Catalyzes the ATP- as well as the pyrophosphate-dependent phosphorylation of a specific serine residue in HPr, a phosphocarrier protein of the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS). HprK/P also catalyzes the pyrophosphate-producing, inorganic phosphate-dependent dephosphorylation (phosphorolysis) of seryl-phosphorylated HPr (P-Ser-HPr) (By similarity). [HPr protein]-L-serine + ATP = [HPr protein]-O-phospho-L-serine + ADP + H(+) [HPr protein]-O-phospho-L-serine + H(+) + phosphate = [HPr protein]-L-serine + diphosphate Homohexamer. The Walker A ATP-binding motif also binds Pi and PPi. Both phosphorylation and phosphorolysis are carried out by the same active site and suggest a common mechanism for both reactions. Belongs to the HPrK/P family.