the general name for any enzyme belonging to the sub-subclass EC 2.7.6, diphosphotransferases, which transfer a diphosphate group from ATP to a named substrate; e.g. GTP pyrophosphokinase (EC 2.7.6.5), which converts guanosine 5′-triphosphate, GTP to guanosine 3′-phosphate 5′-triphosphate, pppGpp.