page_title: Phosphoribulokinase
hierarchical_section_title: Phosphoribulokinase
caption_attribution_description: English: 3d cartoon of phosphoribulokinase from Methanospirillum hungatei (5b3f in RCSB PDB)
context_page_description: Phosphoribulokinase is an essential photosynthetic enzyme that catalyzes the ATP-dependent phosphorylation of ribulose 5-phosphate into ribulose 1,5-bisphosphate, both intermediates in the Calvin Cycle. Its main function is to regenerate RuBP, which is the initial substrate and CO₂-acceptor molecule of the Calvin Cycle. PRK belongs to the family of transferase enzymes, specifically those transferring phosphorus-containing groups to an alcohol group acceptor. Along with ribulose 1,5-bisphosphate carboxylase/oxygenase, phosphoribulokinase is unique to the Calvin Cycle. Therefore, PRK activity often determines the metabolic rate in organisms for which carbon fixation is key to survival. Much initial work on PRK was done with spinach leaf extracts in the 1950s; subsequent studies of PRK in other photosynthetic prokaryotic and eukaryotic organisms have followed. The possibility that PRK might exist was first recognized by Weissbach et al. in 1954; for example, the group noted that carbon dioxide fixation in crude spinach extracts was enhanced by the addition of ATP. The first purification of PRK was conducted by Hurwitz and colleagues in 1956.
context_section_description: Phosphoribulokinase (PRK) (EC 2.7.1.19) is an essential photosynthetic enzyme that catalyzes the ATP-dependent phosphorylation of ribulose 5-phosphate (RuP) into ribulose 1,5-bisphosphate (RuBP), both intermediates in the Calvin Cycle. Its main function is to regenerate RuBP, which is the initial substrate and CO₂-acceptor molecule of the Calvin Cycle. PRK belongs to the family of transferase enzymes, specifically those transferring phosphorus-containing groups (phosphotransferases) to an alcohol group acceptor. Along with ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCo), phosphoribulokinase is unique to the Calvin Cycle. Therefore, PRK activity often determines the metabolic rate in organisms for which carbon fixation is key to survival. Much initial work on PRK was done with spinach leaf extracts in the 1950s; subsequent studies of PRK in other photosynthetic prokaryotic and eukaryotic organisms have followed. The possibility that PRK might exist was first recognized by Weissbach et al. in 1954; for example, the group noted that carbon dioxide fixation in crude spinach extracts was enhanced by the addition of ATP. The first purification of PRK was conducted by Hurwitz and colleagues in 1956.
  ATP + Mg²⁺ - D-ribulose 5-phosphate  ADP + D-ribulose 1,5-bisphosphate
  The two substrates of PRK are ATP and D-ribulose 5-phosphate, whereas its two products are ADP and D-ribulose 1,5-bisphosphate. PRK activity requires the presence of a divalent metal cation like Mg²⁺, as indicated in the reaction above.
caption: 3D cartoon depiction of a phosphoribulokinase protomer from Methanospirillum hungatei
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