NAD-dependent dehydrogenase involved in the biosynthesis of heptose moieties with a hydroxyl group at C6 found on the capsular polysaccharide (CPS) of C.jejuni. Catalyzes the initial oxidation of C4 of the GDP-D-glycero-alpha-D-manno-heptose to form GDP-D-glycero-4-keto-alpha-D-lyxo-heptose in the presence of alpha-ketoglutarate required to recycle the NADH nucleotide. 2-oxoglutarate + GDP-D-glycero-alpha-D-manno-heptose = (S)-2-hydroxyglutarate + GDP-D-glycero-4-keto-alpha-D-lyxo-heptose kcat is 0.64 sec(-1) with GDP-D-glycero-alpha-D-manno-heptose as substrate. kcat is 0.55 sec(-1) with alpha-ketoglutarate as substrate. Capsule biogenesis; capsule polysaccharide biosynthesis. Homotetramer.