In eubacteria ppGpp (guanosine 3'-diphosphate 5'-diphosphate) is a mediator of the stringent response that coordinates a variety of cellular activities in response to changes in nutritional abundance. This enzyme catalyzes the degradation of ppGpp into GDP. It may also be capable of catalyzing the synthesis of ppGpp (By similarity). guanosine 3',5'-bis(diphosphate) + H2O = diphosphate + GDP + H(+) Purine metabolism; ppGpp biosynthesis; ppGpp from GDP: step 1/1. Belongs to the RelA/SpoT family.