DARK REACTIONS-LIGHT INDEPENDENT REACTION

DARK REACTIONS-LIGHT INDEPENDENT REACTION

6.     DARK REACTIONS – LIGHT INDEPENDENT REACTIONS

The ATP and NADPH generated during photophosphorylation reactions are consumed for the fixation of CO2 by the enzymes present in the stroma. Unlike, photophosphorylation, fixation of CO2 is independent of light. Thus, referred as dark reactions.
Most of the angiosperms have the photosynthetic carbon reduction cycle i.e. C3 cycle also as Calvin cycle. Other metabolic pathways associated with C3 cycle are C4 cycle and photorespiration.
6.1.    Calvin cycle
It consists of 3 stages :
6.1.1.    Carboxylation 
The initial acceptor of CO2 is Ribulose-1, 5-bisphosphate. It accepts CO2 and forms 2 molecules of 3-phosphoglycerate i.e. the first stable intermediate and since it is a 3-Carbon compound, the cycle is named as C3 cycle.
6.1.2.    Reduction
Reduction of 3-phosphoglycerate form glyceraldehyde-3-phosphate.
6.1.3.    Regeneration
Regeneration of RuBP from glyceraldehyde-3-phosphate.

Rubisco (Ribulose bisphosphate carboxylase/oxygenase)
Rubisco is a highly abundant protein that consist of 40% of the total soluble protein of the leaves. Its affinity for CO2 is sufficiently high that it allows the carboxylation at low concentration of CO2. The enzyme has oxygenase activity also.
6.2.    The process of the Calvin cycle
In Calvin cycle, 3 molecules of CO2 are fixed thus forming 6 molecules of phosphoglycerate which convert to 6 molecules of triose phosphate. Out of 6 molecules, one is provided to the cell for biosynthetic processes and five regenerates 3 molecules of RuBP utilized for the continuation of the Calvin cycle.
The process occurs as follows:

Similarly, ribose-5-phosphate is converted to ribulose-1,5-bisphosphate in an ATP dependent reaction by the ribulose-5-phosphate kinase.


6.3.    Mechanism of aldolase reaction



6.4.    Mechanism of transketolase reaction

6.5.    Regulation of the Calvin-Benson cycle 
The efficient use of energy in the Calvin-Benson cycle requires the existence of specific regulatory mechanisms ensuring not only that all intermediates in the cycle are present at adequate concentrations in the light, but also that the cycle is turned off when not needed in the dark. Although rubisco plays a critical role in the carbon cycle of the biosphere, its catalytic rate is extremely slow (1-12 CO2 fixations per second). This paradoxical feature was clarified when George Lorimer and colleagues found that rubisco must be activated before acting as a catalyst. Further studies revealed that the CO2 molecule plays a dual role in the activity of Rubisco: CO2 participates in the transformation of the enzyme from an inactive to an active form (modulation) and is the substrate for the carboxylase reaction (catalysis). 
In addition to Rubisco, light controls the activity of four other enzymes of the Calvin-Benson cycle via the ferredoxin-thioredoxin system, which consists of ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxin. The deactivation of the target enzymes in the dark appears to take place by reversal of the reduction (activation) pathway. Oxygen or reactive oxygen species transform reduced thioredoxin (-SH HS) to the oxidized state (-S-S-), which in turn converts the reduced target enzyme to the oxidized state, leading to loss of catalytic activity. 
Excitation of PS I leads to the reduction of ferredoxin and a fraction of electrons are transferred to small protein thioredoxin, which reduces the disulphide groups of certain Calvin cycle enzymes, maintaining them in the active state. In the dark, electron flow to thioredoxin ceases, the sulfhydryl groups of the regulated enzymes become oxidized to the disulphide state and the enzymes are inactivated.
6.6.    Photorespiration
The capacity of Rubisco is to catalyze both the carboxylation and oxygenation of ribulose 1,5-bisphosphate. Carboxylation results in two molecules of 3-phosphoglycerate, whereas oxygenation produces one molecule each of 3-phosphoglycerate and 2-phosphoglycolate. The oxygenation of ribulose 1,5-bisphosphate catalyzed by rubisco initiates a complex series of enzymatic reactions that are compartmentalized in chloroplasts, leaf peroxisomes, and mitochondria. This process is called photorespiration that causes the partial loss of CO2 fixed by C3 cycle. The process initiates with the incorporation of the oxygen molecule in isomer of RuBP that generates 2-Phosphoglycolate and 3-phosphoglycerate in chloroplast by the enzyme rubisco. 2-Phosphoglycolate is further hydrolysed to glycolate by a specific chloroplast phosphatase. Subsequent metabolism of glycolate involves corporation of two other cell organelles;i.e; peroxisomes and mitochondria. Glycolate leaves the chloroplast via specific transporter protein in the envelope membrane and diffusers to the peroxisomes. Glycolate is then oxidised to glyoxylate and hydrogen peroxide by glycolate oxidase. This hydrogen peroxide is destroyed in peroxisomes by the activity of catalase enzyme. Newly formed glyoxylate undergoes transamination for which the amino donor is glutamate. It results in the production of glycine amino acid. Glycine leaves peroxisomes and enters the mitochondria. The glycine decarboxylase enzyme complex influences the conversion of two molecules of glycine and one molecule of NAD+ to one molecule each of seine amino acid, NH4+, NADH and carbon dioxide. The ammonia formed in mitochondria diffuses to chloroplasts, where glutamine synthetase bines with carbon skeletons to form amino acids. The newly formed serine leaves mitochondria and enters the peroxisomes, where it is converted first by transamination to hydroxy pyruvate and then by NADPH dependent reduction to glycerate. Finally, glycerate reverts the chloroplast, where it is phosphorylated to yield 3-PGA by glycerate kinase on the expenditure of ATP currency. Photorespiration involves cycling of carbon and nitrogen. In the Carbon cycle, carbon exits chloroplast in two molecules of glycolate and returns in one molecule of glycerate. In Nitrogen cycle, nitrogen exits chloroplast in one molecule of glutamate and returns in one molecule of ammonia. 
Two molecules of phosphoglycolate (4C) are lost from Calvin cycle by oxygenation of RUBP, finally converted into 3-PGA (3C) and one molecule of CO2. But total organic nitrogen content remains unchanged because the formation of inorganic nitrogen (NH4+) in mitochondria is balanced by the synthesis of glutamine in the chloroplast. The use of NADH in the peroxisome (by hydroxy pyruvate) is balanced by reduction of NAD+ in mitochondria by glycine decarboxylase.
Photorespiration removes toxic metabolic intermediates. Phosphoglycolate and glyoxylate inhibit triose phosphate isomerase that would interfere with the regeneration of ribulose-1,5-biphosphate ultimately declines the rate of photosynthesis. This is the sole pathway in the plant for metabolism of phosphoglycolate. Production of H2O2 during C2 cycle triggers the plant defence response system, that involves cell wall strengthening and activation of phytoalexin biosynthesis. Also, it damages the pathogen by its reactive potential by initiating the hypersensitive response resulting programmed cell death of the attacked cell.
6.7.    Photorespiration: C-2 cycle - Addition of molecular oxygen to ribulose 1–5 bisphosphate by rubisco enzyme is known as photorespiration. Since Rubisco possesses the oxygenase activity. Thus, at high O2 concentration and high temperature, the solubility of all gases decrease while O2 is less affected. It inhibits photosynthesis. This effect was first recognized by Otto Warburg.

Photorespiration produces 3 phosphoglycerates and 2 phosphoglycolates. The 3 phosphoglycerates enter into Calvin cycle. However, the 2 phosphoglycolate is an inhibitor of photosynthesis. The 2 phosphoglycolates are converted into glycolate in the stroma of the chloroplast. The Glycolate is transported to peroxisomes.

The main reactions of photorespiration include three organelles: chloroplast, mitochondria and peroxisomes.
C-2 cycle is important as it dissipates excess ATP and thus prevents damage to the photosynthetic apparatus. It is found to protect C3 plants from photooxidation and photoinhibition.  

6.8.    The C4  cycle
In the leaves of C4 plants, vascular tissues are surrounded by two distinctive photosynthetic cell types, an internal ring of bundle sheath cells, which is wrapped with an outer ring of mesophyll cells. Bundle sheath cells contain starch-rich chloroplasts lacking grana, which differ from those mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. The primary function of Kranz anatomy is to provide a site in which CO2 can be concentrated around RuBisCO, thereby avoiding photorespiration. In order to maintain a significantly higher CO2 concentration in the bundle sheath compared to the mesophyll, the boundary layer of the Kranz has a low conductance to CO2, a property that may be enhanced by the presence of suberin. In this anatomical context, the transport of CO2 from the external atmosphere to the bundle sheath cells proceeds through five successive stages: 


On entry of CO2, fixation of the HCO by PEPCase in the mesophyll cells followed by transport of the 4-carbon acids (malate, aspartate) to bundle sheath cells then decarboxylation of the 4-carbon acids and generation of CO2, which is then reduced to carbohydrate via the Calvin-Benson cycle. After that transport of the 3-carbon backbone (pyruvate or alanine) back to the mesophyll cells and finally regeneration of the HCO acceptor. 
Operation of the C4 cycle requires the cooperative effort of the two distinct chloroplast-containing cell types. The transport process facilitated by plasmodesmata connecting the two cell types generates a much higher concentration of CO2 in bundle sheath cells (the vascular region) than in mesophyll cells. The elevated concentration of CO2 at the carboxylation site of rubisco results in the suppression of ribulose 1,5-bisphosphate oxygenation and hence of photorespiration.
The C4 cycle reduces photorespiration and water loss 
The elevated temperature decreases both the carboxylation capacity of Rubisco and the solubility of CO2, thus limiting the rate of photosynthetic CO2 assimilation in C3 plants. In C4 plants, two features overcome the deleterious effects of high temperature: 
•     First, the affinity of PEPCase for its substrate, HCO3¯, is sufficiently high to saturate the enzyme at the reduced CO2 levels present in warm climates. Further, oxygenase activity is largely suppressed because HCO3¯ does not compete with O2 in the initial carboxylation. This high activity of PEPCase enables C4 plants to reduce their stomatal aperture at high temperatures and thereby conserve water while fixing CO2 at rates equal to or greater than those of C3 plants. 
•     Second, the high concentration of CO2 in bundle sheath cells minimizes the operation of the C2 oxidative photosynthetic cycle. 
Types of C4 pathways
In plants that grow in hot environments, the leaves possess anatomic variations in arrangements of mesophyll cells, bundle sheath cells and vascular bundles. C4 plants show Kranz anatomy. The sieve tubes and the xylem vessels of vascular bundles are surrounded by a sheath of cells known as bundle sheath cells. Mesophyll cells encircle these bundle sheath cells remain in the contact with the intracellular gas space of the leaves. This type of arrangement is an indication of the division of labour. Both types of cells are separated by suberin layer and the borders between cells are penetrated by plasmodesmata that enable the passage of metabolites between the cells.
In some species of Chenopodiaceae Kranz anatomy is not found, rather metabolism occurs in uniform extended cells. At one peripheral end, Phosphoenol-Pyruvate Carboxylase (PEPCO)  is in the cytoplasm and Rubisco in the chloroplast at the other end.


6.8.1.    Three different types of C4 plants are:
NADP-ME Type 
In NADP-ME type C4 plants, that includes maize and sugarcane, chloroplasts of bundle sheath cells are arranged in a centrifugal position with respect to the vascular bundle and have thylakoid membranes with reduced grana stacking. Oxaloacetate is formed by PEP carboxylase in the cytoplasm during the capture of atmospheric CO2, followed by its transport to chloroplasts within mesophyll cells for reduction of oxaloacetate to malate by enzyme NADP-specific malate dehydrogenase. These acids are then exported from mesophyll cells to bundle sheath cells via plasmodesmata, where malate is decarboxylated by NADP-malic enzyme to produce CO2 and reduced NADP to the Calvin cycle. Pyruvic acid is formed as a by-product of decarboxylation. From bundle sheath cell, it is returned to mesophyll chloroplasts and subjected to phosphorylation by pyruvate phosphate dikinase enzyme to generate phosphoenolpyruvate (PEP), the acceptor of inorganic 
C4 NADP-Malic type E.g. Maize and sugarcane
Pyruvate phosphate dikinase enzyme catalyzes the conversion of pyruvate to phosphoenolpyruvate in the mesophyll cells by an unusual reaction. One phosphate residue is transferred from ATP to pyruvate and other residues to a histidine residue of the enzyme specifically on its catalytic site. Thus, a phosphoramide is formed which is transferred to pyruvate forming phosphoenolpyruvate.

6.8.2.    C4 NAD-Malic type E.g. Millets
NAD-ME Type

In NAD-ME type C4 plants that include millets, bundle sheath chloroplasts are composed of thylakoid membranes with developed grana stackings. Both chloroplasts and mitochondria are localized together in a centripetal position with respect to the vascular bundle. The initial product of atmospheric CO2 fixation is oxaloacetate. It gets converted to aspartate via aspartate aminotransferase in the mesophyll cell cytoplasm. This newly formed aspartate is then transported to bundle sheath cell mitochondria, followed by its deamination to OAA by aspartate aminotransferase. This product oxaloacetate is reduced to malate by NAD-malate dehydrogenase and then the malate is decarboxylated by enzyme NAD-ME to feed CO2 to bundle sheath chloroplasts for Calvin cycle. The decarboxylation in bundle sheath cells results in the formation of pyruvate, which is then converted to alanine, for migration towards the mesophyll cells, where it is used for resynthesis of 


6.8.3.    PEP-CK Type 
In PEP-CK type C4 plants that include fast-growing tropical grasses, bundle sheath chloroplasts consist of well- developed grana stacks. The chloroplasts are arranged evenly or in a centrifugal position in bundle sheath cells. The atmospheric CO2 is captured in the form of OAA. It follows conversion of OAA into malate in mesophyll chloroplasts and aspartate in mesophyll cytoplasm. These acids are shuttled in the bundle sheath cells. In bundle sheath chloroplasts, malate is decarboxylated via NAD-ME enzyme. It is aspartate that deaminates and converts back to OAA in the cytoplasm of bundle sheath cells and then is subjected to decarboxylation by enzyme PEP- carboxykinase (PEP-CK) generating CO2 for Calvin cycle. The NADH formed by NAD-ME is oxidized through the mitochondrial electron transport chain to produce ATP by oxidative phosphorylation. The ATP is exported to the cytoplasm, where it is used for the PEP-CK reaction. Of the two decarboxylation products, pyruvate return back to mesophyll chloroplasts via alanine and phosphoenolpyruvate (PEP) migrates directly towards the mesophyll 
C4 phosphoenolpyruvate carboxykinase type: eg. fast-growing tropical grasses

6.8.4.    The significance of C4 cycle
C4 cycle helps the plants to overcome the deleterious effects of high temperature. Since the substrate for PEP carboxylase is HCO3-, thus O2 does not compete in the reaction. It reduces the stomatal aperture thus reducing the transpirational loss and suppress the photorespiration.
6.8.    CAM cycle (Crassulacean acid metabolism)
Crassulacean acid metabolism (CAM) 
The plants showing CAM pathway for carbon reduction are characterized by anatomical features that minimize water loss including thick cuticles, low surface-to-volume ratios, large vacuoles and stomata with small apertures. Along with compound packed mesophyll cells, photoactive stomata in CAM plants enhance the efficiency by restricting CO2 loss during the day. 
In CAM plants, the initial capture of atmospheric CO2 into C4 acids and the final incorporation of CO2 into triose phosphates are spatially close because Kranz anatomy is absent, therefore no differentiation between the sites for both processes but temporally out of phase;i.e; they occur at the different time in the 24-hour light-dark cycle.  Unlike C4 plants, however, CAM species carry out light-dependent reactions and CO2 fixation at different times of the day, rather than in different cells of the leaf. During the night when stomata are open, atmospheric CO2 enters the leaves is captured by cytosolic PEPCase enzyme using phosphoenolpyruvate conversion to oxaloacetate. A cytosolic NAD-malate dehydrogenase converts the oxaloacetate to malate, which is stored in the acid vacuole of mesophyll cell. During the day, the stored malate is transported to the chloroplast and decarboxylated. The released CO2 is made available to the chloroplast for processing via the Calvin-Benson cycle, while the complementary 3-carbon acids are converted to phosphoenolpyruvate via the glycolytic breakdown of stored carbohydrates. 
Changes in the rate of carbon uptake and in enzyme regulation throughout the day create a 24-hour CAM cycle that is divided into four distinct phases: phase I (night), phase II (early morning), phase III (daytime), and phase IV (late afternoon). During the nocturnal phase I, when stomata are open and leaves are respiring, CO2 is captured and stored as malate in the vacuole. CO2 uptake by PEPCase dominates phase I. In the diurnal phase III, when stomata are closed and leaves are photosynthesizing, the stored malate is decarboxylated. This results in high concentrations of CO2 around the active site of rubisco, thereby avoiding the adverse effects of photorespiration. The transient phases II and IV shift the metabolism in preparation for phases III and I, respectively. In phase II, rubisco activity increases, but it decreases in phase IV. In contrast the activity of PEPCase increases in phase IV, but declines in phase II. 
This cycle is typically found in desert plants including members of family Crassulaceae (Bryophyllum), Euphorbiaceae (Euphorbia) and other plants like Pineapple, Vanilla and Agave. In CAM plants the stomata open at night and close during the day. Thus these plants are able to survive the dry environment reducing the transpirational loss.
During night CO2 is accepted by PEP carboxylase and the malate formed is stored in the vacuole. During the day, malate is transported from vacuole to the chloroplast and is decarboxylated to form pyruvate. The released CO2 is fixed by entering the Calvin cycle and NADPH is formed which is used for the synthesis of starch from triose phosphate. Some examples showing the functioning of the CAM cycle during day and night :
6.8.1.    During day

6.8.2.    During night

Malate  →  Malic acid (during the night) for storage because of malic acid unable to cross vacuolar membrane and channels available only for malate and H+.  

The opening of stomata during the night in the hot and drier region adapted plants not only reduces H2O loss but also provides increase  CO2 concentration stored as malic acid via H2CO3 formation. This provides ample of CO2 available for fixation during day time.
Thus demerit of photorespiration i.e. reduced RuBisCO affinity for CO2 due to its low concentration, is avoided by CAM plants easily-just as C4 plants.
Factors affecting photosynthesis
Various environmental factors affect photosynthesis. Light influences the process both in a qualitative and quantitative manner. The action spectrum is a function of the light wavelength it determines maximum production during red light and least in green light. The rate of photosynthesis is greater an intense light then diffused light but at higher light intensity photooxidation of pigments disrupts photosynthetic apparatus.  It is an enzymatic process which requires the optimum temperature to occur efficiently because higher temprature denatures the enzyme and low temperature reduces affinity towards substrate due to a decrease in collisions among them. Increase in carbon dioxide and oxygen concentration boosts the rate up to 1% only. Higher value exhibits toxic effects of these gases resulting in stomatal closure. High oxygen concentration reduces photosynthesis due to photorespiration. Water contributes to this process by donating protons with help of metal ion manganese complex. Apart from these, leaf age and orientation effect the rate of photosynthesis.
6.9.    Pentose phosphate pathway/Hexose Monophosphate shunt Pathway
This cycle is majorly present in the cytosol in animals, while present in both chloroplast and cytosol in plants. PPP generates NADPH i.e. required for lipid and fatty acid biosynthesis. It also provides pentose phosphate i.e a precursor for the ribose and deoxyribose required for the synthesis of nucleic acids. Intermediates of PPP are also significant e.g. erythrose-4-phosphate. i.e. essential for the biosynthesis of flavonoids and aromatic acids. Pentose phosphate pathway is also known as the oxidative pentose phosphate pathway.

6.9.1.    Regulation of oxidative and reductive pentose phosphate pathways
Thioredoxin is a family of proteins with the sequence of the reactive group as Cys-gly-pro-cys. Because of Cys side chains, it occurs in two forms reduced (two –SH groups) and oxidized form (S-S bond). 
All the inactive enzymes of the chloroplast are activated by reduction of thioredoxin which is responsible for providing the signal ‘illumination’(light). Although some isoenzymes are not regulated by thioredoxin e.g. fructose-1,6-bisphosphatase and malate dehydrogenase. In some cases, there are additional sequences at N- or C- terminus which provide cysteine residues. The disulfide bond is formed by oxidation of –SH groups of these cysteine residues by thioredoxin.

6.9.1.1.     Regulation of Rubisco:- Regulation of various enzymes involved in different photosynthetic pathways.
•    Rubisco is regulated by light, pH and Mg2+ ions and is not found to be responsive to the action of thioredoxin.


This increase in pH and Mg2+ ions help in enhancing the activation of Rubisco. After activation, Rubisco binds to another CO2 molecule and then react with 2, 3-enediol form of ribulose-1, 5-bisphosphate thus forming 2-carboxy-3-ketoribito-1, 5-bisphosphate. This compound is very unstable, thus cleaves the bond between 2nd and 3rd carbon of RuBP and Rubisco thus release 2 molecules of 3-phosphoglycerate.
•    Rubisco activase also regulates the activity of Rubisco. Since binding of sugar phosphates like RuBP prevents the carbamylation of Rubisco. These are removed by Rubisco activase.
•    A sugar phosphate carboxyarabinitol-1-phosphate also regulates the activity of Rubisco. It binds to Rubisco during the night and is removed in the morning on increasing flux density.
6.9.1.2.     Regulation of C4 enzymes
1.    PEP Carboxylase (PEP Case)
In darkened leaves, the affinity of PEP carboxylase reduces for its substrate PEP and a low concentration of malate inhibits it.
•    When the leaf is illuminated, a serine protein kinase is activated and the hydroxyl group of a serine residue of the enzyme is phosphorylated thus activating the enzyme.
•    A protein serine phosphatase can hydrolyze the phosphate group thus inactivating the enzyme.
•    Malate can inhibit both phosphorylated as well as non-phosphorylated enzyme although it can inhibit the phosphorylated form of enzyme more.
2.    Pyruvate-phosphate dikinase (PPDK)
This enzyme is inactivated in dark by phosphorylation of a threonine residue.
•    Its phosphorylation requires ADP rather than ATP.
•    Light activates this enzyme by phosphoryl cleavage of the threonine phosphate group.

6.9.1.3.     Gas exchange in the leaf is regulated by stomata
Stomata possess two guard cells that surrounded by subsidiary cells. When osmotic pressure in the guard cells increases, the stomatal pore opens. Its opening is regulated by malate and K+ ions.
H+-ATPase of the plasma membrane of the guard cells transport proton outside the cell leading to hyperpolarization of the membrane. (–70  →  –90 mv)




6.9.1.4.    Diffusion of CO2 across the plasma membrane of chloroplast and role of carbonic anhydrase.
A model for diffusion of CO2 was proposed by taking values of CO2 concentration from C3 and C4 plants in the presence of Rubisco. 

The stomatal aperture is controlled leading to a resistance in diffusion that maintains a diffusion gradient. CO2 diffuses to chloroplast through stomata. For diffusion from the chloroplast membrane, the stroma of chloroplast contains an enzyme carbonic anhydrase. It allows the CO2 to equilibrate with HCO3- ions. Thus increases the concentration of CO2 across the stroma at pH 8.


6.9.1.5. Recycling of phosphoglycerate by oxygenase activity of Rubisco
During the photorespiratory pathway, two molecules RuBP accepts O2 to yield two molecules of 3-phosphoglycerate and two molecules of 2-phosphoglycolate. The 2-phosphoglycolate formed is recycled to form 2-phosphoglycerate. 

•    Glutamate transfers an amino group to glyoxylate in the presence of glutamate-glyoxylate aminotransferase.
•    Serine-glyoxylate aminotransferase catalyzes transamination of glyoxylate by serine.
H2O2 formed during the conversion of glycolate to glyoxylate is converted to water and oxygen in the presence of catalase. Pyridoxal phosphate is bound to the aminotransferase enzymes. It contains an aldehyde group as a reactive group.
Sequential steps of aminotransferase reaction. 

The glycine formed in the above reactions is transported from peroxisomes to mitochondria via pore. Two molecules of glycine are oxidized forming one molecule of serine releasing CO2 and NH4+.. A reducing equivalent is transferred to NAD+. This oxidation of glycine is catalysed by glycine decarboxylase-serine-hydroxymethyl transferase complex.
Glycine decarboxylase-serine-hydroxymethyl transferase is a complex of multi-enzyme. It consists of 4 different subunits:
1.    The H-protein represents the central glycine decarboxylase complex with a prosthetic group lipoic acid amide.
2.    P-protein contains pyridoxal phosphate surrounding the above center.
3.    T-protein with tetrahydrofolate as prosthetic group.
4.    L-protein also known as dihydrolipoamide dehydrogenase.

6.9.2.    Glycine decarboxylase-serine-hydroxymethyl transferase complex


6.9.3.    Triose phosphate-3-phosphoglycerate shuttle
It is a pathway that is utilized for exporting reducing equivalents from chloroplast to cytosol. In the stroma of chloroplast-3-phosphoglycerate forms triose phosphate by the expense of NADPH and ATP. This triose phosphate is transported to the cytosol by a triose phosphate-phosphate translocator in exchange for 3-phosphoglycerate. In cytosol, it is converted back to 3-phosphoglycerate by the generation of NADPH and ATP.
Comparison of expenditure of ATP and NADPH during carboxylation and oxygenation of RuBP.


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