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 (PubMed:6130093, PubMed:8394006). In vitro, can hydrolyze pppGp (PubMed:6130093). Also has exopolyphosphatase activity, catalyzing the release of orthophosphate by processive hydrolysis of the phosphoanyhydride bonds of polyphosphate chains (1000 residues) (PubMed:8394006). guanosine 3'-diphosphate 5'-triphosphate + H2O = guanosine 3',5'-bis(diphosphate) + H(+) + phosphate Requires Mg(2+) and a monovalent cation, with NH(4) preferred over K(+) (PubMed:6130093). KCl, NaCl and NaF salts inhibit the exopolyphosphatase activity (PubMed:8394006). kcat is 0.023 sec(-1) for pppGpp hydrolase activity. kcat is 1.1 sec(-1) for the exopolyphosphatase activity. Optimum pH is 9. Purine metabolism; ppGpp biosynthesis; ppGpp from GTP: step 2/2. Belongs to the GppA/Ppx family. GppA subfamily.