Calvin cycle

The Calvin cycle ( Calvin - Benson cycle or Ribulosebisphosphatzyklus ) is a cyclic sequence of chemical reactions (CO2 ) is reduced to glucose and assimilated by the carbon dioxide. The metabolic pathway found in C3 plants and with additional reactions in all other photosynthetic ( photoautotrophic ) creatures instead; is the dark reaction. He also serves many chemoautotrophic beings to assimilate carbon from carbon dioxide. In analogy to the citric acid cycle of the Calvin cycle is also called the reductive pentose phosphate cycle. The cycle was discovered by the American biochemists Melvin Calvin and AA Benson and named after Melvin Calvin.

The Calvin cycle consists of several periodically arranged enzymatic substeps runs in plants in the stroma of the chloroplasts from, in bacteria, however, in the cytoplasm. The individual steps can be divided into three phases: the fixation of CO2, the reduction in the primary fixation product (3 -phosphoglycerate ) and the regeneration of the CO2 acceptor ( ribulose -1 ,5 -bisphosphate ).

As a reducing agent for the reduction of CO2 in the Calvin cycle, NADPH is oxidized to NADP serves. The reduction is endergonic, serves as an energy source ATP, which emits energy by being split into ADP and phosphate.

In photoautotrophic creatures NADPH and ATP are formed by the so-called light reaction of photosynthesis and asked for the Calvin cycle. In chemoautotrophic creatures NADPH and ATP are formed by the exergonic chemical reactions of their energy metabolism.

• 2.1 Photosynthesis types
• 2.2 pyruvate phosphate dikinase

The individual steps of the cycle,

CO2 fixation

In the first step of the Calvin cycle, CO2 is added by the key enzyme RubisCO to ribulose -1 ,5 -bisphosphate ( RuBP2 ) as acceptor; The highly unstable intermediate decomposes spontaneously into two molecules of 3-phosphoglycerate (3- PG ), the first tangible intermediate for C3 - plants.

The primary fixation product 3- phosphoglycerate is not only an important intermediate in the Calvin cycle, but also occurs in other important points in setting up and dismantling of glucose on: gluconeogenesis and glycolysis in the cytoplasm. It also serves as a precursor to building the strength of memory in the chloroplast. However, before entering the three -PG -mentioned reactions, it is reduced in the next part of the Calvin cycle in the chloroplast to glyceraldehyde -3-phosphate.

Reduction in the primary fixation product (3 -phosphoglycerate )

After phosphorylation, and reduction by a specific glyceraldehyde-3- phosphate dehydrogenase (GAPDH; as reductant instead of NADPH, NADH ) Gluconeogenesemetabolit the glyceraldehyde -3-phosphate (GAP ), a key branch point is produced. Since in each round a molecule of CO2 is fixed after three rounds in the balance sheet is a molecule of triose CAP for biosynthesis available, and is available with dihydroxyacetone phosphate ( DHAP ) in balance. Both of which are also known as triose phosphates. They are the first emerging as assimilation profit carbohydrates and can either

• To the formation of a reserve substance serving polysaccharide starch in the stroma of the chloroplasts of plants are used or
• Be via the intermediate dihydroxyacetone phosphate ( DHAP ) and place in exchange for inorganic phosphate (Pi) discharged into the cytoplasm where they feed into glycolysis or gluconeogenesis, or
• The cytosolic synthesis of the transport sugar sucrose ( cane sugar; see below) and
• The synthesis of the cell wall material can be used cellulose.

With sucrose, other parts of the plant can be provided with sugar via the phloem. Thus, the cycle can begin again, but some of the triose phosphates to the primary acceptor ribulose -1 ,5 -bisphosphate must be regenerated. The purpose of the third part of the Calvin cycle.

Regenerate ribulose -1 ,5 -BP

In the third part of the ring closure of the Calvin cycle via the reductive pentose phosphate pathway occurs. In the fixation of CO2 in three ribulose -1 ,5 -bisphosphate (C5) logically arise six triose phosphates ( C3). But this is only one "real" assimilation profit, must again be the three spent ribulose -1 ,5- bisphosphate regenerated from the other five.

Individual reactions

CO2 fixation

In detail, the CO2 group is added to the C2 atom of the enol form of ribulose -1 ,5 -bisphosphate. The result is an enzyme-bound, hypothetical 3-oxo- acid ( arabinitol; exactly: 2- carboxy-3 -keto- D- arabinol -1 ,5 -bisphosphate ) as an unstable intermediate which ( hydrolyzed by water on the C3 atom ) spontaneously in two molecules of triose precursor 3 -phosphoglycerate (3- PG) decays. In this case, from the aforementioned result arabinitol (C6) only one molecule of D-3- phosphoglycerate (C3) and a three- carbon atoms carbanion ( also C3) which is also transformed by protonation into the primary fixed product phosphoglycerate. Characterized two molecules are generated at phosphoglycerate net per bound carbon dioxide, each of which contains one of the two newly added fixed carbon of the carbon dioxide.

Reduction in the primary fixation product (3 -phosphoglycerate )

The steps on the path of 3- phosphoglycerate to glyceraldehyde -3-phosphate are similar to those of gluconeogenesis and are catalyzed by isoenzymes in chloroplasts. The reaction takes place in two steps. First, the 3 -phosphoglycerate is activated by phosphorylation to 1,3- bisphosphoglycerate. This is consumed by the kinase catalyzes energy in the form of ATP. Thereafter, 1,3 - bisphosphoglycerate be reduced by elimination of the newly introduced phosphate group to glyceraldehyde -3 -phosphate ( GAP). The catalyzing enzyme is light -activated, glyceraldehyde -3-phosphate dehydrogenase. NADPH is required for this step, as a reducing agent. The cytoplasmic enzyme of gluconeogenesis however uses NADH as the reductant.

Regenerate ribulose -1 ,5 -BP

In the reductive pentose phosphate pathway, three molecules of CAP and two molecules of DHAP in a branched reaction sequence through various C3, C4, C6, and C7 - sugar intermediates are finally made ​​in three C5 molecules. These are converted into ribulose -5-phosphate and phosphorylated with ATP to ribulose -1 ,5- bisphosphate. For these processes, especially aldolases transketolases are necessary and also phosphatases.

Reactions of the reductive pentose phosphate pathway ( for 3CO2 ):

• Aldolase: GAP (C3 ) DHAP (C3) → fructose-1 ,6 -BP (C6)
• Fructose-1 ,6- bisphosphate phosphatase: fructose-1 ,6- BP H2O → fructose -6-P Pi
• Transketolase: fructose -6-P (C6 ) GAP ( C3) → erythrose -4- P (C4 ) xylulose -5-P (C5)
• Aldolase: erythrose -4- P (C4 ) DHAP (C3) → Seduheptulose -1 ,7 -BP (C7)
• Sedoheptulose -1 ,7- bisphosphate phosphatase: Seduheptulose -1 ,7- BP H2O → Seduheptulose -7 P Pi
• Transketolase: Seduheptulose -7 -P ( C7 ) GAP ( C3) → xylulose -5-P (C5 ) ribose -5-P (C5)
• Rib5P epimerase 2 xylulose -5-P (C5) 2 → ribulose -5-P (C5)
• Rib5P isomerase: Ribose -5-P (C5) → ribulose -5-P (C5)
• Ribulose -5-phosphate kinase: 3 ribulose 5 -P ( C5 ) 3 ATP → 3 Ribulose -1 ,5 -BP (C5 ) 3 ADP

Sum equation of the Calvin cycle

The three CO2 must be spent nine ATP and six NADPH.

Six molecules of ATP and NADPH can be used for the reduction of six ( six molecules of glyceric acid -3-phosphate can be reduced to six, glyceraldehyde -3-phosphate ). This results in six ADP, phosphate six and six NADP . The other three ATP are consumed for the regeneration of the acceptor ( three molecules of ribulose-5- P for three molecules of ribulose -1 ,5- phosphorylated BP ), it creates three ADP.

A total of nine molecules of ATP are hydrolyzed to ADP molecules nine and eight molecules of phosphate are released in the balance sheet. The remaining ninth phosphate is found in glyceric acid -3-phosphate again.

Regulation of the Calvin cycle

For the activation of some of the enzymes involved in the reactions of light is needed. This includes not only the enzyme RubisCO, which catalyzes the fixation itself. But also enzymes in the reductive part of the Calvin cycle ( glyceraldehyde -3-phosphate dehydrogenase ) and in the regenerative part ( fructose-1 ,6- bisphosphate phosphatase, sedoheptulose -1 ,6- bisphosphate phosphatase, sedoheptulose -1 ,7- bisphosphate phosphatase, and ribulose -5-phosphate kinase). In pure darkness, these enzymes are inactive, as required for the assimilation of energy and reducing equivalents are missing. It is activated via the mechanism of the ferredoxin - thioredoxin system. Thioredoxin is thereby ferredoxin from the light response of the photosynthesis of the disulfide ( SS) - converted into the dithiol (SH ) form. Thioredoxin then in turn reduced disulfide bonds in the various enzymes which are thereby activated. In the dark, the dithiol form of the enzymes of molecular oxygen is re- oxidized to the disulfide form. Here, water is formed.

Carbohydrate formation in plants

After three passes of the Calvin cycle, a molecule of glyceraldehyde -3-phosphate can (CAP ) are diverted from the Calvin cycle for further syntheses in the balance sheet. A central product of assimilation in chloroplasts of plants is starch, which initially deposited in the form of granules ( starch grains ) in the stroma. For this buffer carbohydrates released in the form of triose phosphates, if required, which are then reacted in the cytoplasm to the disaccharide sucrose. Sucrose is the major transport form of carbohydrate that passes through the sieve tubes of the phloem in storage organs from non- photosynthetic cells ( roots, tubers, Mark ). There, the sugar can be reused or stored. Include, for example, for recovery glycolysis non- photosynthetic tissues ( and photosynthetic tissue in the dark) and the synthesis of cellulose, nucleotides and other sugar-containing cellular components. For storage, re-form starch grains (starch granules ) in forms that are characteristic ( spherical, oval, lenticular, spindle - or rod- shaped) for the plant and the tissue.

Photosynthetic types

As stated in photorespiration, the RubisCO is inefficient at normal CO2 partial pressure of the air. C4 plants and CAM plants therefore suppress the side reaction by pre-fixing of CO2. This is made possible by an " ATP-driven CO2 pump ". Catalyzed by a chloroplastic pyruvate phosphate dikinase arises as the primary CO2 acceptor from pyruvate ( Pyr) phosphoenolpyruvate ( PEP). This releases energy in the form of ATP is invested. A cytosolic PEP carboxylase catalyzes the condensation of carbon dioxide in the form of bicarbonate (HCO3-) at PEP. The product is the C4 compound oxaloacetate (OA ).

• In C4 plants OA is transported in the form of L- malate or L- aspartate into an adjacent cell type, the bundle sheath cells. There it is again converted to OA and this C4 compound decarboxylated. The released carbon dioxide is then used as a substrate for RuBisCO and as will be further fixed set forth above.
• In ( obligatory ) CAM plants OA is reduced by malate dehydrogenase, L- malate, and then stored in the vacuoles of the cell under the same power consumption. These processes take place at night. On the day of the stored malate is released again and decarboxylated analogous to C4 plants. The fixation of the carbon dioxide corresponds to the steps described above.

Due to the spatial ( C4 plants ) or temporal ( CAM plants ) Separation of Kohlenstoffdioxidvorfixierung and consumption in the RuBisCO reaction produces locally very high CO2 partial pressures, which counteract photorespiration.

Pyruvate phosphate dikinase

The conversion of phosphoenolpyruvate (PEP ) into pyruvate, a reaction of glycolysis is so exergonic that they ( by expending only one molecule of ATP that is ) can be run in the reverse direction is not right. To phosphorylate pyruvate to PEP, using a chloroplast pyruvate phosphate dikinase ( EC 2.7.9.1 ). This enzyme has to activate the unusual property of a phosphate group by ATP hydrolysis ( to AMP). Mechanistically, this is done by transmitting a Pyrophosphatrestes (PPi ) to the enzyme and its subsequent phosphorolysis after specified in the mapping scheme.