C3 carbon fixation

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Calvin-Benson cycle Calvin-cycle4.svg
Calvin–Benson cycle

C3 carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, the other two being C4 and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction:

Contents

CO2 + H2O + RuBP → (2) 3-phosphoglycerate

This reaction was first discovered by Melvin Calvin, Andrew Benson and James Bassham in 1950. [1] C3 carbon fixation occurs in all plants as the first step of the Calvin–Benson cycle. (In C4 and CAM plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.)

Cross section of a C3 plant, specifically of an Arabidopsis thaliana leaf. Vascular bundles shown. Drawing based on microscopic images courtesy of Cambridge University Plant Sciences Department. Cross section of Arabidopsis thaliana, a C3 plant..jpg
Cross section of a C3 plant, specifically of an Arabidopsis thaliana leaf. Vascular bundles shown. Drawing based on microscopic images courtesy of Cambridge University Plant Sciences Department.

Plants that survive solely on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, carbon dioxide concentrations are around 200 ppm or higher, [2] and groundwater is plentiful. The C3 plants, originating during Mesozoic and Paleozoic eras, predate the C4 plants and still represent approximately 95% of Earth's plant biomass, including important food crops such as rice, wheat, soybeans and barley.

C3 plants cannot grow in very hot areas at today's atmospheric CO2 level (significantly depleted during hundreds of millions of years from above 5000 ppm) because RuBisCO incorporates more oxygen into RuBP as temperatures increase. This leads to photorespiration (also known as the oxidative photosynthetic carbon cycle, or C2 photosynthesis), which leads to a net loss of carbon and nitrogen from the plant and can therefore limit growth.

C3 plants lose up to 97% of the water taken up through their roots by transpiration. [3] In dry areas, C3 plants shut their stomata to reduce water loss, but this stops CO2 from entering the leaves and therefore reduces the concentration of CO2 in the leaves. This lowers the CO2:O2 ratio and therefore also increases photorespiration. C4 and CAM plants have adaptations that allow them to survive in hot and dry areas, and they can therefore out-compete C3 plants in these areas.

The isotopic signature of C3 plants shows higher degree of 13C depletion than the C4 plants, due to variation in fractionation of carbon isotopes in oxygenic photosynthesis across plant types. Specifically, C3 plants do not have PEP carboxylase like C4 plants, allowing them to only utilize ribulose-1,5-bisphosphate carboxylase (Rubisco) to fix CO2 through the Calvin cycle. The enzyme Rubisco largely discriminates against carbon isotopes, evolving to only bind to 12C isotope compared to 13C (the heavier isotope), contributing to more 13C depletion seen in C3 plants compared to C4 plants especially since the C4 pathway uses PEP carboxylase in addition to Rubisco. [4]

Variations

Not all C3 carbon fixation pathways operate at the same efficiency.

Refixation

Bamboos and the related rice have an improved C3 efficiency. This improvement might be due to its ability to recapture CO2 produced during photorespiration, a behavior termed "carbon refixation". These plants achieve refixation by growing chloroplast extensions called "stromules" around the stroma in mesophyll cells, so that any photorespired CO2 from the mitochondria has to pass through the RuBisCO-filled chloroplast. [5]

Refixation is also performed by a wide variety of plants. The common approach involving growing a bigger bundle sheath leads down to C2 photosynthesis. [6]

Synthetic glycolate pathway

C3 carbon fixation is prone to photorespiration (PR) during dehydration, accumulating toxic glycolate products. In the 2000s scientists used computer simulation combined with an optimization algorithm to figure out what parts of the metabolic pathway may be tuned to improve photosynthesis. According to simulation, improving glycolate metabolism would help significantly to reduce photorespiration. [7] [8]

Instead of optimizing specific enzymes on the PR pathway for glycolate degradation, South et al. decided to bypass PR altogether. In 2019, they transferred Chlamydomonas reinhardtii glycolate dehydrogenase and Cucurbita maxima malate synthase into the chloroplast of tobacco (a C3 model organism). These enzymes, plus the chloroplast's own, create a catabolic cycle: acetyl-CoA combines with glyoxylate to form malate, which is then split into pyruvate and CO2; the former in turn splits into acetyl-CoA and CO2. By forgoing all transport among organelles, all the CO2 released will go into increasing the CO2 concentration in the chloroplast, helping with refixation. The end result is 24% more biomass. An alternative using E. coli glycerate pathway produced a smaller improvement of 13%. They are now working on moving this optimization into other C3 crops like wheat. [9]

Related Research Articles

<span class="mw-page-title-main">Photosynthesis</span> Biological process to convert light into chemical energy

Photosynthesis is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabolism. Photosynthesis usually refers to oxygenic photosynthesis, a process that produces oxygen. Photosynthetic organisms store the chemical energy so produced within intracellular organic compounds like sugars, glycogen, cellulose and starches. To use this stored chemical energy, an organism's cells metabolize the organic compounds through cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.

<span class="mw-page-title-main">RuBisCO</span> Key enzyme of photosynthesis involved in carbon fixation

Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the light-independent part of photosynthesis, including the carbon fixation by which atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to energy-rich molecules such as glucose. It emerged approximately four billion years ago in primordial metabolism prior to the presence of oxygen on Earth. It is probably the most abundant enzyme on Earth. In chemical terms, it catalyzes the carboxylation of ribulose-1,5-bisphosphate.

<span class="mw-page-title-main">Crassulacean acid metabolism</span> Metabolic process

Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but they open at night to collect carbon dioxide and allow it to diffuse into the mesophyll cells. The CO2 is stored as four-carbon malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO2, which is then used during photosynthesis. The pre-collected CO2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. This mechanism of acid metabolism was first discovered in plants of the family Crassulaceae.

C<sub>4</sub> carbon fixation Photosynthetic process in some plants

C4 carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960s discovery by Marshall Davidson Hatch and Charles Roger Slack.

<span class="mw-page-title-main">Photorespiration</span> Process in plant metabolism

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. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.

<span class="mw-page-title-main">Ribulose 1,5-bisphosphate</span> Chemical compound

Ribulose 1,5-bisphosphate (RuBP) is an organic substance that is involved in photosynthesis, notably as the principal CO2 acceptor in plants. It is a colourless anion, a double phosphate ester of the ketopentose called ribulose. Salts of RuBP can be isolated, but its crucial biological function happens in solution. RuBP occurs not only in plants but in all domains of life, including Archaea, Bacteria, and Eukarya.

<span class="mw-page-title-main">Biological carbon fixation</span> Series of interconnected biochemical reactions

Biological carbon fixation, or сarbon assimilation, is the process by which living organisms convert inorganic carbon to organic compounds. These organic compounds are then used to store energy and as structures for other biomolecules. Carbon is primarily fixed through photosynthesis, but some organisms use chemosynthesis in the absence of sunlight. Chemosynthesis is carbon fixation driven by chemical energy rather than from sunlight. 

<span class="mw-page-title-main">Calvin cycle</span> Light-independent reactions in photosynthesis

The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in the stroma, the fluid-filled region of a chloroplast outside the thylakoid membranes. These reactions take the products of light-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and reducing power of NADPH from the light dependent reactions to produce sugars for the plant to use. These substrates are used in a series of reduction-oxidation (redox) reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of CO2 to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

The light compensation point (Ic) is the light intensity on the light curve where the rate of photosynthesis exactly matches the rate of cellular respiration. At this point, the uptake of CO2 through photosynthetic pathways is equal to the respiratory release of carbon dioxide, and the uptake of O2 by respiration is equal to the photosynthetic release of oxygen. The concept of compensation points in general may be applied to other photosynthetic variables, the most important being that of CO2 concentration – CO2 compensation point (Γ).Interval of time in day time when light intensity is low due to which net gaseous exchange is zero is called as compensation point.

<span class="mw-page-title-main">3-Phosphoglyceric acid</span> Chemical compound

3-Phosphoglyceric acid (3PG, 3-PGA, or PGA) is the conjugate acid of 3-phosphoglycerate or glycerate 3-phosphate (GP or G3P). This glycerate is a biochemically significant metabolic intermediate in both glycolysis and the Calvin-Benson cycle. The anion is often termed as PGA when referring to the Calvin-Benson cycle. In the Calvin-Benson cycle, 3-phosphoglycerate is typically the product of the spontaneous scission of an unstable 6-carbon intermediate formed upon CO2 fixation. Thus, two equivalents of 3-phosphoglycerate are produced for each molecule of CO2 that is fixed. In glycolysis, 3-phosphoglycerate is an intermediate following the dephosphorylation (reduction) of 1,3-bisphosphoglycerate.

The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in green plants and algae. Photosynthesis can be described by the simplified chemical reaction

<span class="mw-page-title-main">Phosphoenolpyruvate carboxylase</span> Class of enzymes

Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPC; EC 4.1.1.31, PDB ID: 3ZGE) is an enzyme in the family of carboxy-lyases found in plants and some bacteria that catalyzes the addition of bicarbonate (HCO3−) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate:

Malate dehydrogenase (oxaloacetate-decarboxylating) (NADP<sup>+</sup>) Enzyme

Malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+) (EC 1.1.1.40) or NADP-malic enzyme (NADP-ME) is an enzyme that catalyzes the chemical reaction in the presence of a bivalent metal ion:

<span class="mw-page-title-main">Phosphoglycolate phosphatase</span>

Phosphoglycolate phosphatase(EC 3.1.3.18; systematic name 2-phosphoglycolate phosphohydrolase), also commonly referred to as phosphoglycolate hydrolase, 2-phosphoglycolate phosphatase, P-glycolate phosphatase, and phosphoglycollate phosphatase, is an enzyme responsible for catalyzing the conversion of 2-phosphoglycolate into glycolate and phosphate:

The evolution of photosynthesis refers to the origin and subsequent evolution of photosynthesis, the process by which light energy is used to assemble sugars from carbon dioxide and a hydrogen and electron source such as water. It is believed that the pigments used for photosynthesis initially were used for protection from the harmful effects of light, particularly ultraviolet light. The process of photosynthesis was discovered by Jan Ingenhousz, a Dutch-born British physician and scientist, first publishing about it in 1779.

<span class="mw-page-title-main">Fractionation of carbon isotopes in oxygenic photosynthesis</span>

Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.

<span class="mw-page-title-main">Kinetic isotope effects of RuBisCO</span>

The kinetic isotope effect (KIE) of ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) is the isotopic fractionation associated solely with the step in the Calvin-Benson cycle where a molecule of carbon dioxide is attached to the 5-carbon sugar ribulose-1,5-bisphosphate (RuBP) to produce two 3-carbon sugars called 3-phosphoglycerate. This chemical reaction is catalyzed by the enzyme RuBisCO, and this enzyme-catalyzed reaction creates the primary kinetic isotope effect of photosynthesis. It is also largely responsible for the isotopic compositions of photosynthetic organisms and the heterotrophs that eat them. Understanding the intrinsic KIE of RuBisCO is of interest to earth scientists, botanists, and ecologists because this isotopic biosignature can be used to reconstruct the evolution of photosynthesis and the rise of oxygen in the geologic record, reconstruct past evolutionary relationships and environmental conditions, and infer plant relationships and productivity in modern environments.

<span class="mw-page-title-main">2-Phosphoglycolate</span> Chemical compound

2-Phosphoglycolate (chemical formula C2H2O6P3-; also known as phosphoglycolate, 2-PG, or PG) is a natural metabolic product of the oxygenase reaction mediated by the enzyme ribulose 1,5-bisphosphate carboxylase (RuBisCo).

<span class="mw-page-title-main">Alarm photosynthesis</span> Variation of photosynthesis

Alarm photosynthesis is a variation of photosynthesis where calcium oxalate crystals function as dynamic carbon pools, supplying carbon dioxide (CO2) to photosynthetic cells when stomata are partially or totally closed. This biochemical appendance of the photosynthetic machinery is a means to alleviate the perpetual plant dilemma of using atmospheric CO2 for photosynthesis and losing water vapor, or saving water and reducing photosynthesis. The function of alarm photosynthesis seems to be rather auxiliary to the overall photosynthetic performance. It supports a low photosynthetic rate, aiming at the maintenance and photoprotection of the photosynthetic apparatus rather than a substantial carbon gain.

References

  1. ↑ Calvin M (1997). "Forty years of photosynthesis and related activities". Interdisciplinary Science Reviews. 22 (2): 138–148. Bibcode:1997ISRv...22..138C. doi:10.1179/isr.1997.22.2.138.
  2. ↑ Hogan CM (2011). "Respiration". In McGinley M, Cleveland CJ (eds.). Encyclopedia of Earth. Washington, D.C.: National Council for Science and the Environment.
  3. ↑ Raven JA, Edwards D (March 2001). "Roots: evolutionary origins and biogeochemical significance". Journal of Experimental Botany. 52 (Spec Issue): 381–401. doi: 10.1093/jexbot/52.suppl_1.381 . PMID   11326045.
  4. ↑ Alonso-Cantabrana H, von Caemmerer S (May 2016). "Carbon isotope discrimination as a diagnostic tool for C4 photosynthesis in C3-C4 intermediate species". Journal of Experimental Botany. 67 (10): 3109–21. doi:10.1093/jxb/erv555. PMC   4867892 . PMID   26862154.
  5. ↑ Peixoto MM, Sage TL, Busch FA, Pacheco HD, Moraes MG, Portes TA, Almeida RA, Graciano-Ribeiro D, Sage RF (27 March 2021). "Elevated efficiency of C3 photosynthesis in bamboo grasses: A possible consequence of enhanced refixation of photorespired CO2". GCB Bioenergy. 13 (6): 941–954. Bibcode:2021GCBBi..13..941P. doi: 10.1111/gcbb.12819 .
  6. ↑ Sage RF, Khoshravesh R, Sage TL (1 July 2014). "From proto-Kranz to C4 Kranz: building the bridge to C4 photosynthesis". Journal of Experimental Botany. 65 (13): 3341–3356. doi: 10.1093/jxb/eru180 . PMID   24803502.
  7. ↑ Zhu XG, de Sturler E, Long SP (October 2007). "Optimizing the distribution of resources between enzymes of carbon metabolism can dramatically increase photosynthetic rate: a numerical simulation using an evolutionary algorithm". Plant Physiology. 145 (2): 513–26. doi:10.1104/pp.107.103713. PMC   2048738 . PMID   17720759.
  8. ↑ Stracquadanio G, Umeton R, Papini A, Lio P, Nicosia G (2010). "Analysis and Optimization of C3 Photosynthetic Carbon Metabolism". 2010 IEEE International Conference on BioInformatics and BioEngineering. Philadelphia, PA, USA: IEEE. pp. 44–51. doi:10.1109/BIBE.2010.17. hdl: 1721.1/101094 . ISBN   978-1-4244-7494-3. S2CID   5568464.
  9. ↑ 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.
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