Overview of the Calvin cycle and carbon fixation

The Calvin cycle, Calvin–Benson-Bassham (CBB) cycle, reductive pentose phosphate cycle or C3 cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It is also known as the dark reactions.

The cycle was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley^[1] by using the radioactive isotope carbon-14. It is one of the light-independent (dark) reactions used for carbon fixation.

Overview[link]

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. The light-independent Calvin cycle uses the energy from short-lived electronically excited carriers to convert carbon dioxide and water into organic compounds^[2] that can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following biochemical 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 gluconeogenesis and the pentose phosphate pathway, but they are to be found in the chloroplast 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 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:

3 CO₂ + 6 NADPH + 5 H₂O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H⁺ + 6 NADP⁺ + 9 ADP + 8 P_i   (P_i = inorganic phosphate)

Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as C₆H₁₂O₆, this is mainly a convenience to counter the equation of respiration, where six-carbon sugars are oxidized in mitochondria. The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or "triose phosphates," namely, glyceraldehyde-3-phosphate (G3P).

Steps[link]

In the first stage of the Calvin cycle, a CO₂ molecule is incorporated into one of two three-carbon molecules (glyceraldehyde 3-phosphate or G3P), using up two molecules of ATP and two molecules of NADPH, both of which were produced in the light-dependent stage. Three steps are involved.

Calvin cycle step 1 (black circles represent carbon atoms)

Calvin cycle steps 2 and 3 combined

1. The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate, RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction.^[3] The product of the first step is enediol-enzyme complex that can capture CO₂ or O₂. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The CO₂ that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of 3-phosphoglycerate, or 3-PGA, a 3-carbon compound^[4] (also: 3-phosphoglyceric acid, PGA, 3PGA).
2. The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3-PGA by ATP (which was produced in the light-dependent stage). 1,3-Bisphosphoglycerate (1,3BPGA, glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two 3-PGAs are produced for every CO₂ that enters the cycle, so this step utilizes two ATP per CO₂ fixed.)
3. The enzyme G3P dehydrogenase catalyses the reduction of 1,3BPGA by NADPH (which is another product of the light-dependent stage). Glyceraldehyde 3-phosphate (also G3P, GP, TP, PGAL) is produced, and the NADPH itself was oxidized and becomes NADP⁺. Again, two NADPH are utilized per CO₂ fixed.

Regeneration stage of the Calvin cycle

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).

The regeneration stage can be broken down into steps.

1. Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.
2. Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphate (6C). A phosphate ion is lost into solution.
3. Then fixation of another CO₂ generates two more G3P.
4. F6P has two carbons removed by transketolase, giving erythrose-4-phosphate. The two carbons on transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P).
5. E4P and a DHAP (formed from one of the G3P from the second CO₂ fixation) are converted into sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.
6. Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle that are unique to plants) cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.
7. Fixation of a third CO₂ generates two more G3P. The ketose S7P has two carbons removed by transketolase, giving 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 CO₂, with generation of three pentoses that can be converted to Ru5P.
8. R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase. Xu5P is converted into RuP by phosphopentose epimerase.
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.

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 CO₂ molecules. The equation of the overall Calvin cycle is shown diagrammatically below.

The overall equation of the Calvin cycle (black circles represent carbon atoms)

RuBisCO also reacts competitively with O₂ instead of CO₂ 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 +CO₂. 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 CO₂, this process leads to loss of CO₂. C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates, for example, corn.

Products[link]

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. In order 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.^[5]

See also[link]

• Light-independent reaction
• Light-dependent reactions
• Citric acid cycle
• Photorespiration
• C₄ carbon fixation
• Nitrogen fixation

References[link]

Citations

Bibliography

• Bassham JA (2003). "Mapping the carbon reduction cycle: a personal retrospective". Photosyn. Res. 76 (1-3): 35–52. DOI:10.1023/A:1024929725022. PMID 16228564. 
• Diwan, Joyce J. (2005). "Photosynthetic Dark Reaction". Biochemistry and Biophysics, Rensselaer Polytechnic Institute. http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm. 
• Portis, Archie; Parry, Martin (2007). "Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective". Photosynthesis Research 94 (1): 121–143. DOI:10.1007/s11120-007-9225-6. PMID 17665149. http://ddr.nal.usda.gov/dspace/bitstream/10113/3976/1/IND43944177.pdf 

External links[link]

• Khan Academy, video introduction