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MetNet - plant pathway - photorespiration
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Pathway details: photorespiration


General info Interaction details Linked pathways Protein-protein interactions
Notes
  Pathway was created on Mon Jul 29, 2013.
 Contributed by aracyc:
General Information: Rubisco (ribulose bisphosphate carboxylase/oxygenase) is a bifunctional enzyme that catalyzes both the carboxylation and oxygenation of D-ribulose-1,5-bisphosphate (RuBP). The carboxylation process is generally known as CO2 fixation (Calvin-Benson-Bassham cycle) whereas the oxygenation process is known as photorespiration (this pathway). Oxygenation of RuBP leads to the production of one molecule of 3-phosphoglycerate and one of 2-phosphoglycolate. The two substrates (O2 and CO2) are competitive with regard to rubisco, so that an increase in CO2 concentration inhibits oxygenation and vice versa |CITS:[Bowyer97]|. Another important factor influencing the rate of oxygenation is temperature. Photorespiration is enhanced by high temperature. The reasons for this are twofold: (i) as temperatures increase the solubility of CO2 diminishes more rapidly than that of O2 and (ii) the specificity factor (Ω) of Rubisco (which represents a measure of the relative specificity of CO2 with O2) decreases with increasing temperatures between 7C and 35C. Under conditions of low CO2 and high O2 concentrations, RuBP is oxidized to 3-phosphoglycerate and 2-phosphoglycolate. The latter cannot be used in the Calvin-Benson-Bassham cycle and, instead, is salvaged in the photorespiration pathway. This salvage pathway involves three types of organelles: chloroplasts, peroxisomes and mitochondria. The main features of this pathway include: (i) the conversion of two-carbon molecule, 2-phosphoglycolate, into glycine; (ii) the decarboxylation of two glycines, which generates serine, CO2 and NH3; and (iii) the conversion of the three-carbon serine into 3-phosphoglycerate, which re-enters the CO2 fixation Calvin-Benson-Bassham cycle. The first step of the pathway involves the dephosphorylation of 2-phosphoglycolate, which not only recycles Pi within the chloroplast but also prevents accumulation of 2-phosphoglycolate, a potent inhibitor of triose-P isomerase (EC 5.3.1.1). In its dephosphorylated form glycolate is exported to the cytoplasm where it is oxidized in the peroxisomes to glyoxylate. The H2O2 generated during this step is detoxified by catalases (see superoxide radicals degradation; ) in the peroxisome. Glyoxylate is then converted into glycine by two different enzymes: serine:glyoxylate aminotransferase (EC 2.6.1.45) and glutamate:glyoxylate aminotransferase (EC 2.6.1.4). The actual desired outcome is the conversion of glyoxylate into hydroxypyruvate, which is the other products of serine:glyoxylate aminotransferase (). Glycine is in fact converted into serine in the mitochondrion by glycine decarboxylase where it is extremely abundant. Glycine decarboxylase has four different subunit (P, H, T and L), which catalyze the transfer of a methylene group from glycine to tetrahydrofolate with the concomitant release of NH3 and CO2, and production of NADH. [Note: NH3, which is an extremely valuable resource, is efficiently refixed by glutamine synthetase (EC 6.3.1.2) in the chloroplast. Moreover, the release of CO2 during the formation of serine decreases the overall efficiency of photosynthesis.] The methylene group is then transferred to another glycine molecule to form serine by a serine hydroxylmetyltransferase (EC 2.1.2.1). Back in the peroxisome, serine is used to convert glyoxylate into hydroxypyruvate via serine:glyoxylate aminotransferase (EC 2.6.1.45) as mentioned above. The last of the peroxisome steps consists in the reduction of hydroxypyruvate into glycerate by an NADH-dependent hydroxypyruvate reductase. Glycerate is then redirected to the chloroplast where it is phosphorylated to 3-phosphoglycerate and reenters the Calvin cycle. The outcome of photorespiration is loss of CO2 and energy in photosynthetic cells. The biological function of photorespiration is not clear. One possibility is that photorespiration is necessary under conditions of high light intensity and low CO2 concentration (i.e. when stomata is closed under water stress) to dissipate excess ATP and reducing power from the photosynthesis light reactions, thus to prevent damage to the photosynthetic apparatus.
  Parts of this pathway occur in:   cytosol     plastid     nucleus     mitochondrion     peroxisome   multiple locations  


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metabolite [36]
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