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Photorespiration

February 24, 2023

Photorespiration (also known as the oxidative photosynthetic carbon cycle or C2 cycle) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some of the energy produced by photosynthesis. The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin–Benson cycle, but approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin–Benson cycle. This process lowers the efficiency of photosynthesis, potentially lowering photosynthetic output by 25% in C3 plants.[1] Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

Simplified photorespiration cycle
Simplified photorespiration and Calvin cycle

The oxygenation reaction of RuBisCO is a wasteful process because 3-phosphoglycerate is created at a lower rate and higher metabolic cost compared with RuBP carboxylase activity. While photorespiratory carbon cycling results in the formation of G3P eventually, around 25% of carbon fixed by photorespiration is re-released as CO2[2] and nitrogen, as ammonia. Ammonia must then be detoxified at a substantial cost to the cell. Photorespiration also incurs a direct cost of one ATP and one NAD(P)H.

While it is common to refer to the entire process as photorespiration, technically the term refers only to the metabolic network which acts to rescue the products of the oxygenation reaction (phosphoglycolate).

Photorespiratory reactions

 
PhotorespirationFrom left to right: chloroplast, peroxisome, and mitochondrion

Addition of molecular oxygen to ribulose-1,5-bisphosphate produces 3-phosphoglycerate (PGA) and 2-phosphoglycolate (2PG, or PG). PGA is the normal product of carboxylation, and productively enters the Calvin cycle. Phosphoglycolate, however, inhibits certain enzymes involved in photosynthetic carbon fixation (hence is often said to be an 'inhibitor of photosynthesis').[3] It is also relatively difficult to recycle: in higher plants it is salvaged by a series of reactions in the peroxisome, mitochondria, and again in the peroxisome where it is converted into glycerate. Glycerate reenters the chloroplast and by the same transporter that exports glycolate. A cost of 1 ATP is associated with conversion to 3-phosphoglycerate (PGA) (Phosphorylation), within the chloroplast, which is then free to re-enter the Calvin cycle.

Several costs are associated with this metabolic pathway; the production of hydrogen peroxide in the peroxisome (associated with the conversion of glycolate to glyoxylate). Hydrogen peroxide is a dangerously strong oxidant which must be immediately split into water and oxygen by the enzyme catalase. The conversion of 2× 2Carbon glycine to 1× C3 serine in the mitochondria by the enzyme glycine-decarboxylase is a key step, which releases CO2, NH3, and reduces NAD to NADH. Thus, one CO
2 molecule is produced for every two molecules of O
2 (two deriving from RuBisCO and one from peroxisomal oxidations). The assimilation of NH3 occurs via the GS-GOGAT cycle, at a cost of one ATP and one NADPH.

Cyanobacteria have three possible pathways through which they can metabolise 2-phosphoglycolate. They are unable to grow if all three pathways are knocked out, despite having a carbon concentrating mechanism that should dramatically lower the rate of photorespiration (see below).[4]

Substrate specificity of RuBisCO

 
Oxygenase activity of RuBisCO

The oxidative photosynthetic carbon cycle reaction is catalyzed by RuBP oxygenase activity:

RuBP + O
2 → Phosphoglycolate + 3-phosphoglycerate + 2H+

During the catalysis by RuBisCO, an 'activated' intermediate is formed (an enediol intermediate) in the RuBisCO active site. This intermediate is able to react with either CO
2 or O
2. It has been demonstrated that the specific shape of the RuBisCO active site acts to encourage reactions with CO
2. Although there is a significant "failure" rate (~25% of reactions are oxygenation rather than carboxylation), this represents significant favouring of CO
2, when the relative abundance of the two gases is taken into account: in the current atmosphere, O
2 is approximately 500 times more abundant, and in solution O
2 is 25 times more abundant than CO
2.[5]

The ability of RuBisCO to specify between the two gases is known as its selectivity factor (or Srel), and it varies between species,[5] with angiosperms more efficient than other plants, but with little variation among the vascular plants.[6]

A suggested explanation of RuBisCO's inability to discriminate completely between CO
2 and O
2 is that it is an evolutionary relic:[citation needed] The early atmosphere in which primitive plants originated contained very little oxygen, the early evolution of RuBisCO was not influenced by its ability to discriminate between O
2 and CO
2.[6]

Conditions which affect photorespiration

Photorespiration rates are increased by:

Altered substrate availability: lowered CO2 or increased O2

Factors which influence this include the atmospheric abundance of the two gases, the supply of the gases to the site of fixation (i.e. in land plants: whether the stomata are open or closed), the length of the liquid phase (how far these gases have to diffuse through water in order to reach the reaction site). For example, when the stomata are closed to prevent water loss during drought: this limits the CO2 supply, while O
2 production within the leaf will continue. In algae (and plants which photosynthesise underwater); gases have to diffuse significant distances through water, which results in a decrease in the availability of CO2 relative to O
2. It has been predicted that the increase in ambient CO2 concentrations predicted over the next 100 years may lower the rate of photorespiration in most plants by around 50%[citation needed]. However, at temperatures higher than the photosynthetic thermal optimum, the increases in turnover rate are not translated into increased CO2 assimilation because of the decreased affinity of Rubisco for CO2.[7]

Increased temperature

At higher temperatures RuBisCO is less able to discriminate between CO2 and O
2. This is because the enediol intermediate is less stable. Increasing temperatures also lower the solubility of CO2, thus lowering the concentration of CO2 relative to O
2 in the chloroplast.

Biological adaptation to minimize photorespiration

 
Maize uses the C4 pathway, minimizing photorespiration

Certain species of plants or algae have mechanisms to lower uptake of molecular oxygen by RuBisCO. These are commonly referred to as Carbon Concentrating Mechanisms (CCMs), as they increase the concentration of CO2 so that RuBisCO is less likely to produce glycolate through reaction with O
2.

Biochemical carbon concentrating mechanisms

Biochemical CCMs concentrate carbon dioxide in one temporal or spatial region, through metabolite exchange. C4 and CAM photosynthesis both use the enzyme Phosphoenolpyruvate carboxylase (PEPC) to add CO
2 to a 4-Carbon sugar. PEPC is faster than RuBisCO, and more selective for CO
2.

C4

C4 plants capture carbon dioxide in their mesophyll cells (using an enzyme called phosphoenolpyruvate carboxylase which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate (PEP)), forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low to avoid photorespiration. Here, carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way, and the Calvin Cycle proceeds as normal. The CO
2 concentrations in the Bundle Sheath are approximately 10–20 fold higher than the concentration in the mesophyll cells.[6]

This ability to avoid photorespiration makes these plants more hardy than other plants in dry and hot environments, wherein stomata are closed and internal carbon dioxide levels are low. Under these conditions, photorespiration does occur in C4 plants, but at a much lower level compared with C3 plants in the same conditions. C4 plants include sugar cane, corn (maize), and sorghum.

CAM (Crassulacean acid metabolism)

 
Overnight graph of CO2 absorbed by a CAM plant

CAM plants, such as cacti and succulent plants, also use the enzyme PEP carboxylase to capture carbon dioxide, but only at night. Crassulacean acid metabolism allows plants to conduct most of their gas exchange in the cooler night-time air, sequestering carbon in 4-carbon sugars which can be released to the photosynthesizing cells during the day. This allows CAM plants to minimize water loss (transpiration) by maintaining closed stomata during the day. CAM plants usually display other water-saving characteristics, such as thick cuticles, stomata with small apertures, and typically lose around 1/3 of the amount of water per CO
2 fixed.[8]


C2

 
In C2 plants, the mitochondria of mesophyll cells have no glycine decarboxylase (GDC).

C2 photosynthesis (also called glycine shuttle and photorespiratory CO2 pump) is a CCM that works by making use of – as opposed to avoiding – photorespiration. It performs carbon refixation by delaying the breakdown of photorespired glycine, so that the molecule is shuttled from the mesophyll into the bundle sheath. Once there, the glycine is decarboxylated in mitochondria as usual, releasing CO2 and concentrating it to triple the usual concentration.[9]

Although C2 photosynthesis is tradiationally understood as an intermediate step between C3 and C4, a wide variety of plant lineages do end up in the C2 stage without further evolving, showing that it's an evolutionary steady state of its own. C2 may be easier to engineer into crops, as the phenotype requires fewer anatomical changes to produce.[9]

Algae

There have been some reports of algae operating a biochemical CCM: shuttling metabolites within single cells to concentrate CO2 in one area. This process is not fully understood.[10]

Biophysical carbon-concentrating mechanisms

This type of carbon-concentrating mechanism (CCM) relies on a contained compartment within the cell into which CO2 is shuttled, and where RuBisCO is highly expressed. In many species, biophysical CCMs are only induced under low carbon dioxide concentrations. Biophysical CCMs are more evolutionary ancient than biochemical CCMs. There is some debate as to when biophysical CCMs first evolved, but it is likely to have been during a period of low carbon dioxide, after the Great Oxygenation Event (2.4 billion years ago). Low CO
2 periods occurred around 750, 650, and 320–270 million years ago.[11]

Eukaryotic algae

In nearly all species of eukaryotic algae (Chloromonas being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the pyrenoid. Carbon dioxide is concentrated in this compartment using a combination of CO2 pumps, bicarbonate pumps, and carbonic anhydrases. The pyrenoid is not a membrane bound compartment, but is found within the chloroplast, often surrounded by a starch sheath (which is not thought to serve a function in the CCM).[12]

Hornworts

Certain species of hornwort are the only land plants which are known to have a biophysical CCM involving concentration of carbon dioxide within pyrenoids in their chloroplasts.

Cyanobacteria

Cyanobacterial CCMs are similar in principle to those found in eukaryotic algae and hornworts, but the compartment into which carbon dioxide is concentrated has several structural differences. Instead of the pyrenoid, cyanobacteria contain carboxysomes, which have a protein shell, and linker proteins packing RuBisCO inside with a very regular structure. Cyanobacterial CCMs are much better understood than those found in eukaryotes, partly due to the ease of genetic manipulation of prokaryotes.

Possible purpose of photorespiration

Lowering photorespiration may not result in increased growth rates for plants. Photorespiration may be necessary for the assimilation of nitrate from soil. Thus, a lowering in photorespiration by genetic engineering or because of increasing atmospheric carbon dioxide (due to fossil fuel burning) may not benefit plants as has been proposed.[13] Several physiological processes may be responsible for linking photorespiration and nitrogen assimilation. Photorespiration increases availability of NADH, which is required for the conversion of nitrate to nitrite. Certain nitrite transporters also transport bicarbonate, and elevated CO2 has been shown to suppress nitrite transport into chloroplasts.[14] However, in an agricultural setting, replacing the native photorespiration pathway with an engineered synthetic pathway to metabolize glycolate in the chloroplast resulted in a 40 percent increase in crop growth.[15][16][17]

Although photorespiration is much lower in C4 species, it is still an essential pathway – mutants without functioning 2-phosphoglycolate metabolism cannot grow in normal conditions. One mutant was shown to rapidly accumulate glycolate.[18]

Although the functions of photorespiration remain controversial,[19] it is widely accepted that this pathway influences a wide range of processes from bioenergetics, photosystem II function, and carbon metabolism to nitrogen assimilation and respiration. The oxygenase reaction of RuBisCO may prevent CO2 depletion near its active sites[20] and contributes to the regulation of CO2. concentration in the atmosphere[21] The photorespiratory pathway is a major source of hydrogen peroxide (H
2O
2) in photosynthetic cells. Through H
2O
2 production and pyrimidine nucleotide interactions, photorespiration makes a key contribution to cellular redox homeostasis. In so doing, it influences multiple signalling pathways, in particular, those that govern plant hormonal responses controlling growth, environmental and defense responses, and programmed cell death.[19]

It has been postulated that photorespiration may function as a "safety valve",[22] preventing the excess of reductive potential coming from an overreduced NADPH-pool from reacting with oxygen and producing free radicals, as these can damage the metabolic functions of the cell by subsequent oxidation of membrane lipids, proteins or nucleotides. The mutants deficient in photorespiratory enzymes are characterized by a high redox level in the cell,[23] impaired stomatal regulation,[24] and accumulation of formate.[25]

See also

References

  1. ^ Sharkey T (1988). "Estimating the rate of photorespiration in leaves". Physiologia Plantarum. 73 (1): 147–152. doi:10.1111/j.1399-3054.1988.tb09205.x.
  2. ^ Leegood RC (May 2007). "A welcome diversion from photorespiration". Nature Biotechnology. 25 (5): 539–40. doi:10.1038/nbt0507-539. PMID 17483837. S2CID 5015366.
  3. ^ Peterhansel C, Krause K, Braun HP, Espie GS, Fernie AR, Hanson DT, Keech O, Maurino VG, Mielewczik M, Sage RF (July 2013). "Engineering photorespiration: current state and future possibilities". Plant Biology. 15 (4): 754–8. doi:10.1111/j.1438-8677.2012.00681.x. PMID 23121076.
  4. ^ Eisenhut M, Ruth W, Haimovich M, Bauwe H, Kaplan A, Hagemann M (November 2008). "The photorespiratory glycolate metabolism is essential for cyanobacteria and might have been conveyed endosymbiontically to plants". Proceedings of the National Academy of Sciences of the United States of America. 105 (44): 17199–204. Bibcode:2008PNAS..10517199E. doi:10.1073/pnas.0807043105. PMC 2579401. PMID 18957552.
  5. ^ a b Griffiths H (June 2006). "Plant biology: designs on Rubisco". Nature. 441 (7096): 940–1. Bibcode:2006Natur.441..940G. doi:10.1038/441940a. PMID 16791182. S2CID 31190084.
  6. ^ a b c Ehleringer JR, Sage RF, Flanagan LB, Pearcy RW (March 1991). "Climate change and the evolution of C(4) photosynthesis". Trends in Ecology & Evolution. 6 (3): 95–9. doi:10.1016/0169-5347(91)90183-x. PMID 21232434.
  7. ^ Hermida-Carrera, Carmen; Kapralov, Maxim V; Galmés, Jeroni (21 June 2016). "Rubisco catalytic properties and temperature response in crops". Plant Physiology. 171 (4): 2549–61. doi:10.1104/pp.16.01846. PMC 4972260. PMID 27329223.
  8. ^ Taiz L, Zeiger E (2010). "Chapter 8: Photosynthesis: The Carbon Reactions: Inorganic Carbon–Concentrating Mechanisms: Crassulacean Acid Metabolism (CAM)". Plant Physiology (Fifth ed.). Sinauer Associates, Inc. p. 222.
  9. ^ a b Lundgren, Marjorie R. (December 2020). "C 2 photosynthesis: a promising route towards crop improvement?". New Phytologist. 228 (6): 1734–1740. doi:10.1111/nph.16494. PMID 32080851.
  10. ^ Giordano M, Beardall J, Raven JA (June 2005). "CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution". Annual Review of Plant Biology. 56 (1): 99–131. doi:10.1146/annurev.arplant.56.032604.144052. PMID 15862091.
  11. ^ Raven JA, Giordano M, Beardall J, Maberly SC (February 2012). "Algal evolution in relation to atmospheric CO2: carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 367 (1588): 493–507. doi:10.1098/rstb.2011.0212. PMC 3248706. PMID 22232762.
  12. ^ Villarejo A, Martinez F, Pino Plumed M, Ramazanov Z (1996). "The induction of the CO2 concentrating mechanism in a starch-less mutant of Chlamydomonas reinhardtii". Physiologia Plantarum. 98 (4): 798–802. doi:10.1111/j.1399-3054.1996.tb06687.x.
  13. ^ Rachmilevitch S, Cousins AB, Bloom AJ (August 2004). "Nitrate assimilation in plant shoots depends on photorespiration". Proceedings of the National Academy of Sciences of the United States of America. 101 (31): 11506–10. Bibcode:2004PNAS..10111506R. doi:10.1073/pnas.0404388101. PMC 509230. PMID 15272076.
  14. ^ Bloom AJ, Burger M, Rubio Asensio JS, Cousins AB (May 2010). "Carbon dioxide enrichment inhibits nitrate assimilation in wheat and Arabidopsis". Science. 328 (5980): 899–903. Bibcode:2010Sci...328..899B. doi:10.1126/science.1186440. PMID 20466933. S2CID 206525174.
  15. ^ South PF, Cavanagh AP, Liu HW, Ort DR (January 2019). "Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field". Science. 363 (6422): eaat9077. doi:10.1126/science.aat9077. PMC 7745124. PMID 30606819.
  16. ^ Timmer J (7 December 2017). "We may now be able to engineer the most important lousy enzyme on the planet". Ars Technica. Retrieved 5 January 2019.
  17. ^ Timmer J (3 January 2019). "Fixing photosynthesis by engineering it to recycle a toxic mistake". Ars Technica. Retrieved 5 January 2019.
  18. ^ Zabaleta E, Martin MV, Braun HP (May 2012). "A basal carbon concentrating mechanism in plants?". Plant Science. 187: 97–104. doi:10.1016/j.plantsci.2012.02.001. PMID 22404837.
  19. ^ a b Foyer CH, Bloom AJ, Queval G, Noctor G (2009). "Photorespiratory metabolism: genes, mutants, energetics, and redox signaling". Annual Review of Plant Biology. 60 (1): 455–84. doi:10.1146/annurev.arplant.043008.091948. PMID 19575589.
  20. ^ Igamberdiev AU (2015). "Control of Rubisco function via homeostatic equilibration of CO2 supply". Frontiers in Plant Science. 6: 106. doi:10.3389/fpls.2015.00106. PMC 4341507. PMID 25767475.
  21. ^ Igamberdiev AU, Lea PJ (February 2006). "Land plants equilibrate O2 and CO2 concentrations in the atmosphere". Photosynthesis Research. 87 (2): 177–94. doi:10.1007/s11120-005-8388-2. PMID 16432665. S2CID 10709679.
  22. ^ Stuhlfauth T, Scheuermann R, Fock HP (April 1990). "Light Energy Dissipation under Water Stress Conditions: Contribution of Reassimilation and Evidence for Additional Processes". Plant Physiology. 92 (4): 1053–61. doi:10.1104/pp.92.4.1053. PMC 1062415. PMID 16667370.
  23. ^ Igamberdiev AU, Bykova NV, Lea PJ, Gardeström P (April 2001). "The role of photorespiration in redox and energy balance of photosynthetic plant cells: A study with a barley mutant deficient in glycine decarboxylase". Physiologia Plantarum. 111 (4): 427–438. doi:10.1034/j.1399-3054.2001.1110402.x. PMID 11299007.
  24. ^ Igamberdiev AU, Mikkelsen TN, Ambus P, Bauwe H, Lea PJ, Gardeström P (2004). "Photorespiration Contributes to Stomatal Regulation and Carbon Isotope Fractionation: A Study with Barley, Potato and Arabidopsis Plants Deficient in Glycine Decarboxylase". Photosynthesis Research. 81 (2): 139–152. doi:10.1023/B:PRES.0000035026.05237.ec. S2CID 9485316.
  25. ^ Wingler A, Lea PJ, Leegood RC (1999). "Photorespiratory metabolism of glyoxylate and formate in glycine-accumulating mutants of barley and Amaranthus edulis 2". Planta. 207 (4): 518–526. doi:10.1007/s004250050512. S2CID 34817815.

Further reading

  • Stern K (2003). Introductory Plant Biology. New York: McGraw-Hill. ISBN 978-0-07-290941-8.
  • Siedow JN, Day D (2000). "Chapter 14: Respiration and Photorespiration". Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists.

February 24, 2023
photorespiration, also, known, oxidative, photosynthetic, carbon, cycle, cycle, refers, process, plant, metabolism, where, enzyme, rubisco, oxygenates, rubp, wasting, some, energy, produced, photosynthesis, desired, reaction, addition, carbon, dioxide, rubp, c. Photorespiration also known as the oxidative photosynthetic carbon cycle or C2 cycle refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP wasting some of the energy produced by photosynthesis The desired reaction is the addition of carbon dioxide to RuBP carboxylation a key step in the Calvin Benson cycle but approximately 25 of reactions by RuBisCO instead add oxygen to RuBP oxygenation creating a product that cannot be used within the Calvin Benson cycle This process lowers the efficiency of photosynthesis potentially lowering photosynthetic output by 25 in C3 plants 1 Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts leaf peroxisomes and mitochondria Simplified photorespiration cycle Simplified photorespiration and Calvin cycle The oxygenation reaction of RuBisCO is a wasteful process because 3 phosphoglycerate is created at a lower rate and higher metabolic cost compared with RuBP carboxylase activity While photorespiratory carbon cycling results in the formation of G3P eventually around 25 of carbon fixed by photorespiration is re released as CO2 2 and nitrogen as ammonia Ammonia must then be detoxified at a substantial cost to the cell Photorespiration also incurs a direct cost of one ATP and one NAD P H While it is common to refer to the entire process as photorespiration technically the term refers only to the metabolic network which acts to rescue the products of the oxygenation reaction phosphoglycolate Contents 1 Photorespiratory reactions 2 Substrate specificity of RuBisCO 3 Conditions which affect photorespiration 3 1 Altered substrate availability lowered CO2 or increased O2 3 2 Increased temperature 4 Biological adaptation to minimize photorespiration 4 1 Biochemical carbon concentrating mechanisms 4 1 1 C4 4 1 2 CAM Crassulacean acid metabolism 4 1 3 C2 4 1 4 Algae 4 2 Biophysical carbon concentrating mechanisms 4 2 1 Eukaryotic algae 4 2 2 Hornworts 4 2 3 Cyanobacteria 5 Possible purpose of photorespiration 6 See also 7 References 8 Further readingPhotorespiratory reactions Edit PhotorespirationFrom left to right chloroplast peroxisome and mitochondrion Addition of molecular oxygen to ribulose 1 5 bisphosphate produces 3 phosphoglycerate PGA and 2 phosphoglycolate 2PG or PG PGA is the normal product of carboxylation and productively enters the Calvin cycle Phosphoglycolate however inhibits certain enzymes involved in photosynthetic carbon fixation hence is often said to be an inhibitor of photosynthesis 3 It is also relatively difficult to recycle in higher plants it is salvaged by a series of reactions in the peroxisome mitochondria and again in the peroxisome where it is converted into glycerate Glycerate reenters the chloroplast and by the same transporter that exports glycolate A cost of 1 ATP is associated with conversion to 3 phosphoglycerate PGA Phosphorylation within the chloroplast which is then free to re enter the Calvin cycle Several costs are associated with this metabolic pathway the production of hydrogen peroxide in the peroxisome associated with the conversion of glycolate to glyoxylate Hydrogen peroxide is a dangerously strong oxidant which must be immediately split into water and oxygen by the enzyme catalase The conversion of 2 2Carbon glycine to 1 C3 serine in the mitochondria by the enzyme glycine decarboxylase is a key step which releases CO2 NH3 and reduces NAD to NADH Thus one CO2 molecule is produced for every two molecules of O2 two deriving from RuBisCO and one from peroxisomal oxidations The assimilation of NH3 occurs via the GS GOGAT cycle at a cost of one ATP and one NADPH Cyanobacteria have three possible pathways through which they can metabolise 2 phosphoglycolate They are unable to grow if all three pathways are knocked out despite having a carbon concentrating mechanism that should dramatically lower the rate of photorespiration see below 4 Substrate specificity of RuBisCO Edit Oxygenase activity of RuBisCO The oxidative photosynthetic carbon cycle reaction is catalyzed by RuBP oxygenase activity RuBP O2 Phosphoglycolate 3 phosphoglycerate 2H During the catalysis by RuBisCO an activated intermediate is formed an enediol intermediate in the RuBisCO active site This intermediate is able to react with either CO2 or O2 It has been demonstrated that the specific shape of the RuBisCO active site acts to encourage reactions with CO2 Although there is a significant failure rate 25 of reactions are oxygenation rather than carboxylation this represents significant favouring of CO2 when the relative abundance of the two gases is taken into account in the current atmosphere O2 is approximately 500 times more abundant and in solution O2 is 25 times more abundant than CO2 5 The ability of RuBisCO to specify between the two gases is known as its selectivity factor or Srel and it varies between species 5 with angiosperms more efficient than other plants but with little variation among the vascular plants 6 A suggested explanation of RuBisCO s inability to discriminate completely between CO2 and O2 is that it is an evolutionary relic citation needed The early atmosphere in which primitive plants originated contained very little oxygen the early evolution of RuBisCO was not influenced by its ability to discriminate between O2 and CO2 6 Conditions which affect photorespiration EditPhotorespiration rates are increased by Altered substrate availability lowered CO2 or increased O2 Edit Factors which influence this include the atmospheric abundance of the two gases the supply of the gases to the site of fixation i e in land plants whether the stomata are open or closed the length of the liquid phase how far these gases have to diffuse through water in order to reach the reaction site For example when the stomata are closed to prevent water loss during drought this limits the CO2 supply while O2 production within the leaf will continue In algae and plants which photosynthesise underwater gases have to diffuse significant distances through water which results in a decrease in the availability of CO2 relative to O2 It has been predicted that the increase in ambient CO2 concentrations predicted over the next 100 years may lower the rate of photorespiration in most plants by around 50 citation needed However at temperatures higher than the photosynthetic thermal optimum the increases in turnover rate are not translated into increased CO2 assimilation because of the decreased affinity of Rubisco for CO2 7 Increased temperature Edit At higher temperatures RuBisCO is less able to discriminate between CO2 and O2 This is because the enediol intermediate is less stable Increasing temperatures also lower the solubility of CO2 thus lowering the concentration of CO2 relative to O2 in the chloroplast Biological adaptation to minimize photorespiration Edit Maize uses the C4 pathway minimizing photorespiration Certain species of plants or algae have mechanisms to lower uptake of molecular oxygen by RuBisCO These are commonly referred to as Carbon Concentrating Mechanisms CCMs as they increase the concentration of CO2 so that RuBisCO is less likely to produce glycolate through reaction with O2 Biochemical carbon concentrating mechanisms Edit Biochemical CCMs concentrate carbon dioxide in one temporal or spatial region through metabolite exchange C4 and CAM photosynthesis both use the enzyme Phosphoenolpyruvate carboxylase PEPC to add CO2 to a 4 Carbon sugar PEPC is faster than RuBisCO and more selective for CO2 C4 Edit C4 plants capture carbon dioxide in their mesophyll cells using an enzyme called phosphoenolpyruvate carboxylase which catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate PEP forming oxaloacetate This oxaloacetate is then converted to malate and is transported into the bundle sheath cells site of carbon dioxide fixation by RuBisCO where oxygen concentration is low to avoid photorespiration Here carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way and the Calvin Cycle proceeds as normal The CO2 concentrations in the Bundle Sheath are approximately 10 20 fold higher than the concentration in the mesophyll cells 6 This ability to avoid photorespiration makes these plants more hardy than other plants in dry and hot environments wherein stomata are closed and internal carbon dioxide levels are low Under these conditions photorespiration does occur in C4 plants but at a much lower level compared with C3 plants in the same conditions C4 plants include sugar cane corn maize and sorghum CAM Crassulacean acid metabolism Edit Overnight graph of CO2 absorbed by a CAM plant CAM plants such as cacti and succulent plants also use the enzyme PEP carboxylase to capture carbon dioxide but only at night Crassulacean acid metabolism allows plants to conduct most of their gas exchange in the cooler night time air sequestering carbon in 4 carbon sugars which can be released to the photosynthesizing cells during the day This allows CAM plants to minimize water loss transpiration by maintaining closed stomata during the day CAM plants usually display other water saving characteristics such as thick cuticles stomata with small apertures and typically lose around 1 3 of the amount of water per CO2 fixed 8 C2 Edit In C2 plants the mitochondria of mesophyll cells have no glycine decarboxylase GDC C2 photosynthesis also called glycine shuttle and photorespiratory CO2 pump is a CCM that works by making use of as opposed to avoiding photorespiration It performs carbon refixation by delaying the breakdown of photorespired glycine so that the molecule is shuttled from the mesophyll into the bundle sheath Once there the glycine is decarboxylated in mitochondria as usual releasing CO2 and concentrating it to triple the usual concentration 9 Although C2 photosynthesis is tradiationally understood as an intermediate step between C3 and C4 a wide variety of plant lineages do end up in the C2 stage without further evolving showing that it s an evolutionary steady state of its own C2 may be easier to engineer into crops as the phenotype requires fewer anatomical changes to produce 9 Algae Edit There have been some reports of algae operating a biochemical CCM shuttling metabolites within single cells to concentrate CO2 in one area This process is not fully understood 10 Biophysical carbon concentrating mechanisms Edit This type of carbon concentrating mechanism CCM relies on a contained compartment within the cell into which CO2 is shuttled and where RuBisCO is highly expressed In many species biophysical CCMs are only induced under low carbon dioxide concentrations Biophysical CCMs are more evolutionary ancient than biochemical CCMs There is some debate as to when biophysical CCMs first evolved but it is likely to have been during a period of low carbon dioxide after the Great Oxygenation Event 2 4 billion years ago Low CO2 periods occurred around 750 650 and 320 270 million years ago 11 Eukaryotic algae Edit In nearly all species of eukaryotic algae Chloromonas being one notable exception upon induction of the CCM 95 of RuBisCO is densely packed into a single subcellular compartment the pyrenoid Carbon dioxide is concentrated in this compartment using a combination of CO2 pumps bicarbonate pumps and carbonic anhydrases The pyrenoid is not a membrane bound compartment but is found within the chloroplast often surrounded by a starch sheath which is not thought to serve a function in the CCM 12 Hornworts Edit Certain species of hornwort are the only land plants which are known to have a biophysical CCM involving concentration of carbon dioxide within pyrenoids in their chloroplasts Cyanobacteria Edit Cyanobacterial CCMs are similar in principle to those found in eukaryotic algae and hornworts but the compartment into which carbon dioxide is concentrated has several structural differences Instead of the pyrenoid cyanobacteria contain carboxysomes which have a protein shell and linker proteins packing RuBisCO inside with a very regular structure Cyanobacterial CCMs are much better understood than those found in eukaryotes partly due to the ease of genetic manipulation of prokaryotes Possible purpose of photorespiration EditLowering photorespiration may not result in increased growth rates for plants Photorespiration may be necessary for the assimilation of nitrate from soil Thus a lowering in photorespiration by genetic engineering or because of increasing atmospheric carbon dioxide due to fossil fuel burning may not benefit plants as has been proposed 13 Several physiological processes may be responsible for linking photorespiration and nitrogen assimilation Photorespiration increases availability of NADH which is required for the conversion of nitrate to nitrite Certain nitrite transporters also transport bicarbonate and elevated CO2 has been shown to suppress nitrite transport into chloroplasts 14 However in an agricultural setting replacing the native photorespiration pathway with an engineered synthetic pathway to metabolize glycolate in the chloroplast resulted in a 40 percent increase in crop growth 15 16 17 Although photorespiration is much lower in C4 species it is still an essential pathway mutants without functioning 2 phosphoglycolate metabolism cannot grow in normal conditions One mutant was shown to rapidly accumulate glycolate 18 Although the functions of photorespiration remain controversial 19 it is widely accepted that this pathway influences a wide range of processes from bioenergetics photosystem II function and carbon metabolism to nitrogen assimilation and respiration The oxygenase reaction of RuBisCO may prevent CO2 depletion near its active sites 20 and contributes to the regulation of CO2 concentration in the atmosphere 21 The photorespiratory pathway is a major source of hydrogen peroxide H2 O2 in photosynthetic cells Through H2 O2 production and pyrimidine nucleotide interactions photorespiration makes a key contribution to cellular redox homeostasis In so doing it influences multiple signalling pathways in particular those that govern plant hormonal responses controlling growth environmental and defense responses and programmed cell death 19 It has been postulated that photorespiration may function as a safety valve 22 preventing the excess of reductive potential coming from an overreduced NADPH pool from reacting with oxygen and producing free radicals as these can damage the metabolic functions of the cell by subsequent oxidation of membrane lipids proteins or nucleotides The mutants deficient in photorespiratory enzymes are characterized by a high redox level in the cell 23 impaired stomatal regulation 24 and accumulation of formate 25 See also EditC3 photosynthesis C4 photosynthesis CAM photosynthesisReferences Edit Sharkey T 1988 Estimating the rate of photorespiration in leaves Physiologia Plantarum 73 1 147 152 doi 10 1111 j 1399 3054 1988 tb09205 x Leegood RC May 2007 A welcome diversion from photorespiration Nature Biotechnology 25 5 539 40 doi 10 1038 nbt0507 539 PMID 17483837 S2CID 5015366 Peterhansel C Krause K Braun HP Espie GS Fernie AR Hanson DT Keech O Maurino VG Mielewczik M Sage RF July 2013 Engineering photorespiration current state and future possibilities Plant Biology 15 4 754 8 doi 10 1111 j 1438 8677 2012 00681 x PMID 23121076 Eisenhut M Ruth W Haimovich M Bauwe H Kaplan A Hagemann M November 2008 The photorespiratory glycolate metabolism is essential for cyanobacteria and might have been conveyed endosymbiontically to plants Proceedings of the National Academy of Sciences of the United States of America 105 44 17199 204 Bibcode 2008PNAS 10517199E doi 10 1073 pnas 0807043105 PMC 2579401 PMID 18957552 a b Griffiths H June 2006 Plant biology designs on Rubisco Nature 441 7096 940 1 Bibcode 2006Natur 441 940G doi 10 1038 441940a PMID 16791182 S2CID 31190084 a b c Ehleringer JR Sage RF Flanagan LB Pearcy RW March 1991 Climate change and the evolution of C 4 photosynthesis Trends in Ecology amp Evolution 6 3 95 9 doi 10 1016 0169 5347 91 90183 x PMID 21232434 Hermida Carrera Carmen Kapralov Maxim V Galmes Jeroni 21 June 2016 Rubisco catalytic properties and temperature response in crops Plant Physiology 171 4 2549 61 doi 10 1104 pp 16 01846 PMC 4972260 PMID 27329223 Taiz L Zeiger E 2010 Chapter 8 Photosynthesis The Carbon Reactions Inorganic Carbon Concentrating Mechanisms Crassulacean Acid Metabolism CAM Plant Physiology Fifth ed Sinauer Associates Inc p 222 a b Lundgren Marjorie R December 2020 C 2 photosynthesis a promising route towards crop improvement New Phytologist 228 6 1734 1740 doi 10 1111 nph 16494 PMID 32080851 Giordano M Beardall J Raven JA June 2005 CO2 concentrating mechanisms in algae mechanisms environmental modulation and evolution Annual Review of Plant Biology 56 1 99 131 doi 10 1146 annurev arplant 56 032604 144052 PMID 15862091 Raven JA Giordano M Beardall J Maberly SC February 2012 Algal evolution in relation to atmospheric CO2 carboxylases carbon concentrating mechanisms and carbon oxidation cycles Philosophical Transactions of the Royal Society of London Series B Biological Sciences 367 1588 493 507 doi 10 1098 rstb 2011 0212 PMC 3248706 PMID 22232762 Villarejo A Martinez F Pino Plumed M Ramazanov Z 1996 The induction of the CO2 concentrating mechanism in a starch less mutant of Chlamydomonas reinhardtii Physiologia Plantarum 98 4 798 802 doi 10 1111 j 1399 3054 1996 tb06687 x Rachmilevitch S Cousins AB Bloom AJ August 2004 Nitrate assimilation in plant shoots depends on photorespiration Proceedings of the National Academy of Sciences of the United States of America 101 31 11506 10 Bibcode 2004PNAS 10111506R doi 10 1073 pnas 0404388101 PMC 509230 PMID 15272076 Bloom AJ Burger M Rubio Asensio JS Cousins AB May 2010 Carbon dioxide enrichment inhibits nitrate assimilation in wheat and Arabidopsis Science 328 5980 899 903 Bibcode 2010Sci 328 899B doi 10 1126 science 1186440 PMID 20466933 S2CID 206525174 South PF Cavanagh AP Liu HW Ort DR January 2019 Synthetic glycolate metabolism pathways stimulate crop growth and productivity in the field Science 363 6422 eaat9077 doi 10 1126 science aat9077 PMC 7745124 PMID 30606819 Timmer J 7 December 2017 We may now be able to engineer the most important lousy enzyme on the planet Ars Technica Retrieved 5 January 2019 Timmer J 3 January 2019 Fixing photosynthesis by engineering it to recycle a toxic mistake Ars Technica Retrieved 5 January 2019 Zabaleta E Martin MV Braun HP May 2012 A basal carbon concentrating mechanism in plants Plant Science 187 97 104 doi 10 1016 j plantsci 2012 02 001 PMID 22404837 a b Foyer CH Bloom AJ Queval G Noctor G 2009 Photorespiratory metabolism genes mutants energetics and redox signaling Annual Review of Plant Biology 60 1 455 84 doi 10 1146 annurev arplant 043008 091948 PMID 19575589 Igamberdiev AU 2015 Control of Rubisco function via homeostatic equilibration of CO2 supply Frontiers in Plant Science 6 106 doi 10 3389 fpls 2015 00106 PMC 4341507 PMID 25767475 Igamberdiev AU Lea PJ February 2006 Land plants equilibrate O2 and CO2 concentrations in the atmosphere Photosynthesis Research 87 2 177 94 doi 10 1007 s11120 005 8388 2 PMID 16432665 S2CID 10709679 Stuhlfauth T Scheuermann R Fock HP April 1990 Light Energy Dissipation under Water Stress Conditions Contribution of Reassimilation and Evidence for Additional Processes Plant Physiology 92 4 1053 61 doi 10 1104 pp 92 4 1053 PMC 1062415 PMID 16667370 Igamberdiev AU Bykova NV Lea PJ Gardestrom P April 2001 The role of photorespiration in redox and energy balance of photosynthetic plant cells A study with a barley mutant deficient in glycine decarboxylase Physiologia Plantarum 111 4 427 438 doi 10 1034 j 1399 3054 2001 1110402 x PMID 11299007 Igamberdiev AU Mikkelsen TN Ambus P Bauwe H Lea PJ Gardestrom P 2004 Photorespiration Contributes to Stomatal Regulation and Carbon Isotope Fractionation A Study with Barley Potato and Arabidopsis Plants Deficient in Glycine Decarboxylase Photosynthesis Research 81 2 139 152 doi 10 1023 B PRES 0000035026 05237 ec S2CID 9485316 Wingler A Lea PJ Leegood RC 1999 Photorespiratory metabolism of glyoxylate and formate in glycine accumulating mutants of barley and Amaranthus edulis 2 Planta 207 4 518 526 doi 10 1007 s004250050512 S2CID 34817815 Further reading EditStern K 2003 Introductory Plant Biology New York McGraw Hill ISBN 978 0 07 290941 8 Siedow JN Day D 2000 Chapter 14 Respiration and Photorespiration Biochemistry and Molecular Biology of Plants American Society of Plant Physiologists Retrieved from https en wikipedia org w index php title Photorespiration amp oldid 1139248114, wikipedia, wiki, book, books, library,

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