HPr kinase/phosphorylase, a Walker motif A-containing bifunctional sensor enzyme controlling catabolite repression in Gram-positive bacteria.

Carbon catabolite repression (CCR) in Gram-positive bacteria is regulated by the bifunctional enzyme HPr kinase/phosphorylase (HprK/P). This enzyme catalyses the ATP- as well as the pyrophosphate-dependent phosphorylation of Ser-46 in HPr, a phosphocarrier protein of a sugar transport and phosphorylation system. HprK/P also catalyses the pyrophosphate-producing, inorganic phosphate-dependent dephosphorylation (phosphorolysis) of seryl-phosphorylated HPr (P-Ser-HPr). P-Ser-HPr functions as catabolite co-repressor by interacting with the LacI/GalR-type repressor, catabolite control protein A (CcpA), and allowing it to bind to operator sites preceding catabolite-regulated transcription units. HprK/P thus indirectly controls the expression of about 10% of the genes of Gram-positive bacteria. The two antagonistic activities of HprK/P are regulated by 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. Biochemical and structural studies revealed that HprK/P exhibits no similarity to eukaryotic protein kinases and that it contains a Walker motif A (or P-loop) as nucleotide binding site. Interestingly, HprK/P has a structural fold resembling that in kinases phosphorylating certain low molecular weight substrates such as nucleosides, nucleotides or oxaloacetate. The structures of the complexes of HprK/P with HPr and P-Ser-HPr have also been determined, which allowed proposing a detailed mechanism for the kinase and phosphorylase functions of HprK/P.