Supplies heptaprenyl diphosphate, the precursor for the side chain of the isoprenoid quinone menaquinone-7 (MQ-7). (2E,6E)-farnesyl diphosphate + 4 isopentenyl diphosphate = all-trans-heptaprenyl diphosphate + 4 diphosphate Binds 2 Mg(2+) ions per subunit. Heterodimer of component I and II. Belongs to the FPP/GGPP synthase family.