Involved in the biosynthesis of 3-deoxy-D-manno-octulosonate (KDO), a unique 8-carbon sugar component of lipopolysaccharides (LPSs). KdsD is not essential in the KDO biosynthesis and can be substituted by GutQ. Catalyzes the reversible aldol-ketol isomerization between D-ribulose 5-phosphate (Ru5P) and D-arabinose 5-phosphate (A5P). D-arabinose 5-phosphate = D-ribulose 5-phosphate Completely inhibited by 10 uM of nickel, copper, cadmium and mercury ions. Inhibited by zinc with an IC(50) of 1-3 uM. Metal ion inhibition may be a mechanism to control activity in vivo. Optimum pH is 8.4. 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.