr-266 decreases InhA activity and likely negatively regulates biosynthesis of mycolic acids and growth of the bacterium (PubMed:20864541) (By similarity). InhA activity is likely inhibited by activated isoniazid, hexadecynoyl-CoA and octadecynoyl-CoA, which also block the biosynthesis of mycolic acids (PubMed:10708367). The antitubercular pro-drug isoniazid (INH) is oxidatively activated by the catalase-peroxidase KatG and then covalently binds NAD to form an adduct that inhibits the activity of InhA (By similarity). The inhibitory adduct is the isonicotinic-acyl-NADH where the isonicotinic-acyl group replaces the 4S (and not the 4R) hydrogen of NADH (By similarity). Similarly, the antitubercular pro-drugs ethionamide (ETH) and prothionamide (PTH) are activated by the flavoprotein monooxygenase EthA, and forms an adduct with NAD (ETH-NAD and PTH-NAD, respectively) that is a tight-binding inhibitor of InhA (By similarity). Lipid metabolism; mycolic acid biosynthesis. Homodimer. Homotetramer. Is phosphorylated in vivo (PubMed:20864541). Phosphorylation on Thr-266 decreases enzymatic activity (By similarity). Belongs to the short-chain dehydrogenases/reductases (SDR) family. FabI subfamily.