Calvin cycle 206015 225545056 2008-07-14T06:36:56Z DorganBot 2844817 robot Adding: [[hu:Calvin-ciklus]] [[Image:calvin-cycle3.png|thumb|right|300px| Overview of the Calvin cycle and carbon fixation]] The '''Calvin cycle''' (or '''Calvin-Benson-Bassham cycle''' or carbon fixation) is a series of [[biochemistry|biochemical]] reactions that takes place in the [[stroma]] of [[chloroplast]]s in [[photosynthesis|photosynthetic]] [[organism]]s. It was discovered by [[Melvin Calvin]], [[James Bassham]] and [[Andrew Benson]] at the [[University of California, Berkeley]] .<ref>{{cite journal |author=Bassham J, Benson A, Calvin M |title=The path of carbon in photosynthesis |url=http://www.jbc.org/cgi/reprint/185/2/781.pdf |journal=J Biol Chem |volume=185 |issue=2 |pages=781–7 |year=1950 |pmid=14774424}}</ref> It is one of the [[light-independent reaction]]s or dark reactions. ==Overview== During photosynthesis, [[light]] [[energy]] is used to generate chemical [[free energy]], stored in glucose. 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 compound]]s 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 [[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 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 [[primary production|net productivity]]. The sum of reactions in the Calvin cycle is the following: :3 CO<sub>2</sub> + 6 [[Nicotinamide adenine dinucleotide phosphate|NADPH]] + 5 H<sub>2</sub>O + 9 [[Adenosine triphosphate|ATP]] → C<sub>3</sub>H<sub>5</sub>O<sub>3</sub>-PO<sub>3</sub><sup>2-</sup> + 2 H<sup>+</sup> + 6 [[Nicotinamide adenine dinucleotide phosphate|NADP<sup>+</sup>]] + 9 [[Adenosine diphosphate|ADP]] + 8 P<sub>i</sub> :OR :3 CO<sub>2</sub> + 6 C<sub>21</sub>H<sub>29</sub>N<sub>7</sub>O<sub>17</sub>P<sub>3</sub> + 5 H<sub>2</sub>O + 9 C<sub>10</sub>H<sub>16</sub>N<sub>5</sub>O<sub>13</sub>P<sub>3</sub> → C<sub>3</sub>H<sub>5</sub>O<sub>3</sub>-PO<sub>3</sub><sup>2-</sup> + 2 H<sup>+</sup> + 6 [[Nicotinamide adenine dinucleotide phosphate|NADP<sup>+</sup>]] + 9 C<sub>10</sub>H<sub>15</sub>N<sub>5</sub>O<sub>10</sub>P<sub>2</sub> + 8 P<sub>i</sub> <!--NADP+ from above,should be 6 C21H29N7O17P3 + H12O32 = 6 C21H31N7O22.333P3 OR 6 C21H31N7O(67/3)P3 As this author does not yet believe that this oxidation reaction should cause such a large change in composition, can anyone else verify this formula and insert either 6 C21H31N7O22.333P3 OR 6 C21H31N7O(67/3)P3 in place of NADP+. Note:C21H29N7O17P3 is taken from the wiki article on NADPH. I may have reversed the data, such that NADP+ = C21H29N7O17P3, and NADPH = C21H29N7O17P3 - H12O32 = 6 C21H27N7O11.666P3 OR 6 C21H31N7O(35/3)P3. --> It should be noted that hexose (six carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>, 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," specifically, glyceraldehyde-3-phosphate. ==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) in a two-step reaction [http://www.farazdaghi.com/papers/html/photosynthesis.htm]. Rubisco is a large, slow enzyme averaging 3 substrate per second compared to 1000/s for most other enzymes 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 [[Adenosine triphosphate|ATP]] (which was produced in the light-dependent stage). [[1,3-bisphosphoglycerate]] (glycerate-1,3-bisphosphate) and [[Adenosine diphosphate|ADP]] are the products. (However, note that two PGAs are produced for every CO<sub>2</sub> that enters the cycle, so this step utilizes 2[[Adenosine triphosphate|ATP]] per CO<sub>2</sub> fixed. # The enzyme G3P dehydrogenase catalyses the [[redox|reduction]] of 1,3BPGA by [[NADPH]] (which is another product of the light-dependent stage). [[Glyceraldehyde 3-phosphate]] (also G3P, GP) is produced, and the NADPH itself was oxidized and becomes NADP<sup>+</sup>. Again, two NADPH are utilized per CO<sub>2</sub> fixed. (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 a G3P and a DHAP into [[fructose-6-phosphate]] (6C). A phosphate ion is lost into solution. # Then fixation of another CO<sub>2</sub> generates two more G3P. # 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). # E4P and a DHAP (formed from one of the G3P from the second CO<sub>2</sub> fixation) are converted into [[sedoheptulose-1,7-bisphosphate]] (7C) by aldolase enzyme. # Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle which are unique to plants) cleaves [[sedoheptulose-1,7-bisphosphate]] into [[sedoheptulose-7-phosphate]], releasing an inorganic phosphate ion into solution. # Fixation of a third CO<sub>2</sub> 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 CO<sub>2</sub>, with generation of three pentoses which can be converted to Ru5P. # R5P is converted into [[ribulose-5-phosphate]] (Ru5P, RuP) by [[phosphopentose isomerase]]. Xu5P is converted into RuP by [[phosphopentose epimerase]]. # 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 6 G3P produced, three RuBP (5C) are made totalling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 CO<sub>2</sub>. [[RuBisCO]] also reacts competitively with O<sub>2</sub> instead of CO<sub>2</sub> in ''[[photorespiration]]''. The rate of photorespiration is higher at high temperatures. "[[photorespiration]]" turns RuBP into 3PGA and 2-phosphoglycolate, a 2-carbon molecule which can be converted via glycolate and glyoxalate to glycine. Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine +CO<sub>2</sub>. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3PGA. Obviously photorespiration has very negative consequences for the plant, because rather than fixing CO<sub>2</sub>, this process leads to loss of CO<sub>2</sub>. [[C4 carbon fixation]] evolved to circumvent photorespiration, but can only occur in certain plants living in very warm or tropical climates. ==Products of the Calvin cycle== The immediate product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P) and water. 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. Hexose isomerase converts about half of the F6P molecules in to [[glucose-6-phosphate]]. These are phosphorescent and the [[glucose]] can be used to form [[starch]], which is stored in, for example, [[potato]]es, or [[cellulose]] used to build up [[cell walls]]. Glucose, with [[fructose]], forms [[sucrose]], a non-reducing sugar which is a stable storage [[sugar]], unlike glucose. ==See also== *[[Citric Acid Cycle]] *[[Photorespiration]] *[[C4 carbon fixation]] *[[Nitrogen Fixation]] ==References== {{reflist}} * Bassham, J.A. (2003). Mapping the carbon reduction cycle: a personal retrospective. ''Photosynthesis Research'', volume 76, pages 25-52 (see: {{Entrez Pubmed|16228564}}).Mario Otmman (1998) * Diwan, Joyce J. (2005). ''Photosynthetic Dark Reaction'' at [http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm] {{biology-footer}} {{BranchesofChemistry}} [[Category:Photosynthesis|Calvin cycle]] [[ca:Cicle de Calvin]] [[cs:Calvinův cyklus]] [[da:Calvin-cyklus]] [[de:Calvin-Zyklus]] [[es:Ciclo de Calvin]] [[fr:Cycle de Calvin]] [[gl:Ciclo de Calvin]] [[it:Ciclo di Calvin]] [[he:מעגל קלווין]] [[hu:Calvin-ciklus]] [[nl:Calvincyclus]] [[ja:カルビン回路]] [[no:Calvinsyklusen]] [[pl:Cykl Calvina]] [[pt:Ciclo de Calvin]] [[ru:Восстановительный пентозофосфатный цикл]] [[sv:Calvin-cykeln]] [[zh:卡爾文循環]]