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Photorespiration
Photorespiration
Photorespiration
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Simplified photorespiration cycle
Simplified photorespiration and Calvin cycle

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.

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

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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

[edit]
Oxygenase activity of RuBisCO

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

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

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

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Part of a series on the
Carbon cycle
By regions
Carbon dioxide
Forms of carbon
Metabolic pathways
Carbon respiration
Carbon pumps
Carbon sequestration
Methane
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Other

Photorespiration rates are affected by:

Altered substrate availability: lowered CO2 or increased O2

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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

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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

[edit]
Maize uses the C4 pathway, minimizing photorespiration

The vast majority of plants are C3, meaning they photorespire when necessary. Certain species of plants or algae have mechanisms to lower the 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

[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 CO
2 to a 4-carbon sugar. PEPC is faster than RuBisCO, and more selective for CO
2.

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 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)

[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 CO
2 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 traditionally 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 is 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 CO
2 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 that are known to have a biophysical CCM involving concentration of carbon dioxide within pyrenoids in their chloroplasts.[13]

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

[edit]

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 may not benefit plants as has been proposed.[14] 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.[15] 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.[16][17][18]

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.[19]

Although the functions of photorespiration remain controversial,[20] 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[21] and contributes to the regulation of CO2. concentration in the atmosphere[22] 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.[20]

It has been postulated that photorespiration may function as a "safety valve",[23] preventing the excess of reductive potential coming from an overreduced NADPH-pool from reacting with oxygen and producing free radicals (oxidants), 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,[24] impaired stomatal regulation,[25] and accumulation of formate.[26]

See also

[edit]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Photorespiration

View on Grokipedia
from Grokipedia
Photorespiration is a light-dependent metabolic process in oxygenic photosynthetic organisms, particularly C3 , where the ribulose-1,5-bisphosphate carboxylase/oxygenase () catalyzes the oxygenation of ribulose-1,5-bisphosphate (RuBP) instead of its , producing one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate (2-PG). This 2-PG is then recycled through a multi-step pathway involving nine enzymatic reactions across three cellular compartments—chloroplasts, peroxisomes, and mitochondria—ultimately yielding 3-PGA for reintegration into the Calvin-Benson cycle, while releasing CO₂ and NH₃ and consuming ATP and reducing equivalents. The process serves as an essential repair mechanism to detoxify the inhibitory 2-PG byproduct of 's oxygenase activity, preventing metabolic imbalances and cellular damage under ambient atmospheric conditions where O₂ competes with CO₂ at the 's . Despite its critical function, photorespiration is often described as inefficient because it competes directly with photosynthetic CO₂ fixation, leading to a net loss of fixed carbon—recovering only about 75% of the carbon from 2-PG while releasing one-quarter as CO₂—and consuming approximately 25-30% of the ATP and NADPH produced by in C3 plants under atmospheric CO₂ levels of approximately 426 ppm (as of November 2025). This carbon and energy cost can reduce by 20–50% in warm, arid environments where photorespiration rates increase due to higher O₂ and Rubisco's affinity for O₂ relative to CO₂. Key enzymes in the pathway include glycolate oxidase in peroxisomes, which oxidizes glycolate to glyoxylate and generates H₂O₂; decarboxylase complex in mitochondria, responsible for CO₂ release during conversion to serine; and serine:glyoxylate aminotransferase, facilitating shuttling. Beyond carbon recycling, photorespiration plays multifaceted roles in , acting as an energy sink to dissipate excess reducing power from the photosynthetic and mitigate under high-light or stress conditions such as and . It also supports stress tolerance by providing precursors for synthesis, including for production, and maintains balance across organelles by exporting reducing equivalents from chloroplasts. Mutants deficient in photorespiratory enzymes, such as those lacking glycolate oxidase or hydroxypyruvate reductase, exhibit lethal phenotypes in normal air but survive in elevated CO₂ (1–2%), underscoring its indispensability for survival in photoautotrophs. Recent research as of 2025 highlights potential engineering strategies, such as synthetic bypass pathways that metabolize glycolate directly in chloroplasts—demonstrating up to 40% increases in in model plants like —and alternative pathways in crops like that improve and uptake.

Overview and Significance

Definition and Basic Process

Photorespiration is a occurring in photoautotrophs such as , , and , where the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase () catalyzes the oxygenation of ribulose-1,5-bisphosphate (RuBP) in competition with its by CO₂, yielding one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate (2-PG). Unlike the Calvin-Benson cycle, which fixes carbon productively, this oxygenation reaction results in no net carbon gain and eventual release of CO₂, effectively reversing aspects of photosynthetic carbon assimilation. The simplified oxygenation reaction can be represented as: RuBP+O2→3-PGA+2-PG\text{RuBP} + \text{O}_2 \rightarrow \text{3-PGA} + \text{2-PG} This dual activity of RuBisCO underscores photorespiration's origin in the inherent oxygenase function of the enzyme. The basic process of photorespiration spans multiple organelles, including chloroplasts, peroxisomes, and mitochondria, beginning with O₂ competing with CO₂ at the RuBisCO active site within the chloroplast. The resulting 2-PG is dephosphorylated to glycolate, which is exported to peroxisomes for oxidation by glycolate oxidase and conversion to glycine, before glycine enters mitochondria for further processing that releases CO₂ and produces serine. This pathway salvages approximately 75% of the carbon from 2-PG back into 3-PGA for reuse in the Calvin cycle, but at the expense of ATP and reducing equivalents, with the remaining 25% lost as CO₂. The phenomenon was first characterized in the through studies on oxygen inhibition of , with Barry Osmond and collaborators describing it as the "photosynthetic carbon oxidation cycle" during a pivotal that integrated biochemical and physiological insights. This naming emphasized its light-dependent oxidative nature, distinguishing it from dark respiration.

Metabolic Impact on Plants

Photorespiration imposes substantial negative impacts on carbon fixation in C3 plants by competing with the carboxylation reaction of , resulting in a reduction of by 20-50% under current atmospheric conditions of approximately 425 ppm CO₂ (as of 2025) and 21% O₂. This process consumes ATP and NADPH that would otherwise support carbon assimilation, yielding no net carbon gain while releasing up to 25% of previously fixed CO₂ back into the atmosphere. Consequently, it exacerbates water use inefficiency, as plants must maintain higher to compensate for CO₂ loss, increasing rates without proportional gains in productivity. Quantifiable effects of photorespiration are evident in its contribution to photosynthetic transport and overall plant performance. In temperate climates, photorespiration can account for approximately 25% of the transport flux during in C3 leaves, diverting resources from productive carbon fixation. This leads to reduced accumulation and significant yield penalties in major crops; for instance, in and , photorespiration is estimated to cause 20-40% losses in potential yield under ambient conditions. Under standard atmospheric levels of 21% O₂ and 425 ppm CO₂ (as of 2025), the photorespiration rate reaches about 25% of the rate in C3 plants, highlighting its pervasive limitation on net CO₂ assimilation. The broader implications of photorespiration extend to global and interactions with . By constraining carbon fixation efficiency, it contributes to inherent limits in C3 crop yields, affecting for staple grains that dominate human diets. Furthermore, photorespiration intensifies under rising temperatures, as higher thermal conditions favor the oxygenation reaction over , potentially amplifying yield losses in warming climates and underscoring the need for adaptations like C4 photosynthesis in vulnerable regions.

Molecular and Biochemical Basis

RuBisCO Enzyme and Its Dual Activity

, or ribulose-1,5-bisphosphate carboxylase/oxygenase, is the most abundant on and serves as the primary catalyst for carbon fixation in , while also exhibiting an unintended oxygenase activity that initiates photorespiration. As the largest found in s of photosynthetic eukaryotes, it is a complex hexadecameric protein with a molecular weight of approximately 540–550 kDa. The is composed of eight large subunits, each encoded by the chloroplast rbcL and weighing about 50–55 kDa, and eight small subunits, encoded by the nuclear rbcS and ranging from 12–18 kDa. The large subunits form the catalytic core, where the active site coordinates a magnesium (Mg²⁺) essential for substrate binding and catalysis, while the small subunits primarily stabilize the assembly and modulate activity. This structural organization is characteristic of Form I , predominant in plants and , whereas Form II variants, found in certain and lacking small subunits, consist of simpler dimeric or octameric large subunit assemblies. The dual functionality of stems from its ability to catalyze two competing reactions at the same : , which fixes CO₂ onto ribulose-1,5-bisphosphate (RuBP) to produce two molecules of 3-phosphoglycerate for the Calvin-Benson-Bassham cycle, and oxygenation, which fixes O₂ onto RuBP to generate 3-phosphoglycerate and 2-phosphoglycolate, the latter triggering photorespiration. This bifunctional nature arises because the enzyme's , stabilized by Mg²⁺ and residues from the large subunit, cannot fully discriminate between CO₂ and O₂ as substrates, leading to a in . Despite this inefficiency, with a carboxylation turnover rate typically ranging from 3 to 10 s⁻¹ in higher , remains indispensable for autotrophic carbon assimilation. The enzyme's catalytic rate is notably slow compared to other metabolic enzymes, necessitating high cellular concentrations to sustain photosynthetic flux. RuBisCO's evolutionary origins trace back to ancient prokaryotes around 3.5 billion years ago, likely emerging in a low-oxygen atmosphere where its oxygenase activity was minimal, allowing it to evolve as a dominant CO₂-fixing despite later inefficiencies under rising atmospheric O₂ levels. Prokaryotic ancestors, including bacterial Form II types, provided the foundational large subunit structure, which was later augmented by small subunits in eukaryotic lineages to enhance stability and . This retention across diverse taxa underscores RuBisCO's critical role in global primary productivity, even as its dual activity imposes metabolic costs in modern oxygenated environments.

Substrate Specificity and Reaction Kinetics

The substrate specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is quantified by the specificity factor τ\tau, which represents the ratio of its carboxylase activity relative to its oxygenase activity. This factor is defined as τ=VcKoVoKc\tau = \frac{V_c K_o}{V_o K_c}, where VcV_c and VoV_o are the maximum velocities for carboxylation and oxygenation, respectively, and KcK_c and KoK_o are the Michaelis constants for CO2_2 and O2_2. In C3 plants, τ\tau typically ranges from 80 to 100, indicating a modest preference for CO2_2 over O2_2, though this value varies across species and environmental conditions. Key kinetic parameters further illustrate RuBisCO's substrate preferences. The Michaelis constant KmK_m for CO2_2 is approximately 9-15 μ\muM in C3 plant RuBisCO, while for O2_2 it is around 300-500 μ\muM. Despite the higher KmK_m for O2_2, oxygenation remains competitive under atmospheric conditions because the dissolved concentration of O2_2 in the stroma (~250 μ\muM at 21% atmospheric O2_2) greatly exceeds that of CO2_2 (~10-12 μ\muM at 400 ppm atmospheric CO2_2), owing to differences in gas in aqueous environments. RuBisCO's activity and specificity are modulated by several biochemical factors. RuBisCO activase (Rca) plays a crucial role by using to remove inhibitory sugar phosphates (such as xylulose-1,5-bisphosphate) from the , thereby maintaining carboxylase efficiency and preventing inhibition under fluctuating light conditions. Activation also requires elevated stromal pH (around 8) and Mg2+^{2+} concentrations (5-10 mM), which promote carbamylation of a residue in the , stabilizing the enzyme's catalytically active form; these conditions arise during photosynthetic illumination via proton pumping and fluxes. Genetic variations influence specificity, with some algal RuBisCOs exhibiting higher τ\tau values (up to 238 in ), reflecting evolutionary adaptations to low-O2_2 aquatic environments. Temperature significantly affects specificity, as τ\tau decreases with rising temperatures due to a greater increase in the oxygenase catalytic efficiency (Vo/KoV_o / K_o) compared to the carboxylase (Vc/KcV_c / K_c); for instance, in many C3 plants, τ\tau can drop by 20-30% from 15°C to 35°C, thereby favoring photorespiration in warmer climates.

Detailed Photorespiratory Pathway

Initiation and Key Enzymatic Steps

Photorespiration initiates in the chloroplast through the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the reaction of ribulose-1,5-bisphosphate (RuBP) with molecular oxygen to produce one molecule of 3-phosphoglycerate (3-PGA) and one molecule of 2-phosphoglycolate (2-PG). This oxygenation reaction competes with the carboxylase activity essential for the Calvin-Benson cycle, occurring under conditions of high O₂/CO₂ ratios within the chloroplast. The 3-PGA can directly enter the Calvin-Benson cycle for further processing, while 2-PG represents a toxic byproduct that must be salvaged to prevent cellular damage. In the subsequent chloroplast-localized step, 2-PG is dephosphorylated to form glycolate by the enzyme phosphoglycolate phosphatase (PGLP, EC 3.1.3.18), which hydrolyzes the phosphate ester using a magnesium-dependent mechanism. This reaction is crucial for generating the primary photorespiratory metabolite, glycolate, which is then exported from the to the via specific transporters. The photorespiratory pathway, spanning the , , and , ultimately recovers approximately 75% of the carbon from 2-PG as 3-PGA for reintegration into photosynthetic metabolism. Within the peroxisome, glycolate is oxidized to glyoxylate by glycolate oxidase (GO, also known as GOX, EC 1.1.3.15), a flavin-dependent that transfers electrons to O₂, generating (H₂O₂) as a . The H₂O₂ is rapidly detoxified by (CAT, EC 1.11.1.6) to water and O₂, preventing in the . Glyoxylate is then transaminated to by glutamate:glyoxylate aminotransferase (GGAT, EC 2.6.1.4), utilizing glutamate as the amino donor and producing 2-oxoglutarate. Key enzymes in these early photorespiratory steps include glycolate oxidase (GO), which drives the committed oxidation; serine:glyoxylate aminotransferase (SGAT), involved in related reactions; and hydroxypyruvate reductase (HPR), which participates in downstream peroxisomal reduction processes. produced here is transported to the for further metabolism, marking the transition from peroxisomal processing.

Metabolite Cycling and Energy Costs

In the mitochondrial phase of photorespiration, two molecules of , derived from peroxisomal glycolate oxidation, are converted to one molecule of serine by the glycine decarboxylase complex (GDC) and (SHMT, EC 2.1.2.1). This multi-enzyme complex consists of four components: the P-protein ( decarboxylase, requiring ), H-protein (lipoamide , with as a cofactor), T-protein (aminomethyltransferase), and L-protein (lipoamide ). The reaction also requires (TPP) and produces CO₂, NH₃, and 5,10-methylene-tetrahydrofolate (CH₂-THF), with the overall stoichiometry given by: 2 glycine+THF→serine+CO2+NH3+CH2-THF2 \text{ glycine} + \text{THF} \rightarrow \text{serine} + \text{CO}_2 + \text{NH}_3 + \text{CH}_2\text{-THF} The decarboxylation step releases one CO₂ per two glycines, representing a net carbon loss of 25% from the original two phosphoglycolate molecules that initiated the pathway. GDC serves as the rate-limiting enzyme in this phase, and mutants deficient in GDC activity exhibit glycine accumulation, leading to photoinhibition and lethality under ambient CO₂ conditions. Serine exits the mitochondria and returns to the , where serine:glyoxylate aminotransferase converts it to hydroxypyruvate. Hydroxypyruvate is then reduced to glycerate by hydroxypyruvate reductase using NADH as the cofactor. Glycerate is transported into the , where glycerate kinase phosphorylates it to 3-phosphoglycerate (3-PGA) in an ATP-dependent reaction. The resulting 3-PGA enters the Calvin-Benson cycle to facilitate RuBP regeneration, thereby closing the photorespiratory loop and recovering three-quarters of the carbon from the initial phosphoglycolates. The metabolite cycling in photorespiration imposes significant energy costs, consuming 3.5 ATP and 2 NADPH equivalents per two molecules of 2-phosphoglycolate processed back to RuBP. This expenditure arises primarily from the ATP used in , the NADPH in hydroxypyruvate reduction, and the refixation of released NH₃ via the glutamine synthetase-glutamate synthase cycle, in addition to costs for RuBP regeneration. The net effect of the full cycle can be simplified as: 2 RuBP+2 O2→3 RuBP+ CO2+ H2O2 \text{ RuBP} + 2 \text{ O}_2 \rightarrow 3 \text{ RuBP} + \text{ CO}_2 + \text{ H}_2\text{O} with no net carbon fixation and the aforementioned energy drain. A key intermediate outcome is the net conversion 2 glycolate→ serine+ CO22 \text{ glycolate} \rightarrow \text{ serine} + \text{ CO}_2, underscoring the partial carbon recovery despite the losses.

Environmental and Physiological Factors

Effects of CO2 and O2 Levels

Photorespiration in C3 plants is modulated by the competitive binding of CO2 and O2 at the active site of the RuBisCO enzyme, where low CO2 concentrations or elevated O2 levels favor the oxygenation reaction over carboxylation, leading to increased rates of photorespiration. Under low atmospheric CO2 levels below 200 ppm, the ratio of CO2 to O2 declines, enhancing RuBisCO's oxygenase activity and thereby elevating photorespiration, which can reduce net photosynthetic efficiency by promoting the release of fixed carbon as CO2. Conversely, O2 concentrations above the ambient atmospheric level of 21% further intensify this competition, resulting in higher oxygenation rates and greater photorespiratory losses, as observed in experimental conditions that simulate hyperoxic environments. Atmospheric CO2 levels have risen significantly since pre-industrial times, from approximately 280 ppm to around 427 ppm as of November 2025, altering the balance of photorespiration in natural ecosystems. This increase partially suppresses photorespiration by improving the CO2:O2 ratio at , with elevated CO2 concentrations (e.g., 500–600 ppm in controlled settings) reducing photorespiratory rates and boosting net in C3 s by 30–50%. In current ambient conditions of 427 ppm CO2 and 21% O2, photorespiration accounts for a substantial portion of carbon loss, but further elevation to levels like 700–1000 ppm can nearly halve these losses, enhancing productivity. Physiologically, plays a critical role in regulating internal CO2 availability, linking water stress to heightened photorespiration; during , stomatal closure limits CO2 influx, lowering intercellular CO2 concentrations and shifting RuBisCO toward oxygenation, which exacerbates photorespiratory carbon loss. This effect is measurable through techniques, where O2 inhibition manifests as increased apparent respiration and reduced CO2 assimilation rates under low CO2 or high O2 scenarios. For instance, reducing ambient O2 to 2% nearly eliminates photorespiration, resulting in a approximately 50% increase in net photosynthetic rates in C3 plants by minimizing at RuBisCO.

Influence of Temperature and Other Conditions

Temperature exerts a profound influence on photorespiration primarily through its effects on gas solubilities and the kinetic properties of . Higher temperatures decrease the of CO₂ in aqueous solutions more rapidly than that of O₂, thereby lowering the effective CO₂/O₂ ratio within the and favoring the oxygenase activity of . This shift contributes to an increase in the relative rate of photorespiration. Additionally, the specificity factor (τ) of , which measures its preference for CO₂ over O₂, decreases with rising temperature due to differential activation energies for and oxygenation reactions; between 20°C and 30°C, the Q₁₀ () for the oxygenase reaction is approximately 1.78, compared to 1.26 for the carboxylase reaction, resulting in a roughly 20-30% reduction in τ per 10°C increase. In C₃ plants, net is typically optimal at 20-25°C under ambient conditions, but above 30°C, photorespiration becomes dominant, potentially accounting for up to 25% or more of flow and reducing . This temperature-induced dominance arises from the combined impacts on and , leading to greater competition from oxygenation. Recent modeling studies incorporating these effects predict that warming climates could exacerbate photorespiration, contributing to 6-16% yield losses in major crops by 2050, even as rising atmospheric CO₂ partially mitigates the impact. Beyond , other environmental conditions modulate photorespiration rates. High light intensity enhances the regeneration of RuBP, increasing its availability as a substrate for and thereby amplifying photorespiratory flux under conditions where oxygenation is favored, such as low CO₂ availability. Similarly, stress induces stomatal closure to conserve , which restricts CO₂ into the and mimics low internal CO₂ levels, thereby promoting photorespiration as an alternative sink for photosynthetic reductants. Interactions between these factors can intensify photorespiration further. In crops like , the combination of heat and exerts synergistic negative effects on carbon assimilation, with stomatal limitations under compounding the kinetic biases toward oxygenation at higher temperatures. These compounded stresses highlight the vulnerability of C₃ photosynthesis to climate variability.

Biological Adaptations

Biochemical Carbon-Concentrating Pathways

Biochemical carbon-concentrating mechanisms in plants evolved to elevate CO₂ concentrations around RuBisCO, thereby suppressing photorespiration through enzymatic pathways that concentrate CO₂ without relying on physical compartments. These pathways primarily include the C₄, crassulacean acid metabolism (CAM), and C₂ photosynthesis systems, which achieve this by spatially or temporally separating initial CO₂ fixation from the Calvin cycle. The C₄ pathway represents a spatial carbon-concentrating mechanism characterized by the initial fixation of CO₂ in mesophyll cells by phosphoenolpyruvate (PEP) carboxylase, forming the four-carbon compound oxaloacetate, which is then reduced to malate or transaminated to aspartate. These C₄ acids are transported to bundle sheath cells, where decarboxylation releases high concentrations of CO₂ proximal to RuBisCO, minimizing the oxygenase activity and photorespiration. This pathway, first elucidated by Hatch and Slack in the 1960s, operates in over 60 plant families and is prevalent in tropical grasses. C₄ photosynthesis is classified into three main biochemical subtypes based on the decarboxylation enzyme: NADP-malic enzyme (NADP-ME) type, where NADP-malic enzyme decarboxylates malate in bundle sheath chloroplasts; NAD-malic enzyme (NAD-ME) type, involving mitochondrial NAD-malic enzyme and additional aspartate-malate shuttles; and phosphoenolpyruvate carboxykinase (PEPCK) type, which uses PEPCK for partial decarboxylation of oxaloacetate-derived compounds, often in combination with malic enzyme activities. In contrast, the CAM pathway employs temporal separation of CO₂ fixation, primarily in succulents such as cacti and agaves, where stomata open nocturnally to fix CO₂ via PEP carboxylase into oxaloacetate and subsequently malate, which is stored in vacuoles. During the day, with stomata closed to conserve , malate is decarboxylated—typically by NADP-malic enzyme or NAD-malic enzyme—releasing CO₂ for in the chloroplasts, thus concentrating CO₂ and reducing photorespiration under arid conditions. This adaptation enhances water-use efficiency in environments where daytime would otherwise limit . The C₂ pathway, observed in C₃-C₄ intermediate species like certain Flaveria and Moricandia, functions as a photorespiratory bypass by relocating to bundle sheath cell mitochondria, where the releases CO₂ at higher concentrations near . This reduces net CO₂ loss from photorespiration by concentrating the released CO₂, serving as an evolutionary intermediate toward full C₄ without a complete C₄ cycle. These biochemical pathways suppress photorespiration to negligible levels, enabling C₄ and CAM plants to achieve photosynthetic rates 50% higher than C₃ plants under optimal conditions, with C₄ crops like and exhibiting approximately 50% greater water- and nitrogen-use efficiencies due to reduced and more efficient CO₂ assimilation. Recent advances in , such as installing C₄ biochemical traits into through the C4 Rice Project, have demonstrated up to 20% yield improvements in field trials of transformed lines, highlighting potential for enhancing staple crop productivity amid rising atmospheric CO₂.

Biophysical Carbon-Concentrating Mechanisms

In , biophysical carbon-concentrating mechanisms (CCMs) primarily rely on carboxysomes, which are icosahedral protein microcompartments that encapsulate and enzymes within a selective shell composed of proteins like CcmK, CcmL, and CcmO. These structures facilitate the dehydration of accumulated (HCO₃⁻) into CO₂, elevating the local CO₂ concentration around to levels up to 100 times higher than in the , thereby minimizing the enzyme's oxygenation activity. The CCM is supported by active uptake of HCO₃⁻ through plasma membrane transporters, including the high-affinity sodium-dependent SbtA (a member of the BASS superfamily) and the low-affinity, high-flux BicA, as well as the ATP-dependent BCT1 complex. These transporters enable to thrive in CO₂-limited environments by maintaining intracellular HCO₃⁻ pools that are then channeled into carboxysomes. Eukaryotic algae employ pyrenoids as analogous biophysical compartments, where forms dense, phase-separated aggregates often surrounded by a sheath that restricts metabolite and enhances CO₂ retention. In chlorophytes such as , the limiting CO₂-inducible protein B (LCIB), a carbonic anhydrase-like enzyme, localizes to the periphery to dehydrate into CO₂, preventing its leakage while facilitating accumulation via multiple transporters like HLA3, LCI1, and CCP1. This setup creates a localized CO₂ microenvironment exceeding 20-fold ambient levels, optimizing efficiency. In diatoms, transporters and carbonic anhydrases in the and envelopes support the CCM, concentrating CO₂ around . Hornworts, unique among land plants, host symbiotic cyanobacteria within thalloid cavities, where the cyanobionts differentiate into heterocysts—specialized cells with thick walls that house carboxysomes and maintain elevated internal CO₂ via their native CCM. This symbiosis integrates the biophysical CCM of , providing the hornwort host with access to concentrated CO₂ derived from cyanobacterial HCO₃⁻ uptake and conversion, in addition to fixed nitrogen, thereby supporting the plant's own pyrenoid-based CCM under fluctuating environmental CO₂. These biophysical CCMs activate under low external CO₂ concentrations below 100 ppm, induced by environmental cues such as and shifts, dramatically reducing photorespiration rates to less than 5% of gross photosynthetic carbon fixation.

Evolutionary Roles and Modern Implications

Potential Protective Functions

Photorespiration plays a protective role in detoxifying (ROS) generated during , particularly through the production of (H₂O₂) by glycolate in peroxisomes, which is subsequently decomposed by to prevent oxidative damage. This process minimizes ROS accumulation under conditions of high or low CO₂, where the oxygenase activity of increases, thereby safeguarding cellular components from peroxidation. In addition to ROS management, photorespiration prevents under excess light by consuming ATP and NADPH, acting as a for excess reducing power and maintaining the balance in chloroplasts and mitochondria. This dissipation of photochemical energy avoids over-reduction of the photosynthetic , particularly protecting from during high . Photorespiration also integrates with metabolism by recycling released during decarboxylation in mitochondria, facilitating its reassimilation via the /glutamate synthase cycle and preventing loss. This linkage supports overall and couples carbon and fluxes in C₃ . Under stress conditions such as and , photorespiration enhances by promoting thermotolerance and reducing ROS-induced ; for instance, the cat2 , deficient in peroxisomal , accumulates excess H₂O₂ from photorespiration under long-term stress, leading to increased oxidative and compared to wild-type . Similarly, mutants with reduced activities of photorespiratory enzymes like decarboxylase and serine:glyoxylate aminotransferase exhibit greater photosynthetic inhibition and reliance on alternative photoprotective mechanisms, such as enhanced , during , underscoring photorespiration's role in mitigating stress-related . Recent studies further indicate that photorespiration maintains cellular balance and supports metabolic stability under fluctuating environmental conditions, such as variable light, buffering against perturbations that could otherwise impair growth. The evolutionary persistence of photorespiration is tied to the approximately 2.4 billion years ago, when oxygenic by elevated atmospheric O₂ levels, necessitating mechanisms to recycle the toxic 2-phosphoglycolate byproduct of Rubisco's oxygenase reaction. This pathway originated in early oxyphotobacteria around 3.2–2.5 billion years ago and has been conserved through endosymbiotic events, enabling photoautotrophs to thrive in oxygenated environments despite its carbon costs.

Strategies to Suppress Photorespiration

Efforts to suppress photorespiration in crops have primarily focused on breeding and to enhance and yield, particularly in C3 plants where photorespiration can reduce productivity by 20-50% under current atmospheric conditions. Breeding approaches leverage natural variation in RuBisCO specificity (τ), selecting variants with higher affinity for CO2 over O2 to minimize oxygenation reactions. For instance, introducing high-τ RuBisCO from or into has demonstrated improved CO2 fixation rates, though challenges remain in maintaining enzyme stability and assembly in plant chloroplasts. Additionally, interspecific hybridization with C3-C4 intermediate species, such as Flaveria, aims to incorporate partial carbon-concentrating mechanisms that reduce photorespiratory CO2 release by recapturing glycine-derived CO2 in bundle sheath cells, offering a pathway for breeding reduced-photorespiration traits into major crops like . Genetic engineering strategies target photorespiratory bypasses to redirect glycolate metabolism, avoiding energy-intensive mitochondrial steps. Overexpression of glycolate dehydrogenase (GDH) in peroxisomes, derived from sources like , converts glycolate directly to glyoxylate without glycine formation, thereby bypassing ammonia release and associated nitrogen recycling costs; this approach in and has shown up to 15-20% increases in photosynthetic rates under ambient CO2. The RIPE project has pioneered introducing cyanobacterial CO2-concentrating mechanism (CCM) genes, such as the bicarbonate transporter BCT1, into chloroplasts, elevating internal CO2 levels to suppress oxygenation; field trials from 2017 to 2023 reported 20-40% higher and seed yield compared to wild-type plants, with sustained benefits under fluctuating and . Synthetic biology extends these efforts through comprehensive pathway redesigns. The C4 Rice Project, an international consortium, engineers full C4 photosynthesis into by introducing bundle sheath anatomy and C4 cycle enzymes (e.g., PEPC and PPDK) to concentrate CO2 around , effectively eliminating photorespiration; while not yet complete, modeling predicts 50% yield gains, targeting field deployment by 2030. Photorespiratory bypasses using E. coli-derived enzymes, such as malate synthase and hydroxypyruvate reductase, have been installed in and to metabolize glycolate in chloroplasts or peroxisomes, reducing CO2 loss by 25-30% and boosting dry matter accumulation in greenhouse trials. Despite these advances, challenges include metabolic trade-offs, such as altered balance or slowed growth under low light, which can offset gains in some environments. Recent field trials in 2024-2025, including synthetic bypasses in under heat stress (up to 35°C), have demonstrated yield increases of up to 19% without penalties to overall , highlighting resilience to variability. Global initiatives, including RIPE and C4 , underscore these strategies' role in enhancing amid rising temperatures and CO2 levels, potentially adding billions of tons to annual crop production. As of 2025, a synthetic glycolate bypass (GCBG) in has shown an average 19% yield increase under natural field conditions, integrating carbon and for improved . Recent reviews summarize ongoing field trial results for various bypasses, emphasizing their potential and challenges.

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