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, the 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 activated by fructose 1,6-bisphosphate (FBP) only at low concentrations of enzyme or at high ATP concentrations. Homohexamer, arranged as bilayered trimers. Constitutively expressed, with or without glucose in the growth medium. 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. Was originally (PubMed:10714994 and PubMed:11904409) 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.