Catalyzes the ATP- as well as probably 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 bifunctional HPr kinase/phosphorylase is a sensor enzyme that plays a major role in the regulation of carbon metabolism and sugar transport: it probably 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) at low ATP concentrations, and is inhibited by inorganic phosphate (Pi). Moreover, FBP, phosphoenolpyruvate and 2-phosphoglycerate, but not fructose 1-P, fructose 6-P, and ribulose 1,5-bisphosphate protect kinase activity against inhibition by Pi. Dephosphorylation of P-Ser-HPr by S.salivarius HPrK/P is strictly dependent on the presence of Pi, and is inhibited by FBP. FBP seems to modulate HPrK/P activities by enhancing affinity of the active site for ATP and, conversely, lowering the affinity for Pi. Optimum pH is 7.5. Active from pH 5.5 to 9.5. At pH 5.5, exhibits less than 15% of its optimal activity. 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.