Catalyzes the interconversion between ADP-D-glycero-beta-D-manno-heptose and ADP-L-glycero-beta-D-manno-heptose via an epimerization at carbon 6 of the heptose. ADP-D-glycero-beta-D-manno-heptose = ADP-L-glycero-beta-D-manno-heptose Binds 1 NADP(+) per subunit. NAD(+) can substitute for NADP(+), but enzymatic activity is reduced. Completely inhibited by ADP and ADP-glucose, and partially inhibited by ATP and NADH. Optimum pH is 5.5-9.5. Optimum temperature is 42 degrees Celsius. Nucleotide-sugar biosynthesis; ADP-L-glycero-beta-D-manno-heptose biosynthesis; ADP-L-glycero-beta-D-manno-heptose from D-glycero-beta-D-manno-heptose 7-phosphate: step 4/4. Bacterial outer membrane biogenesis; LPS core biosynthesis. Homopentamer. By heat shock. Contains a large N-terminal NADP-binding domain associated with a modified Rossman fold, and a smaller C-terminal substrate-binding domain. Essential for E.coli viability at elevated temperatures. Insertional inactivation of the gene by the Tn5 transposon results in E.coli being unable to form colonies at temperatures above 43 degrees Celsius. Belongs to the NAD(P)-dependent epimerase/dehydratase family. HldD subfamily. Was originally thought to be a homohexamer.