Is a metabolite-sensitive enzyme that catalyzes the ATP-as well as probably the pyrophosphate-dependent phosphorylation of Ser-47 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 regulatory role of HPrK/P in the physiology of M.pneumoniae is not known yet. [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 Contrary to HPrK/P of B.subtilis and other bacteria, that of M.pneumoniae is active as a kinase at very low ATP concentrations in the absence of fructose 1,6-bisphosphate (FBP). Kinase activity is slightly activated by FBP, and inhibited by inorganic phosphate (Pi), but FBP prevents kinase inhibition by Pi. Dephosphorylation of P-Ser-HPr by M.pneumoniae HPrK/P is strictly dependent on the presence of Pi, and is inhibited by FBP. This unique mode of control of HPrK/P activity is proposed to reflect the parasitic lifestyle of M.pneumoniae, that is strictly adapted to its ecological niche on nutrient-rich human mucous membranes. Homohexamer, arranged as bilayered trimers. 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. Contrary to HPrK/P of other bacteria, that of M.pneumoniae has a very high affinity for ATP (Kd=5.3 microM), explaining kinase activity even at low ATP concentrations. Belongs to the HPrK/P family. Was originally (PubMed:12368461 and PubMed:12589763) called HPr kinase/phosphatase, but P-Ser-HPr dephosphorylation was shown in several bacteria 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.