night. There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH. 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. 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. 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. 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 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. References Citations Bibliography Further reading Rubisco Activase, from the Plant Physiology Online website Thioredoxins, from the Plant Physiology Online website External links The Biochemistry of the Calvin Cycle at Rensselaer Polytechnic Institute The Calvin Cycle and the Pentose Phosphate Pathway from Biochemistry, Fifth Edition by Jeremy M. Berg, John L. Tymoczko and Lubert Stryer. Published by W. H. Freeman and Company (2002). Photosynthesis Metabolism