Part of the mycobacterial fatty acid elongation system FAS-II, which is involved in mycolic acid biosynthesis (PubMed:11932442). Catalyzes the NADPH-dependent reduction of beta-ketoacyl derivatives, the second step of the FAS-II elongation cycle (PubMed:9802011, PubMed:11932442, PubMed:17059223, PubMed:18155153, PubMed:19685079). May preferentially metabolize long-chain substrates (C8-C20) (PubMed:11932442). Can use CoA derivatives as substrates in vitro (PubMed:9802011, PubMed:11932442, PubMed:17059223, PubMed:18155153). a (3R)-hydroxyacyl-[ACP] + NADP(+) = a 3-oxoacyl-[ACP] + H(+) + NADPH a (3R)-3-hydroxyacyl-CoA + NADP(+) = a 3-oxoacyl-CoA + H(+) + NADPH (3R)-3-hydroxybutanoyl-CoA + NADP(+) = acetoacetyl-CoA + H(+) + NADPH (3R)-hydroxyoctanoyl-CoA + NADP(+) = 3-oxooctanoyl-CoA + H(+) + NADPH (3R)-3-hydroxydodecanoyl-CoA + NADP(+) = 3-oxododecanoyl-CoA + H(+) + NADPH Phosphorylation alters the activity, and subsequently mycolic acid biosynthesis (PubMed:20178986). In vitro, activity is efficiently inhibited by isoniazid. Acts by forming an isonicotinoyl-NADP adduct that binds to the MabA active site (PubMed:14693546). kcat is 4.3 sec(-1) with beta-ketododecanoyl-CoA as substrate. kcat is 3.5 sec(-1) with beta-ketooctanoyl-CoA as substrate. kcat is 1.9 sec(-1) with acetoacetyl-CoA as substrate. kcat is 2.6 sec(-1) with NADPH as substrate (PubMed:11932442). kcat is 7 sec(-1) with acetoacetyl-CoA as substrate (PubMed:17059223). Lipid metabolism; mycolic acid biosynthesis. Homotetramer (PubMed:11932442, PubMed:12079383, PubMed:15977159, PubMed:17642518). Homodimer in solution (PubMed:17059223, PubMed:18155153). Shows a significant rearrangement of the active site between a closed inactive conformation and an open and active form in presence of NADP. The C-terminus adopts a particular conformation that locks the conformational changes. Efficiently phosphorylated in vitro by several M.tuberculosis Ser/Thr protein kinases (STPKs), including PknA, PknB, PknD, PknE, and PknL. Thr-191 represents the primary phosphorylation site in vivo. Recombinant protein expressed in E.coli. Restores respiratory growth of S.cerevisiae oar1 deletion mutant. Belongs to the short-chain dehydrogenases/reductases (SDR) family.