The Calvin-Benson cycle is the source of almost all the organic carbon in the biosphere. It uses the ATP and NADPH generated by the photosynthetic light-dependent reactions to fix carbon dioxide into organic carbon. In the dark, the light-dependent reactions are inactive and in response the Calvin-Benson cycle is down regulated. The small chloroplast protein CP12 is responsible for Calvin-Benson cycle down regulation; in the light it is intrinsically disordered and in the dark it becomes ordered due to the oxidation of two disulfide bridges. This oxidation causes CP12 to form an inhibited enzyme complex with Calvin- Benson cycle enzymes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK). I have expressed and purified complexes of CP12, GAPDH and PRK from the cyanobacterium Thermosynechococcus elongatus for structure and function studies. I solved structures of GAPDH and GAPDH-CP12 using X-ray crystallography producing structures of full length CP12 and in collaboration with Dr. Nita Shah and Dr. Doryen Bubeck we solved the first structure of the GAPDH-CP12-PRK complex using cryo-electron microscopy. In conjunction with enzyme assays, these structures reveal how CP12 inhibits enzyme activity and how it senses the nucleotide and redox state of the chloroplast to regulate GAPDH and PRK, furthering our understanding Calvin-Benson cycle regulation and carbon fixation as a whole.