Editing Calvin cycle (section)

==Light-dependent regulation==
{{main|Light-dependent reactions}}
These reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH.<ref>{{Cite journal |last1=Schreier |first1=Tina |last2=Hibberd |first2=Julian |author-link2=Julian Hibberd |date=1 March 2019 |title=Variations in the Calvin–Benson cycle: selection pressures and optimization? |url=https://doi.org/10.1093/jxb/erz078 |journal=Journal of Experimental Botany |publication-date=27 March 2019 |volume=70 |issue=6 |pages=1697–1701 |doi=10.1093/jxb/erz078 |pmid=30916343 |pmc=6436154 |via=Oxford Academic}}</ref>

There are two regulation systems at work when the cycle must be turned on or off: the [[thioredoxin]]/[[ferredoxin]] activation system, which activates some of the cycle enzymes; and the [[RuBisCo]] enzyme activation, active in the Calvin cycle, which involves its own activase.<ref>{{Citation |last1=Konwarh |first1=Rocktotpal |title=Chapter 7 - Exemplary evidence of bio-nano crosstalk between carbon dots and plant systems |date=2022-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780323902601000024 |work=Carbon Dots in Agricultural Systems |pages=155–173 |editor-last=Khan |editor-first=Raju |access-date=2023-04-22 |publisher=Academic Press |language=en |isbn=978-0-323-90260-1 |last2=Abda |first2=Ebrahim M. |last3=Haregu |first3=Simatsidk |last4=Singh |first4=Anand Pratap |editor2-last=Murali |editor2-first=S. |editor3-last=Gogoi |editor3-first=Satyabrat}}</ref>

The thioredoxin/ferredoxin system activates the enzymes glyceraldehyde-3-P dehydrogenase, glyceraldehyde-3-P phosphatase, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and ribulose-5-phosphatase kinase, which are key points of the process. This happens when light is available, as the ferredoxin protein is reduced in the [[photosystem I]] complex of the thylakoid electron chain when electrons are circulating through it.<ref>{{cite journal |author1=Besse, I  |author2=Buchanan, B | title = Thioredoxin-linked animal and plant processes: the new generation | journal = [[Bot. Bull. Acad. Sin.]] | year = 1997 | volume = 38 | pages = 1–11 }}</ref> Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle enzymes by severing a [[cystine]] bond found in all these enzymes. This is a dynamic process as the same bond is formed again by other proteins that deactivate the enzymes. The implications of this process are that the enzymes remain mostly activated by day and are deactivated in the dark when there is no more reduced ferredoxin available.{{Citation needed|date=December 2023}}

The enzyme RuBisCo has its own, more complex activation process. It requires that a specific [[lysine]] amino acid be [[carbamylated]] to activate the enzyme. This lysine binds to [[RuBP]] and leads to a non-functional state if left uncarbamylated. A specific activase enzyme, called [[RuBisCO#By RuBisCO activase|RuBisCo activase]], helps this carbamylation process by removing one proton from the lysine and making the binding of the carbon dioxide molecule possible. Even then the RuBisCo enzyme is not yet functional, as it needs a magnesium ion bound to the lysine to function. This magnesium ion is released from the thylakoid lumen when the inner pH drops due to the active pumping of protons from the electron flow. RuBisCo activase itself is activated by increased concentrations of ATP in the stroma caused by its [[phosphorylation]].<ref>{{Cite journal |last1=Ruuska |first1=Sari A. |last2=Andrews |first2=T. John |last3=Badger |first3=Murray R. |last4=Price |first4=G. Dean |last5=von Caemmerer |first5=Susanne |date=2000-02-01 |title=The Role of Chloroplast Electron Transport and Metabolites in Modulating Rubisco Activity in Tobacco. Insights from Transgenic Plants with Reduced Amounts of Cytochrome b/f Complex or Glyceraldehyde 3-Phosphate Dehydrogenase |journal=Plant Physiology |language=en |volume=122 |issue=2 |pages=491–504 |doi=10.1104/pp.122.2.491 |pmid=10677442 |pmc=58886 |issn=1532-2548}}</ref>