Catalyzes the formation of isopentenyl diphosphate (IPP), the building block of all isoprenoids. Has lower activity with isopentenyl thiolophosphate (ISP). Has low activity with dimethylallyl phosphate (DMAP), 1-butyl phosphate (BP) and 3-buten-1-yl phosphate (BEP). Has no significant activity with geranyl phosphate (in vitro). ATP + isopentenyl phosphate = ADP + isopentenyl diphosphate Optimum pH is 6.5-8.5. Optimum temperature is 70 degrees Celsius. Homodimer. Belongs to the isopentenyl phosphate kinase family.