D-arabinose 5-phosphate + H2O + phosphoenolpyruvate = 3-deoxy-alpha-D-manno-2-octulosonate-8-phosphate + phosphate Carbohydrate biosynthesis; 3-deoxy-D-manno-octulosonate biosynthesis; 3-deoxy-D-manno-octulosonate from D-ribulose 5-phosphate: step 2/3. Belongs to the KdsA family.