Photorespiration

The photorespiration (Greek φῶς Phos, light; Latin: Respiratio, breathing), and oxidative photosynthetic carbon cycle or (oxidative ) C2 cycle is a metabolic pathway in organisms that operate an oxygenic photosynthesis (plants, algae, cyanobacteria ). Here, carbon dioxide is released in a light-dependent reaction and consumes oxygen as breathing. Therefore, it refers to the pathway as " light respiration". This name inspired by the cellular respiration ( " dark respiration " ), as it also creates carbon dioxide and oxygen is consumed. However, both processes have nothing to do with each other.

The photorespiration can occur in the course of Kohlenstoffdioxidfixierung in the Calvin cycle during photosynthesis. Normally, the key enzyme involved RuBisCO uses carbon dioxide as a substrate, but alternatively, it also accepts oxygen. This produces the toxic metabolite 2- phosphoglycolate which can no longer be used in the Calvin cycle and must therefore be transformed by other biochemical reactions. In higher plants, these reactions take place in three closely adjacent cell compartments: the chloroplast, the peroxisome and the mitochondrion. It is essentially a recovery process.

The photorespiratorische pathway is one of the most wasteful processes on Earth.

Discovery history

The first records of the effect of photorespiration taken by Otto Warburg. He observed in 1920 in the freshwater alga Chlorella that Kohlenstoffdioxidaufnahme can be inhibited by oxygen. The scientist John P. Decker was able to demonstrate here is that in the presence of light and oxygen increased carbon dioxide is released. Without the biochemical processes to know, the two scientists were able to conclude on the importance of oxygen in the photorespiration.

Further steps in the Enlightenment were concerned with the role of glycolate as one of the first metabolites of photorespiration and treated the influence of oxygen and carbon dioxide in the process.

Thus Warburg and Günter Krippahl were able to show in 1960 that high oxygen levels cause the formation of glycolate. This was two years later by James A. Bassham and Martha Kirk confirmed in Chlorella, which could also measure an oxygen-dependent increase in glycolate and 2- phosphoglycolate. Israel Zelitch suspected in 1964 that glycolate constitutes an important intermediate in the photorespiration. In addition, Bassham and Kirk observed that oxygen inhibit photosynthesis ( inhibit ) can.

The oxygen -induced formation of glycolate is reversed by high carbon dioxide concentrations, as demonstrated by Bermingham and employees in the following year. This indicates that oxygen and carbon dioxide, to compete with each other. In 1966, these results were validated by the working group to Krotkov Gleb: the oxygen-dependent inhibition of photosynthesis decreases due to rising CO2 concentrations.

The first theory about the photorespiratory pathway was proposed in 1971 by Nathan E. Tolbert.

Based on the previous observations and our own investigations were George E. Bowes and William L. Ogren demonstrate the same year with the isolated soybean enzyme RuBisCO ( ribulose 1,5 bisphosphate carboxylase / oxygenase ) that both carbon dioxide and oxygen as substrates for RuBisCO serve. At this time called RuBisCO still " ribulose bisphosphate carboxylase ", since only their carboxylative, so kohlenstoffassimilierende function was known. RuBisCO reacts with oxygen instead of carbon dioxide, 2- phosphoglycolate is formed. Thus, the enzyme has both a carboxylase, as well as a Oxygenasefunktion, thus the name used today comes about. The abbreviation for RuBisCO, the enzyme was introduced in 1979 by David Eisenberg at a seminar.

These conclusions were initially accepted only reluctantly. The working group could Tolbert (14C -ribulose -1 ,5 -bisphosphate and 18O2 ) prove with radioactively labeled substrates for the detection of RuBisCO Oxygenasereaktion doubt in 1973, however. From these results and the previous work and theories Tolbert is listed as the discoverer of photorespiration in the literature.

Further studies in the following years treated the measurements of the reaction kinetics of RuBisCO, the influence of temperature on the photorespiration and the characterization of all the enzymes involved. Knowledge of the translocators for the exchange of metabolites between organelles, however, are still limited and remain the subject of research.

Biochemistry of Oxygenasefunktion

Green plants, algae and cyanobacteria ( " blue-green algae " ) take on carbon dioxide to build it in the Calvin cycle carbs. The first step is carried out by the enzyme ribulose -1 ,5- bisphosphate carboxylase / oxygenase ( RuBisCO ), which catalyzes the addition of CO2 to ribulose -1 ,5- bisphosphate (1, see Fig upper arm ). This will generate two molecules of 3 -phosphoglycerate (3, top panel ) is formed and further processed in the Calvin cycle.

As a side reaction RuBisCO also accepts oxygen ( " Oxygenasereaktion " ), which in addition to 3 -phosphoglycerate (3) the 2- phosphoglycolate ( 4) is formed ( see figure lower branch ). 3 -phosphoglycerate flows regularly into the Calvin cycle. However, 2- phosphoglycolate can not be mounted directly to a carbohydrate, yet it is required for the metabolism of some kind. Green algae, for example, distinguish its dephosphorylated form, glycolate, with a good supply of CO2 from ( photosynthetic Glycolatexkretion ).

Photorespiration is a metabolic pathway, the 2- phosphoglycolate transferred through a series of reactions in the 3-phosphoglycerate, and thus counteracts the loss of carbon. This regeneration of nine enzymes are needed; in higher plants it is the participation of the cytosol in the chloroplast, rather than in the peroxisome and the mitochondrion.

The formation of 2 - phosphoglycolate in chloroplasts is carried out by RuBisCO exclusively in the light, as the enzyme is not active in the dark. This explains the first part of the name of photorespiration. Thus photorespiration always runs parallel to the Calvin cycle.

Cause and temperature dependence

The side reaction RuBisCO and the associated occurrence of the photorespiratory pathway is because RuBisCO, both CO2 and O2 as reacting as substrate. Although the affinity of RuBisCO for CO2 is higher than O2, the KM - value is for CO2 9 micromoles / l, for O2 350 micromoles / l and thus CO2 over O2 preferred, however, the concentration of oxygen in the water is 20 times higher than that of CO2. Thus, every fourth to every second molecule ribulose -1 ,5 -bisphosphate reacted with oxygen instead of carbon dioxide.

As RuBisCO can use both carbon dioxide and oxygen as a substrate, the Oxygenasereaktion of RUBISCO occurs more frequently, the higher the oxygen concentration. This is for example the case when the temperature rises. Although generally the solubility of a gas falls with increasing temperature, it does so more than O2 ( see Table ) in CO2. As a result, the ratio of dissolved CO2 reduced to dissolved O2 with increasing temperature; for CO2 fixation, it is so unfavorable. In addition, the slit apertures of the sheet are closed so as to reduce the water loss of the plant at higher temperature. This means that less CO2 is emitted into the cell, while the local O2 content is increased by photolysis. High temperatures favor consequently photorespiration.

Expiry of photorespiration

The Glycolatweg

The resulting chloroplast 2- phosphoglycolate is first reacted with a 2 - phosphoglycolate phosphatase (PGP, EC 3.1.3.18 ) to glycolate. The enzyme activity is essential for plants, if the enzyme is missing, for example, that under natural CO2 concentrations can not grow. Although plants also have a cytosolic PGP only chloroplast isozyme plays a role in photorespiration.

Glycolate is transported and passes through porin -like channels in the peroxisome by a glycolate - glycerate antiporter from the chloroplasts. Assimilating in leaf cells of higher plants there is a special type of peroxisome, which is why it is referred to as Blattperoxisom.

In peroxisome glycolate is oxidized by a flavin mononucleotide (FMN )-dependent glycolate oxidase (EC 1.1.3.15 ) to glyoxylate. The enzyme is present as tetra-or octamer of identical subunits. While corn has only one copy of the gene of this enzyme, five copies have been identified in Arabidopsis. If this gene copies are removed, these plants can only grow under high CO2 concentrations. In the oxidation to glyoxylate oxygen is consumed, which points to the second part of the concept of photorespiration: As is generally in the aerobic respiration, oxygen is consumed, and then carbon dioxide are released (see further below).

In step produces hydrogen peroxide ( H2O2), which is toxic for the cell. Therefore, H2O2 by catalase to water and oxygen ( O2) is reduced.

The glyoxylate is converted by two enzymes:

• As α -keto acid is glyoxylate by the serine - glyoxylate transaminase ( EC 2.6.1.45 ), a homodimer, transamination to glycine; the donor of the NH2 group is formed only below L -serine. The enzyme preferred as nitrogen donor L- serine; for the photorespiratory pathway, it is essential.
• Another molecule of glyoxylate is converted by a glutamate - glyoxylate aminotransferase ( EC 2.6.1.4 ) to glycine, using as amino group donor L- glutamate ( Glu) serves. This produces α -ketoglutarate. This aminotransferase can use in addition to L - glutamate and L- alanine as the N- donor.

The two glycine molecules produced are eventually transported into the mitochondrion. There, they unite in a tetrahydrofolic acid (THF )-dependent reaction to form a molecule of L- serine. In this case both a glycine decarboxylase complex ( GDC ) and a serine hydroxymethyltransferase ( SHMT, EC 2.1.2.1 ) are involved (see figure above). GDC consists of three functional enzymes, a decarboxylating Glycindehydrogenase ( EC 1.4.4.2 ), a Aminomethyltransferase ( EC 2.1.2.10 ) and a Dihydrolipoyldehydrogenase ( EC 1.8.1.4 ). GDC deaminated and decarboxylated glycine under consumption of NAD . The water formed in this step, CO2 may come in the form of bicarbonate from the mitochondrion. In C3 plants 30-50 % of the released carbon dioxide are again refixed by RuBisCO, escapes the rest. SHMT finally linked the methyl group of the first glycine, water, and a further molecule of glycine to L-serine.

Both enzyme complexes, GDC and SHMT are highly concentrated in the matrix of plant cells mitochondria before and are sensitive to oxidation. The released ammonium (NH4 ) is not lost and is still in the photorespiratory nitrogen cycle regenerates (see section below).

L- serine is deaminated to hydroxypyruvate into the peroxisome by the serine - glyoxylate transaminase described above for transport. This reduces the consumption of NADH to D- glycerate, which catalyzes a NAD -dependent hydroxypyruvate reductase ( EC 1.1.1.81 ). Back in the chloroplast D- glycerate is converted to 3 -phosphoglycerate by a Glyceratkinase ( GKK, EC 2.7.1.31 ), one molecule of ATP is invested. 3 -phosphoglycerate then enters regularly into the Calvin cycle. Interestingly, the GKK is the only known enzyme that produces 3 -phosphoglycerate; Contrast, bacteria use a kinase, is formed in the 2- phosphoglycerate.

The exchange of metabolites involved is done either by translocators or porins ( peroxisomes ).

Regeneration of glutamate ( photorespiratorischer nitrogen cycle )

In the photorespiratory pathway, glutamate is converted to α -ketoglutarate in the peroxisome. The amino group is split off but later in the mitochondrion by the glycine - Decarboxylasekomplex in the form of NH4 ( ammonium ). Ammonium itself cytotoxic effects at higher concentrations, but may not be easily excreted. For plant growth is frequently limited by the availability of nitrogen.

This ammonium is not lost as a valuable source of nitrogen and glutamate is regenerated, followed by further reactions in chloroplasts. Thither NH4 enters through an ammonium transporter, possibly by simple diffusion. L- glutamate (3, see Figure ) and NH4 are converted to L- glutamine ( 2) then with ATP consumption by the glutamine synthetase ( GS, EC 6.3.1.2 ). The latter is reacted with α -ketoglutarate ( 1) by a ferredoxin -dependent glutamine -oxoglutarate aminotransferase ( GOGAT, glutamate synthase, EC 1.4.7.1 ) to two molecules of L - glutamate. At the same time two molecules are oxidized ferredoxin ( Fdox ). α -ketoglutarate enters in exchange for malate in the chloroplasts ( DIT1 translocator ), while glutamate is transported back into the peroxisome by a malate - glutamate translocator ( DiT2 ). There it is again for the transamination of glyoxylate available.

Almost everything released ammonium is reattached through this cycle, only about 0.1 ‰ lost. This regeneration consumed a total of two molecules of ferredoxin and one molecule of ATP:

Metabolism in cyanobacteria

Cyanobacteria ( blue-green algae ) are the only known bacteria which operate an oxygenic photosynthesis. Use the Calvin cycle for the fixation of carbon dioxide. For a long time it was assumed for two reasons that photorespiration does not occur in cyanobacteria. First cyanobacteria carbon dioxide accumulate actively in by carboxysomes, so that RuBisCO hardly reacts with oxygen and photorespiration therefore not in any case occurs in appreciable extent. Second, it was believed that when small amounts of glycolate should form, this would excreted as in green algae and did not have to be regenerated in any form.

Now it is known that cyanobacteria also have a 2- phosphoglycolate pathway. This begins the same way as in plants with the reaction of 2 - phosphoglycolate to glycolate via glyoxylate. For the second step using the filamentous nitrogen-fixing cyanobacterium Anabaena as well as occurring in the sea Prochlorococcus marinus in the oxidation of glycolate to glyoxylate, a plant-like glycolate. In contrast, Synechocystis utilizes a glycolate dehydrogenase, which consumes NADH and in which no hydrogen peroxide is formed.

Glyoxylate may - depending on the nature of the cyanobacterium - are metabolized differently; there were three identified to partially overlapping pathways. So it is oxidized in some few cyanobacteria by a Glyoxylatoxidase of oxygen consumption to oxalate. Oxalate is then converted by a Oxalatdecarboxylase and a formate to two molecules of carbon dioxide, while a molecule of NADH is formed ( Decarboxylierungsweg ). This path is not used so that the return of carbon, on the contrary, it is released as a result.

Other cyanobacteria form hydroxypyruvate from glyoxylate. This is done either via a plant-like mechanism (plant- like pathway, see section above). Alternatively, some cyanobacteria can convert two molecules of glyoxylate by a Glyoxylatcarboligase and a Tartronsäuresemialdehydreduktase under NADH - hydroxypyruvate consumption, thereby also creates carbon dioxide and as an intermediate Tartronatsemialdehyd ( Glyceratweg ).

In Synechocystis and Anabaena is hydroxypyruvate - as in plants - phosphorylated by a plant-like Glyceratkinase to 3 -phosphoglycerate with ATP consumption. But other cyanobacteria first form 2 -phosphoglycerate, which is subsequently isomerized to 3-phosphoglycerate.

In Synechocystis, these three overlapping metabolic pathways occur even on together. Only when all three pathways are interrupted Synechocystis requires high carbon dioxide concentrations for survival - analogously to plants in which the photorespiratorische path is interrupted.

Biological prevention

The photorespiration is a costly process in which increased ATP and reducing equivalents are invested. Without photorespiration would be metabolized per mol CO2 fixed 3 mol ATP and 2 mol of NADPH. In the event that the ratio of carboxylation to oxygenation is 1 to 0.25, the fixed consumption per mol CO2 increased to 5.375 mol ATP and 3.5 mol of NADPH.

This increased consumption reduces the efficiency of photosynthesis. Only sufficiently high CO2 partial pressures (e.g., 1 % CO 2) does not occur and so that no efficiency loss Oxygenasereaktion photosynthesis. Thus, photorespiration, per se, an energetically unfavorable More investment for the C3 plant dar. It is estimated that the carbon gain in the Calvin cycle could be higher by about 30% without photorespiration. In that RuBisCO is the most abundant protein on earth photorespiration is classified as one of the even wasteful processes in the world.

In the course of evolution, various mechanisms have evolved to avoid the costly side reaction of RuBisCO in particular with regard to lower CO2 concentrations. So have the aquatic green algae and cyanobacteria kohlenstoffdioxidkonzentrierende mechanisms such as pyrenoids or carboxysomes. These have the function of carbon dioxide to enrich the RuBisCO molecule around. This creates such a high local concentrations of CO2, hardly reacts at the RuBisCO with oxygen.

In land plants arose due to changed climatic conditions of the C4 and CAM metabolism. This is an ATP-driven CO2 pump to reason with which they actively increase the CO2 concentration in the tissue and thus saturate RuBisCO with carbon dioxide. You suffer with a temperature increase hardly losses of photosynthesis efficiency, since photorespiration occurs only in small sizes. They consequently have a higher net rate of fixation than C3 plants. For C4 plants include, for example, sugar cane, sorghum, maize and many weeds, which are found in hot locations.

Importance

Through the photorespiratory path are all organisms that operate an oxygenic photosynthesis (plants, algae, cyanobacteria ), a pathway to keep the carbon loss due to the Oxygenasereaktion of RuBisCO as low as possible.

In the multi-stage process three C atoms for the Calvin cycle are of two molecules of 2- phosphoglycolate provided again and released a molecule of CO2. If this is not reattached CO2 molecule in the Calvin cycle, is the loss of carbon atoms due to photorespiration 25%.

So that the primary function of the photorespiratory lies in the recovery of the carbon. In most green C3 plants, and C4 plants such as maize, as well as in cyanobacteria of the pathway is even essential, so indispensable.

C4 plants and cyanobacteria accumulate carbon dioxide while active, so that photorespiration does not occur as much as in C3 plants. Nevertheless, one can also observe there the photorespiratory pathway. This means that photorespiration has not disappeared in the course of evolution, despite kohlenstoffdioxidkonzentrierender mechanisms and must therefore provide some additional benefits:

Evolution

Forerunner of today's cyanobacteria were the first organisms with oxygenic photosynthesis a. So with them RuBisCO also been exposed to oxygen. 2- phosphoglycolate was probably also produced there since the CO2 -concentrating mechanisms, such as the carboxysome, in evolution occurred much later (probably 360-300 million years ago, in which the oxygen concentration increased in the atmosphere). Plants have lost over time for the Glyceratweg enzymes, which in many cyanobacteria still occurs today. The photorespiration has remained intact. Even in picoplankton, whose genome is greatly reduced (eg Prochlorococcus or Synechococcus ), the genes have been preserved for photorespiration. The C2 cycle of today's cyanobacteria was either already present at the beginning, or evolved quite early in the first Protocyanobakterien.

After endosymbiotic the present plants and algae back to precursor of the cyanobacteria, so that RuBisCO reached in these organisms. RuBisCO all organisms (bacteria, algae, plants) have in common that they accept both carbon dioxide and oxygen as a substrate. Through a specificity constant we can specify how much CO2 RuBisCO preferred as substrate compared to O2. This constant is the product of two ratios: the Michaelis constant KM and the maximum speed vmax constants ( see table).

In the course of evolution, the affinity of RuBisCO for CO2 was only slightly improved. Probably the catalytic site of the enzyme has been optimized in the time when the atmospheric O2 concentration was still very low and was thus able to exert any selective pressure. Consequently RuBisCO could not be significantly improved by the subsequent increase in the oxygen concentration.

Impact on the world nutrition

In C3 plants, photorespiration occurs especially in hot and dry environmental conditions, crop yields in the regions - especially in regard to global warming and the growing world population - reduces. Therefore, a part of the research on a genetic reduction of photorespiration or an introduction of new metabolic pathways, making the crop yields could be increased oriented.

Various attempts to increase the specificity of RuBisCO with respect to carbon dioxide, resulting in lower conversion rates. Thus, the rate of photosynthesis deteriorated. Another strategy aims RuBisCO other species, such as the bacterial type II RuBisCO, bring in C3 plants. But this failed is because the newly introduced RuBisCO not composed to form a functional enzyme.

Therefore Another part of the research attempts to suppress not photorespiration in C3 plants. Instead, it follows the aim to convert C3 plants in C4 plants, because there photorespiration hardly occurs and take advantage of C4 plants in water and nitrogen deficiency C3 plants, particularly with rising temperatures. One of these projects is the so-called C4 rice from which in commercial rice, a C3 plant, C4 photosynthesis is to be introduced.