Catalyzes the conversion of pppGpp to ppGpp. Guanosine pentaphosphate (pppGpp) is a cytoplasmic signaling molecule which together with ppGpp controls the 'stringent response', an adaptive process that allows bacteria to respond to amino acid starvation, resulting in the coordinated regulation of numerous cellular activities. guanosine 3'-diphosphate 5'-triphosphate + H2O = guanosine 3',5'-bis(diphosphate) + H(+) + phosphate Purine metabolism; ppGpp biosynthesis; ppGpp from GTP: step 2/2. Belongs to the GppA/Ppx family. GppA subfamily.