Catalyzes the second (last) step of the biosynthesis of Kdo8N (8-amino-3,8-dideoxy-D-manno-octulosonate) from Kdo (3-deoxy-D-manno-octulosonate). 2-oxoglutarate + 8-amino-3,8-dideoxy-alpha-D-manno-octulosonate = 3,8-dideoxy-8-oxo-alpha-D-manno-octulosonate + L-glutamate Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. A double kdnA/kdnB deletion mutant shows increased sensitivity to polymyxin B and bile salts. Kdo8N is found in lipopolysaccharides of members of the Shewanella genus. Belongs to the DegT/DnrJ/EryC1 family.