Catalyzes the ATP- as well as the pyrophosphate-dependent phosphorylation of 'Ser-46' 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). The two antagonistic activities of HprK/P are regulated by several intracellular metabolites, which change their concentration in response to the absence or presence of rapidly metabolisable carbon sources (glucose, fructose, etc.) in the growth medium. Therefore, by controlling the phosphorylation state of HPr, HPrK/P is a sensor enzyme that plays a major role in the regulation of carbon metabolism and sugar transport: it mediates carbon catabolite repression (CCR), and regulates PTS-catalyzed carbohydrate uptake and inducer exclusion. [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 Kinase activity is slightly activated by fructose 1,6-bisphosphate (FBP), and inhibited by inorganic phosphate (Pi), but FBP prevents kinase inhibition by Pi. Dephosphorylation of P-Ser-HPr is slightly inhibited by FBP. Homohexamer, arranged as bilayered trimers. Six HPr molecules bind to the hexamer at sites that overlap two of its subunits. The Walker A ATP-binding motif also binds Pi and PPi. The truncated form that consists of amino acids 128-319 exhibits in vitro enzymatic activities identical to those of the full-length protein, and also forms a hexamer. 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. Was originally (PubMed:10762262) called HPr kinase/phosphatase, but P-Ser-HPr dephosphorylation was shown (PubMed:12359880) to follow a quite unique mechanism, in which Pi instead of H(2)O is used for the nucleophilic attack on the phosphoryl group. P-Ser-HPr dephosphorylation is therefore not a phosphohydrolysis but a phospho-phosphorolysis reaction, and the bifunctional enzyme was dubbed HPr kinase/phosphorylase.