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37-451: Though it is also called the "dark reaction", the Calvin cycle does not actually occur in the dark or during night time. This is because the process requires NADPH , which is short-lived and comes from light-dependent reactions . In the dark, plants instead release sucrose into the phloem from their starch reserves to provide energy for the plant. The Calvin cycle thus happens when light

74-602: A fluorescence emission which peaks at 445-460 nm (violet to blue). NADP has no appreciable fluorescence. NADPH provides the reducing agents, usually hydrogen atoms, for biosynthetic reactions and the oxidation-reduction involved in protecting against the toxicity of reactive oxygen species (ROS), allowing the regeneration of glutathione (GSH). NADPH is also used for anabolic pathways, such as cholesterol synthesis , steroid synthesis, ascorbic acid synthesis, xylitol synthesis, cytosolic fatty acid synthesis and microsomal fatty acid chain elongation . The NADPH system

111-459: A fluorescent product that can be used conveniently for quantitation. Conversely, NADPH and NADH are degraded by acidic solutions while NAD /NADP are fairly stable to acid. Many enzymes that bind NADP share a common super-secondary structure named named the "Rossmann fold". The initial beta-alpha-beta (βαβ) fold is the most conserved segment of the Rossmann folds. This segment is in contact with

148-464: A net gain of one G3P molecule per three CO 2 molecules (as would be expected from the number of carbon atoms involved). The regeneration stage can be broken down into a series of steps. 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 CO 2 molecules. The equation of

185-414: A source of one-carbon units to sustain nucleotide synthesis and redox homeostasis in mitochondria. Mitochondrial folate cycle has been recently suggested as the principal contributor to NADPH generation in mitochondria of cancer cells. NADPH can also be generated through pathways unrelated to carbon metabolism. The ferredoxin reductase is such an example. Nicotinamide nucleotide transhydrogenase transfers

222-530: Is a metabolite that occurs as an intermediate in several central pathways of all organisms. With the chemical formula H(O)CCH(OH)CH 2 OPO 3 , this anion is a monophosphate ester of glyceraldehyde . D-glyceraldehyde 3-phosphate is formed from the following three compounds in reversible reactions: Compound C05378 at KEGG Pathway Database. Enzyme 4.1.2.13 at KEGG Pathway Database. Compound C00111 at KEGG Pathway Database. Compound C00118 at KEGG Pathway Database. The numbering of

259-473: Is a series of biochemical redox reactions that take place in the stroma of chloroplast in photosynthetic organisms . The cycle was discovered in 1950 by Melvin Calvin , James Bassham , and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14 . Photosynthesis occurs in two stages in a cell. In the first stage, light-dependent reactions capture

296-416: Is also of some importance since this is how glycerol (as DHAP) enters the glycolytic and gluconeogenic pathways. Furthermore, it is a participant in and a product of the pentose phosphate pathway . | Click on genes, proteins and metabolites below to link to respective articles. During plant photosynthesis , 2 equivalents of glycerate 3-phosphate (GP; also known as 3-phosphoglycerate) are produced by

333-425: Is also responsible for generating free radicals in immune cells by NADPH oxidase . These radicals are used to destroy pathogens in a process termed the respiratory burst . It is the source of reducing equivalents for cytochrome P450 hydroxylation of aromatic compounds , steroids , alcohols , and drugs . NADH and NADPH are very stable in basic solutions, but NAD and NADP are degraded in basic solutions into

370-404: Is available independent of the kind of photosynthesis ( C3 carbon fixation , C4 carbon fixation , and crassulacean acid metabolism (CAM) ); CAM plants store malic acid in their vacuoles every night and release it by day to make this process work. The reactions of the Calvin cycle are closely coupled to the thylakoid electron transport chain, as the energy required to reduce the carbon dioxide

407-402: Is available, as the ferredoxin protein is reduced in the photosystem I complex of the thylakoid electron chain when electrons are circulating through it. Ferredoxin then binds to and reduces the thioredoxin protein, which activates the cycle enzymes by severing a cystine bond found in all these enzymes. This is a dynamic process as the same bond is formed again by other proteins that deactivate

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444-446: Is less well understood, but with all the similar proteins the process should work in a similar way. NADPH is produced from NADP . The major source of NADPH in animals and other non-photosynthetic organisms is the pentose phosphate pathway , by glucose-6-phosphate dehydrogenase (G6PDH) in the first step. The pentose phosphate pathway also produces pentose, another important part of NAD(P)H, from glucose. Some bacteria also use G6PDH for

481-475: Is provided by NADPH produced during the light dependent reactions . The process of photorespiration , also known as C2 cycle, is also coupled to the Calvin cycle, as it results from an alternative reaction of the RuBisCO enzyme, and its final byproduct is another glyceraldehyde-3-P molecule. The Calvin cycle , Calvin–Benson–Bassham (CBB) cycle , reductive pentose phosphate cycle (RPP cycle) or C3 cycle

518-594: Is regenerated for the Calvin cycle to continue. G3P is generally considered the prime end-product of photosynthesis and it can be used as an immediate food nutrient, combined and rearranged to form monosaccharide sugars, such as glucose , which can be transported to other cells, or packaged for storage as insoluble polysaccharides such as starch . 6 CO 2 + 6 RuBP (+ energy from 12 ATP and 12 NADPH) →12 G3P (3-carbon) 10 G3P (+ energy from 6 ATP ) → 6 RuBP (i.e. starting material regenerated) 2 G3P → glucose (6-carbon). Glyceraldehyde 3-phosphate occurs as

555-531: Is synthesized before NADPH is. Such a reaction usually starts with NAD from either the de-novo or the salvage pathway, with NAD kinase adding the extra phosphate group. ADP-ribosyl cyclase allows for synthesis from nicotinamide in the salvage pathway, and NADP phosphatase can convert NADPH back to NADH to maintain a balance. Some forms of the NAD kinase, notably the one in mitochondria, can also accept NADH to turn it directly into NADPH. The prokaryotic pathway

592-401: Is the oxidized form. NADP is used by all forms of cellular life. NADP is essential for life because it is needed for cellular respiration. NADP differs from NAD by the presence of an additional phosphate group on the 2' position of the ribose ring that carries the adenine moiety . This extra phosphate is added by NAD kinase and removed by NADP phosphatase. In general, NADP

629-557: The Entner–Doudoroff pathway , but NADPH production remains the same. Ferredoxin–NADP reductase , present in all domains of life, is a major source of NADPH in photosynthetic organisms including plants and cyanobacteria. It appears in the last step of the electron chain of the light reactions of photosynthesis . It is used as reducing power for the biosynthetic reactions in the Calvin cycle to assimilate carbon dioxide and help turn

666-732: The ADP portion of NADP. Therefore, it is also called an "ADP-binding βαβ fold". In 2018 and 2019, the first two reports of enzymes that catalyze the removal of the 2' phosphate of NADP(H) in eukaryotes emerged. First the cytoplasmic protein MESH1 ( Q8N4P3 ), then the mitochondrial protein nocturnin were reported. Of note, the structures and NADPH binding of MESH1 ( 5VXA ) and nocturnin ( 6NF0 ) are not related. Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate , also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P , GA3P , GADP , GAP , TP , GALP or PGAL ,

703-490: The Calvin cycle is the following: Hexose (six-carbon) sugars are not products of the Calvin cycle. Although many texts list a product of photosynthesis as C 6 H 12 O 6 , this is mainly for convenience to match the equation of aerobic 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). In

740-418: 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

777-457: 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. These reactions do not occur in the dark or at night. There is a light-dependent regulation of the cycle enzymes, as the third step requires NADPH. There are two regulation systems at work when the cycle must be turned on or off: the thioredoxin / ferredoxin activation system, which activates some of

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814-472: The carbon atoms indicates the fate of the carbons according to their position in fructose 6-phosphate. Compound C00111 at KEGG Pathway Database. Enzyme 5.3.1.1 at KEGG Pathway Database. Compound C00118 at KEGG Pathway Database. Compound C00118 at KEGG Pathway Database. Enzyme 1.2.1.12 at KEGG Pathway Database. Reaction R01063 at KEGG Pathway Database. Compound C00236 at KEGG Pathway Database. D-glyceraldehyde 3-phosphate

851-692: The carbon dioxide into glucose. It has functions in accepting electrons in other non-photosynthetic pathways as well: it is needed in the reduction of nitrate into ammonia for plant assimilation in nitrogen cycle and in the production of oils. There are several other lesser-known mechanisms of generating NADPH, all of which depend on the presence of mitochondria in eukaryotes. The key enzymes in these carbon-metabolism-related processes are NADP-linked isoforms of malic enzyme , isocitrate dehydrogenase (IDH), and glutamate dehydrogenase . In these reactions, NADP acts like NAD in other enzymes as an oxidizing agent. The isocitrate dehydrogenase mechanism appears to be

888-417: The cell cytosol , 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 when they have no net productivity . The sum of reactions in

925-453: The cycle enzymes; and the RuBisCo enzyme activation, active in the Calvin cycle, which involves its own activase. The thioredoxin/ferredoxin system activates the enzymes glyceraldehyde-3-P dehydrogenase, glyceraldehyde-3-P phosphatase, fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, and ribulose-5-phosphatase kinase, which are key points of the process. This happens when light

962-402: The energy of light and use it to make the energy-storage molecule ATP and the moderate-energy hydrogen carrier NADPH . The Calvin cycle uses these compounds to convert carbon dioxide and water into organic compounds 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

999-542: The enzymes. The implications of this process are that the enzymes remain mostly activated by day and are deactivated in the dark when there is no more reduced ferredoxin available. The enzyme RuBisCo has its own, more complex activation process. It requires that a specific lysine amino acid be carbamylated to activate the enzyme. This lysine binds to RuBP and leads to a non-functional state if left uncarbamylated. A specific activase enzyme, called RuBisCo activase , helps this carbamylation process by removing one proton from

1036-609: The first stage of the Calvin cycle, a CO 2 molecule is incorporated into one of two three-carbon molecules ( glyceraldehyde 3-phosphate or G3P), where it uses up two molecules of ATP and two molecules of NADPH , which had been produced in the light-dependent stage. The three steps involved are: 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 CO 2 molecule produces two G3P molecules, three CO 2 molecules produce six G3P molecules, of which five are used to regenerate RuBP , leaving

1073-487: The first step of the light-independent reactions when ribulose 1,5-bisphosphate (RuBP) and carbon dioxide are catalysed by the rubisco enzyme. The GP is converted to D-glyceraldehyde 3-phosphate (G3P) using the energy in ATP and the reducing power of NADPH as part of the Calvin cycle . This returns ADP , phosphate ions Pi, and NADP + to the light-dependent reactions of photosynthesis for their continued function. RuBP

1110-429: 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 most enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway , but the enzymes in the Calvin cycle are found in the chloroplast stroma instead of

1147-443: The hydrogen between NAD(P)H and NAD(P) , and is found in eukaryotic mitochondria and many bacteria. There are versions that depend on a proton gradient to work and ones that do not. Some anaerobic organisms use NADP -linked hydrogenase , ripping a hydride from hydrogen gas to produce a proton and NADPH. Like NADH , NADPH is fluorescent . NADPH in aqueous solution excited at the nicotinamide absorbance of ~335 nm (near UV) has

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1184-458: The lysine and making the binding of the carbon dioxide molecule possible. Even then the RuBisCo enzyme is not yet functional, as it needs a magnesium ion bound to the lysine to function. This magnesium ion is released from the thylakoid lumen when the inner pH drops due to the active pumping of protons from the electron flow. RuBisCo activase itself is activated by increased concentrations of ATP in

1221-448: The major source of NADPH in fat and possibly also liver cells. These processes are also found in bacteria. Bacteria can also use a NADP-dependent glyceraldehyde 3-phosphate dehydrogenase for the same purpose. Like the pentose phosphate pathway, these pathways are related to parts of glycolysis . Another carbon metabolism-related pathway involved in the generation of NADPH is the mitochondrial folate cycle, which uses principally serine as

1258-690: The overall Calvin cycle is shown diagrammatically below. RuBisCO also reacts competitively with O 2 instead of CO 2 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 plus CO 2 . 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

1295-403: The oxygenation reaction. This improved specificity evolved after RuBisCO incorporated a new protein subunit. 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. For

1332-434: The plant, because, rather than fixing CO 2 , this process leads to loss of CO 2 . C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates—corn, for example. Furthermore, RuBisCOs catalyzing the light-independent reactions of photosynthesis generally exhibit an improved specificity for CO 2 relative to O 2 , in order to minimize

1369-416: The stroma caused by its phosphorylation . NADPH Nicotinamide adenine dinucleotide phosphate , abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions , such as the Calvin cycle and lipid and nucleic acid syntheses, which require NADPH as a reducing agent ('hydrogen source'). NADPH is the reduced form, whereas NADP

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