Structure of the photosynthetic Calvin-Benson-Bassham sedoheptulose-1,7-bisphosphatase SBPase from the model microalga Chlamydomonas reinhardtii

The Calvin-Benson-Bassham cycle (CBBC) performs carbon fixation in photosynthetic organisms. Among the eleven enzymes that participate in the pathway, sedoheptulose-1,7-bisphosphatase (SBPase) is expressed in photo-autotrophs and contributes to the regeneration of ribulose-1,5-bisphosphate, the carbon fixation co-substrate used by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While SBPase is structurally similar to fructose-1,6-bisphosphatase (FBPase) involved in both neoglucogenesis and the CBBC, it exclusively functions in the CBBC and is indispensable for building a productive cycle. In this study we report the first structure of an SBPase from a chlorophyte, the model unicellular green microalga Chlamydomonas reinhardtii. By combining experimental and computational structural analyses, we describe the topology, conformations and quaternary structure of Chlamydomonas reinhardtii SBPase. We identify active site residues and locate sites of redox-and phospho-post-translational modifications that contribute to enzymatic functions. Finally, we observe that CrSBPase adopts distinct oligomeric states that may dynamically contribute to the control of its activity.