HPr(Ser) kinase (IPR003755) is a bifunctional HPr serine kinase/phosphorylase (HprK/P). It catalyzes the phosphorylation of specific serine residues in HPr, as well as the dephosphorylation of seryl-phosphorylated HPr (P-Ser-HPr). HPr is a phosphate carrier protein that is a component of the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS). HprK/P regulates sugar transport and carbon metabolism. A number of substances inside cells change their levels depending on whether metabolic carbon sources (like glucose and fructose) are present or not. This affects the two opposing functions of HprK/P.