Photorespiration (also known as oxidative photosynthesis carbon cycle , or C 2 photosynthesis ) refers to a process in a metabolic plant where RuBisCO oxygenates RuBP enzyme, causing some of the energy generated by photosynthesis to be in vain. The desired reaction is the addition of carbon dioxide to the RuBP (carboxylation), a key step in the Calvin-Benson cycle, but about 25% of the reaction by RuBisCO instead adds oxygen to RuBP (oxygenation), creating products that can not be used in the Calvin-Benson cycle. This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C 3 plants. Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.
The oxygenation reaction of RuBisCO is a waste process because 3-phosphoglycerate is made at a reduced level and a higher metabolic cost than RuBP carboxylase activity. While carbon yields of photorespiratory cycling in the formation of G3P eventually, there is still a net loss of carbon (about 25% of carbon remains with photosynthesis released as CO 2 ) and nitrogen, as ammonia. Ammonia should 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 whole process as photorespiration, technically the term refers only to metabolic tissues that act to salvage products from oxygenation (phosphoglycolate) reactions.
Video Photorespiration
Reaksi fotorespirasi
The addition of molecular oxygen to ribulose-1,5-bisphosphate produces 3-phosphoglycerate (PGA) and 2-phosphoglycolic acid (2PG, or PG). PGA is a normal product of carboxylation, and productively enters the Calvin cycle. Phosphoglycolate, however, inhibits certain enzymes involved in the fixation of photosynthetic carbon (hence it is often referred to as 'photosynthetic inhibitors'). It is also relatively difficult to recycle: in high plants, it is saved by a series of reactions in peroxisomes, mitochondria, and again in peroxisomes where it is converted into glycerate. The glycerate goes back into the chloroplast and with the same transporter that exports glycolate. The cost of 1 ATP is associated with the conversion to 3-phosphoglycerate (PGA), in chloroplasts, which are then free to re-enter the Calvin cycle.
There are some costs associated with this metabolic pathway; one of which is the production of hydrogen peroxide in peroxisomes (related to the conversion of glycolate to glyoxylate). Hydrogen peroxide is a very dangerous oxidant that must be broken down into water and oxygen by the catalase enzyme. Conversion 2ÃÆ'â € "2 Carbon glycine into 1 C3 serine in mitochondria by glycine-decarboxylase enzyme is a key step, which releases CO 2 , NH 3 , and reduces NAD to NADH. Thus, 1 CO
2 molecules are produced for every 2 molecules O
2 (two derived from RuBisCO activity, the third from peroxisomal oxidation). The assimilation of NH 3 occurs through the GS-GOGAT cycle, at the cost of one ATP and one NADPH.
Cyanobacteria have three possible pathways through which they can metabolize 2-phosphoglycolate. They can not grow if all three pathways are eliminated, although they have a carbon concentration mechanism that should dramatically reduce photorespiration rates (see below).
Maps Photorespiration
RuBisCO substance's peculiarity
The reaction of the oxidative photosynthetic carbon cycle is catalyzed by RuBP oxygenase activity:
- RuBP O
2 -> Phosphoglycolate 3-phosphoglycerate 2 H
During catalysis by RuBisCO, 'active' intermediates are formed (an enediol intermediate) at the RuBisCO active site. These intermediates can react with CO
2 or O
2 . It has been shown that the specific form of the active site RuBisCO acts to induce reactions with CO
2 . Although there is a significant "failure" rate (~ 25% of the reaction is oxygenation rather than carboxylation), it represents significant support from CO
2 , when the relative abundance of two gases is taken into account: in the current atmosphere, O
2 approximately 500 times more, and in the solution O < span>
2 is 25 times more than CO
2 . RuBisCO's ability to determine between two gases is known as the selectivity factor (or Srel), and it varies between species, with angiosperms more efficient than other plants, but with slight variation among vascular plants.
Suggested explanation of RuBisCO's inability to fully distinguish between CO
2 and O
2 is that this is a relic of evolution: The early atmosphere in which primitive plants derive very little oxygen, RuBisCO's early evolution was not influenced by its ability to distinguish between O
2 and CO
2 .
Conditions that increase photorespiration
The level of photorespiration is enhanced by:
Change substrate availability: decrease CO 2 or increase O 2
These influencing factors include the atmospheric abundance of the two gases, the gas supply to the fixation site (ie in a land plant: whether the stomata is open or closed), the length of the liquid phase (how far this gas has spread through the water to reach the reaction site). For example, when the stomata is closed to prevent water loss during the dry season: this limits the supply of CO 2 , while O
2 production in leaves will continue. In algae (and plants that photosynthesize underwater); the gas must spread a significant distance through water, resulting in a decrease in the availability of CO 2 relative to O
2 . It has been predicted that a predicted increase in ambient CO 2 substrate concentration over the next 100 years can reduce photorespiration rates in most plants by about 50%.
Temperature increases
At higher temperatures, RuBisCO is less able to distinguish between CO 2 and O
2 . This is because enediol intermediate is less stable. Increased temperature also reduces CO 2 solubility, thus reducing CO 2 concentration relative to O
2 in chloroplast.
Biological adaptation to minimize photorespiration
Certain plant species or algae have mechanisms to reduce the absorption of molecular oxygen by RuBisCO. These are commonly referred to as Carbon Concentrating Mechanisms (CCMs), as they increase the concentration of CO
2 so RuBisCO is less likely to produce glycolate by reaction with O
2 .
The biochemical carbon concentration mechanism
Biochemical CCMs concentrate carbon dioxide in a temporal or spatial region, through exchange of metabolites. C 4 and CAM photosynthesis both use Phosphoenolpyruvate carboxylase (PEPC) enzyme to add CO
2 to 3-Carbon sugar. PEPC is faster than RuBisCO, and more selective for CO
2 .
C 4
C 4 plants capture carbon dioxide in their mesophyll cells (using an enzyme called phosphoenolpyruvate carboxylase that catalyzes the combination of carbon dioxide with a compound called phosphoenolpyruvate (PEP)), forming oxaloacetate. This oxaloacetate is then converted to malate and transported to the bundle sheath cell (where carbon dioxide fixation by RuBisCO) in which the oxygen concentration is low to avoid photorespiration. Here, carbon dioxide is removed from malate and combined with RuBP by RuBisCO in the usual way, and the Calvin Cycle goes as usual. The CO
2 Concentration in Bundle Sheath is approximately 10-20 times higher than concentration in mesophyll cells.
The ability to avoid photorespiration makes this plant stronger than any other plant in a dry and hot environment, where the stomata is closed and the internal carbon dioxide level is low. Under these conditions, photorespiration occurs in C 4 plants, but at a much lower rate compared to C 3 plants under the same conditions. C 4 plants including sugar cane, corn (corn), and sorghum. CAM (Crassulacean_acid_metabolism) CAM (Crassulacean acid metabolism) <
CAM plants, such as cactus and succulent plants, also use PEP carboxylase enzymes to capture carbon dioxide, but only at night. Crassulacean acid metabolism allows plants to do most of their gas exchange in the cooler night air, absorbing the carbon in the 4-carbon sugar that can be released into photosynthetic cells during the day. This allows the CAM plant to reduce water loss (transpiration) by maintaining closed stomata during the day. CAM plants usually feature other water-saving characteristics, such as thick cuticles, stomata with small holes, and usually lose about 1/3 of the amount of water per CO
2 fixed.
Alga
There are several reports of algae operating the CCM biochemistry: transmitting metabolites in a single cell to center CO 2 in one area. This process is not fully understood.
Biophysical carbon concentration mechanism
The type of carbon concentration mechanism (CCM) depends on the compartment located inside the cell where CO 2 is dismantled, and where RuBisCO is highly expressed. In many species, biophysical CCM is only induced under low carbon dioxide concentrations. The biophysical CCM is more ancient evolutionary than biochemical CCMs. There is some debate as to when the first biophysical CCM evolved, but it may have occurred during periods of low carbon dioxide, after the Great Oxygenation Event (2.4 billion years ago). Low CO
2 750, 650, and 320-270 million years ago.
alukaryotic alga
In almost all species of eukaryotic algae ( Chloromonas being an important exception), after CCM induction, ~ 95% of solid RuBisCO is packed into one subcellular compartment: pyrenoid. Carbon dioxide is concentrated in this compartment using a combination of CO 2 pumps, bicarbonate pumps, and carbonic anhydrases. Pyrenoids are not bound membrane compartments, but are found in chloroplasts, often surrounded by a starch sheath (not considered to be functioning in CCM). Hornworts
Specific horn species are the only terrestrial plants known to have a biophysical CCM involving the concentration of carbon dioxide in the pyrenoids in its chloroplasts.
Cyanobacteria
Cyanobacterial CCMs are in principle similar to those found in algal algae and horn algae, but the compartments in which concentrated carbon dioxide have some structural differences. Instead of pyrenoids, cyanobacteria contain carboxysomes, which have a protein shell, and a protein linker that packs RuBisCO inside with a very regular structure. Cyanobacterial CCMs are much better understood than those found in eukaryotes, in part because of the ease of genetic manipulation of prokaryotes.
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Possible photorespiratory goals
Reducing photorespiration may not result in an increase in growth rate for plants. Photorespiration may be necessary for the assimilation of nitrate from the soil. Thus, photorespiration reduction by genetic engineering or due to increased atmospheric carbon dioxide (due to burning of fossil fuels) may not benefit the plant as proposed. Some physiological processes may be responsible for linking photorespiration and nitrogen assimilation. Photorespiration increases the availability of NADH, which is necessary for the conversion of nitrates to nitrites. Certain nitrite transporters also carry bicarbonate, and an increase in CO 2 has been shown to suppress nitrite transport to chloroplasts.
Although photorespiration is greatly reduced in species C 4 , it is still an important pathway - mutants without metabolic function 2-phosphoglycolic acid can not grow under normal conditions. One mutant proved to accumulate glycolic fast.
Although the function of photorespiration is controversial, it is widely accepted that this pathway affects the various processes of bioenergetics, the function of photosystem II, and the metabolism of carbon to nitrogen assimilation and respiration. The photorespiratory pathway is the main source of hydrogen peroxide ( H
2 O In line photosynthesis cell. Through H
2 O > 2 production and interaction of pyridine nucleotides, photorespiration makes a key contribution to homeostasis redox mobile. Thus, it affects multiple signal pathways, in particular, those that regulate hormonal response of plants controlling growth, environmental and defense response, and programmed cell death.
Another theory postulates that it can serve as a "safety valve", preventing the excessive reductive potential derived from a reduced NADPH pool from reacting with oxygen and generating free radicals, as this can damage the cell's metabolic function with subsequent oxidation of membrane lipids, proteins or nucleotides.
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See also
- C 3 photosynthesis
- C 4 photosynthesis
- Photosynthesis CAM
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References
Source of the article : Wikipedia
2 O In line photosynthesis cell. Through H
2 O > 2 production and interaction of pyridine nucleotides, photorespiration makes a key contribution to homeostasis redox mobile. Thus, it affects multiple signal pathways, in particular, those that regulate hormonal response of plants controlling growth, environmental and defense response, and programmed cell death.
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