Abstract
Photosynthetic CO2 fixation supports life on Earth and is a fundamental source of food, fuels and chemicals for human society. In the vast majority of photosynthetic organisms, carbon fixation is operated by the Calvin-Benson (CB) cycle, a pathway that has been extensively studied at physiological, biochemical and structural level. It consists of 13 distinct reactions catalyzed by 11 enzymes1. In land plants and algae, the CB cycle takes place in the chloroplast, a specialized organelle operating the photosynthetic process. Despite decades of efforts, one last enzyme, the phosphoribulokinase (PRK), remains uncharacterized at atomic-scale while the structure of the other ten enzymes have been solved or confidentially modeled by close homology from the structure of isoforms found in other pathways. PRK together with ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) enzyme, is exclusive of the CB cycle. In opposition to RuBisCO, whose structure has been solved from diverse sources2, the structure of PRK remains unknown despite its characterization is crucial to better understand and control photosynthesis.
Here, we report for the first time the crystal structures of redox-sensitive PRK from two model species: the green alga Chlamydomonas reinhardtii (CrPRK) and the land plant Arabidopsis thaliana (AtPRK). The enzyme is an elongated homodimer characterized by a large central β-sheet of eighteen strands, extending between the two catalytic sites positioned at the dimer edges. The structure was also studied in solution through the combination of size exclusion chromatography and small-angle X-ray scattering (SEC-SAXS) and no appreciable differences were found.
This study completes the description at atomic level of the 11 CB cycle enzymes providing a rational basis for catalytic improvement of carbon fixation in chloroplasts.