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. Capsule biogenesis; capsule polysaccharide biosynthesis. 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. Belongs to the SIS family. GmhA subfamily.