Abstract
Phosphoribulokinase (PRK) catalyses the ATP-dependent phosphorylation of ribulose 5-phosphate to give ribulose 1,5-bisphosphate. Regulation of this reaction in response to light controls carbon fixation during photosynthesis. Here, the crystal structure of PRK from the cyanobacterium Synechococcus sp. strain PCC 6301 is presented. The enzyme is dimeric and has an α/β-fold with an 18-stranded β-sheet at its core. Interestingly, a disulfide bond is found between Cys40 and the P-loop residue Cys18, revealing the structural basis for the redox inactivation of PRK activity. A second disulfide bond appears to rigidify the dimer interface and may thereby contribute to regulation by the adaptor protein CP12 and glyceraldehyde-3-phosphate dehydrogenase.