Phosphoheptose isomerase is a member of the SIS (Sugar ISomerase) superfamily. Phosphoheptose isomerase catalyzes the isomerization of sedoheptulose 7-phosphate into D-glycero-D-mannoheptose 7-phosphate. This is the first step of the biosynthesis of gram-negative bacteria inner core lipopolysaccharide precursor, L-glycero-D-mannoheptose (Gmh).