Involved in the biosynthesis of 3-deoxy-D-manno-octulosonate (KDO), a unique 8-carbon sugar component of lipopolysaccharides (LPSs). Catalyzes the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P) (By similarity). D-arabinose 5-phosphate = D-ribulose 5-phosphate Carbohydrate biosynthesis; 3-deoxy-D-manno-octulosonate biosynthesis; 3-deoxy-D-manno-octulosonate from D-ribulose 5-phosphate: step 1/3. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homotetramer. Belongs to the SIS family. GutQ/KpsF subfamily. Extended N-terminus.