Calvin cycle

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Overview of the Calvin cycle and carbon fixation

The Calvin cycle (or Calvin-Benson cycle or carbon fixation) is a series of biochemical reactions that takes place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin and Andrew Benson at the University of California, Berkeley. James Bassham also made important contributions to elucidating this pathway. It is one of the light-independent reactions.

Overview

During photosynthesis, light energy is used to generate chemical free energy, stored in ATP and NADPH. The light-independent Calvin cycle, also (misleadingly) known as the "dark reaction" or "dark stage", uses the energy from short-lived electronically-excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals which feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.

The enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as glycolysis and gluconeogenesis, but they are to be found in the chlorophlast stroma instead of the cell cytoplasm, 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 operating in reverse to respiration, which would create a continuous cycle of carbon dioxide being reduced to carbohydrates, and carbohydrates being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity.

The sum of reactions in the Calvin cycle is the following:

6 CO₂ + 12 NADPH + 12 H⁺ + 18 ATP → C₆H₁₂O₆ + 6 H₂O + 12 NADP⁺ + 18 ADP + 18 P_i

*Steps of the Calvin cycle**

The enzyme RuBisCO catalyses the carboxylation of Ribulose-1,5-bisphosphate, a 5 carbon compound, by carbon dioxide (a total of 6 carbons). Two molecules of glycerate-3-phosphate, a 3-carbon compound, are created. (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA)

The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATP (which was produced in the light-dependent stage). 1,3-bisphosphoglycerate (glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two PGAs are produced for every CO₂ that enters the cycle, so this step happens twice.)

The enzyme G3P dehydrogenase catalyses the reduction of 1,3BPGA by NADPH (which was another product of the light-dependent stage). Glyceraldehyde-3-phosphate (also G3P, GP) is produced, and the NADPH itself was oxidised and hence becomes NADP⁺.

(Simplified versions of the Calvin cycle integrate the remaining steps, except for the last one, into one general step - the regeneration of RuBP - also, one G3P would exit here.)

Triose phosphate isomerase converts some G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.

Aldolase and Fructose-1,6-bisphosphatase convert some of these two into fructose-6-phosphate (6C). A phosphate ion is lost to ADP.

Up to this point, as per the overall equation given above, 6 carbon dioxide molecules would have been converted, with the use of 6 RuBP, 12 ATP and 12 NADPH, to 12 G3P molecules. One F6P, (= 2 G3P) then exits the cycle, while 10 of these G3P molecules continue, giving a ratio of 1:5 G3P. Obviously, the ratio of carbon dioxide entering the cycle to RuBP already present is also 1:5.

F6P is then combined with another G3P (total 9C) and then cleaved into xylulose-5-phosphate (X5P) and erythrose-4-phosphate by transketolase.

E4P and DHAP are converted into sedoheptulose-7-phosphate (7C) by S1,7BPase. A phosphate ion is lost to ADP.

S7P is then combined with another G3P (total 10C) and then cleaved into another X5P and ribose-5-phosphate (R5P) again by transketolase.

X5P is converted into ribulose-5-phosphate (Ru5P, RuP) by epimerase. R5P is also converted into RuP by ribose isomerase.

Finally, phosphoribulokinase phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.

All the G3P produced earlier is converted into RuBP (5C), so 10 G3Ps (30C, 10 phosphates) were needed to produce 6 RuBPs (30C, 6 phosphates). 6 ATPs were also needed in the last step, giving a total of 18 ATPs used up per 6 CO₂s. However, four phosphate ions are lost and these also form ATP. The energy in those ATPs is used to drive some of the reactions.

At high temperatures, RuBisCO will react with O₂ instead of CO₂ in photorespiration. This turns RuBP into 3PGA and 2-phosphoglycolate, a 2-carbon molecule which can be converted into 3PGA, some of which will exit the Calvin cycle. However, if this continues the RuBP will eventually be depleted, which slows down the cycle if electrons are entering from the light-dependent reaction too quickly.

Products of the Calvin cycle

The two G3P molecules (or one F6P molecule) which have exited the cycle are used to make carbohydrates. In simplified versions of the Calvin cycle they may be converted to F6P after exit, but this conversion is also part of the cycle. Hexose isomerase converts about half of the F6P molecules into glucose-6-phosphate. These are dephosphorylated and the glucose can be used to form starch, which is stored in, for example, potatoes, or cellulose used to build up cell walls. Other glucose, with fructose, forms sucrose, the plant sugar.

References

Bassham, J.A. (2003). Mapping the carbon reduction cycle: a personal retrospective. Photosynthesis Research, volume 76, pages 25-52 (see: Entrez PubMed [16228564]).

Diwan, Joyce J. (2005). Photosynthetic Dark Reaction at [link]

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