Involved in the biosynthesis of isoprenoids. Catalyzes the 1,3-allylic rearrangement of the homoallylic substrate isopentenyl (IPP) to its allylic isomer, dimethylallyl diphosphate (DMAPP). isopentenyl diphosphate = dimethylallyl diphosphate Homooctamer. Dimer of tetramers. Belongs to the IPP isomerase type 2 family.