Part of the mycobacterial fatty acid elongation system FAS-II, which is involved in mycolic acid biosynthesis (By similarity). Catalyzes the NADPH-dependent reduction of beta-ketoacyl derivatives, the second step of the FAS-II elongation cycle (By similarity). Has a preference for longer substrates (PubMed:29717709). Can use CoA derivatives as substrates in vitro (PubMed:29717709). 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 kcat is 1.14 sec(-1) with acetoacetyl-CoA as substrate. kcat is 5.72 sec(-1) with beta-keto-octanoyl-CoA as substrate. Lipid metabolism; mycolic acid biosynthesis. Homotetramer. Upon NADPH binding, several residues are pushed away from the active site, allowing the enzyme to adopt an open conformation. The transition from an NADPH-bound to an NADP(+)-bound form is likely to facilitate release of the product. Belongs to the short-chain dehydrogenases/reductases (SDR) family. Truncated N-terminus.