Calvin cycle

The Calvin cycle, Calvin–Benson–Bassham (CBB) cycle, reductive pentose phosphate cycle (RPP cycle) or C3 cycle is a series of

The enzymes in the Calvin cycle are functionally equivalent to most enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway, but the enzymes in the Calvin cycle are found in the chloroplast stroma instead of the cell cytosol, separating the reactions. They are activated in the light (which is why the name "dark reaction" is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions when they have no net productivity.^{[citation needed]}

The sum of reactions in the Calvin cycle is the following:^{[citation needed]}

3 CO
₂ + 6

glyceraldehyde-3-phosphate (G3P) + 6 NADP⁺ + 9 ADP + 8 P_i   (P_i = inorganic phosphate

)

1. The enzyme
   3-phosphoglycerate (also written as 3-phosphoglyceric acid, PGA, 3PGA, or 3-PGA), a 3-carbon compound.^[7]
2. The enzyme
   1,3-Bisphosphoglycerate (glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two 3-PGAs are produced for every CO
   ₂ that enters the cycle, so this step utilizes two ATP per CO
   ₂ fixed.)^{[citation needed}
   ]
3. The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyses the reduction of 1,3BPGA by NADPH (which is another product of the light-dependent stage). Glyceraldehyde 3-phosphate (also called G3P, GP, TP, PGAL, GAP) is produced, and the NADPH itself is oxidized and becomes NADP⁺. Again, two NADPH are utilized per CO
   ₂ fixed.^{[citation needed]}

The next stage in the Calvin cycle is to regenerate RuBP. Five G3P molecules produce three RuBP molecules, using up three molecules of ATP. Since each CO₂ molecule produces two G3P molecules, three CO₂ molecules produce six G3P molecules, of which five are used to regenerate RuBP, leaving a net gain of one G3P molecule per three CO₂ molecules (as would be expected from the number of carbon atoms involved).^{[citation needed]}

The regeneration stage can be broken down into a series of steps.

1. Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.^{[citation needed}
   ]
2. fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphate (6C). A phosphate ion is lost into solution.^{[citation needed}
   ]
3. Then fixation of another CO
   ₂ generates two more G3P.^[citation needed]
4. F6P has two carbons removed by
   xylulose-5-phosphate (Xu5P).^{[citation needed}
   ]
5. E4P and a DHAP (formed from one of the G3P from the second CO
   ₂ fixation) are converted into
   sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.^{[citation needed}
   ]
6. sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.^{[citation needed}
   ]
7. Fixation of a third CO
   ₂ generates two more G3P. The ketose S7P has two carbons removed by
   ribose-5-phosphate (R5P), and the two carbons remaining on transketolase are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 CO
   ₂, with generation of three pentoses that can be converted to Ru5P.^{[citation needed}
   ]
8. R5P is converted into
   phosphopentose isomerase. Xu5P is converted into RuP by phosphopentose epimerase.^{[citation needed}
   ]
9. Finally, phosphoribulokinase (another plant-unique enzyme of the pathway) phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.^{[citation needed]}

Thus, of six G3P produced, five are used to make three RuBP (5C) molecules (totaling 15 carbons), with only one G3P available for subsequent conversion to hexose. This requires nine ATP molecules and six NADPH molecules per three CO
₂ molecules. The equation of the overall Calvin cycle is shown diagrammatically below.^{[citation needed]}

RuBisCO also reacts competitively with O
₂ instead of CO
₂ in photorespiration. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3-PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine. Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine plus CO
₂. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3-PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing CO
₂, this process leads to loss of CO
₂. C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example. Furthermore, RuBisCOs catalyzing the light-independent reactions of photosynthesis generally exhibit an improved specificity for CO₂ relative to O₂, in order to minimize the oxygenation reaction. This improved specificity evolved after RuBisCO incorporated a new protein subunit.^[8]

Products

The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2 NADP⁺. (ADP and NADP⁺ are not really "products". They are regenerated and later used again in the light-dependent reactions). Each G3P molecule is composed of 3 carbons. For the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated. So, 5 out of 6 carbons from the 2 G3P molecules are used for this purpose. Therefore, there is only 1 net carbon produced to play with for each turn. To create 1 surplus G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle. To make one glucose molecule (which can be created from 2 G3P molecules) would require 6 turns of the Calvin cycle. Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs.^[9]

Light-dependent regulation

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.^[10]

There are two regulation systems at work when the cycle must be turned on or off: the

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.^[12] 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]}

The enzyme RuBisCo has its own, more complex activation process. It requires that a specific