Calvin cycle

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 with James Bassham also contributing.^[1] It is one of the light-independent reactions or dark reactions.

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). Rubisco is a large, slow enzyme averaging 3 substrate per second compared to 1000/s of most other enzyme in the Calvin cycle. 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 into solution.

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-1,7-bisphosphate (7C) by a transaldolase enzyme.

• S17BPase cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.

• 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 phosphopentose 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.

The immediate product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P). Two G3P molecules (or one F6P molecule) that have exited the cycle are used to make larger carbohydrates. In simplified versions of the Calvin cycle they may be converted to F6P or F5P after exit, but this conversion is also part of the cycle.