Catalyzes the isomerization of sedoheptulose 7-phosphate in D-glycero-D-manno-heptose 7-phosphate. 2 D-sedoheptulose 7-phosphate = D-glycero-alpha-D-manno-heptose 7-phosphate + D-glycero-beta-D-manno-heptose 7-phosphate Binds 1 zinc ion per subunit. Carbohydrate biosynthesis; D-glycero-D-manno-heptose 7-phosphate biosynthesis; D-glycero-alpha-D-manno-heptose 7-phosphate and D-glycero-beta-D-manno-heptose 7-phosphate from sedoheptulose 7-phosphate: step 1/1. Homotetramer. The reaction produces a racemic mixture of D-glycero-alpha-D-manno-heptose 7-phosphate and D-glycero-beta-D-manno-heptose 7-phosphate. B.aphidicola is not able to make lipopolysaccharides (LPS), and except lpcA and kdtB, the genes responsible for the biosynthesis of the LPS components are missing from the genome. Belongs to the SIS family. GmhA subfamily.