<P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

Where do light dependent and light independent reactions take place