Subsystem: Photorespiration (oxidative C2 cycle)
This subsystem's description is:
Subsystem still under construction. It has been created by Andreas Weber, whose exquisite contribution we gratefully acknowledge; it is currently being curated by SvetaG.
Photorespiration (oxidative photosynthetic C2 cycle) is a salvage pathway for 2-phosphoglycolate (2-PG), the product of the oxygenase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), to the Calvin cycle intermediate phosphoglycerate. Photorespiration is a light-dependent process reminiscent of mitochondrial respiration regarding its gas exchange, because O2 is taken up and CO2 released (reviewed in Reumann and Weber, 2006).
In plants this pathway is highly compartmentalized and involves reactions in chloroplasts, peroxisomes, and mitochondria (see diagram). The H2O2-producing enzyme glycolate oxidase, catalase, and several aminotransferases of the photorespiratory cycle are located in peroxisomes, with catalase representing the major constituent of the peroxisomal matrix in photosynthetic tissues. Identification of all the enzymes involved in this process has recently been completed, but very little is known still about the metabolite transporters for the exchange of photorespiratory intermediates between the organelles involved, and about the regulation of this pathway (Reumann and Weber, 2006). In brief, the photorespiratory reactions first continue in the chloroplast stroma by dephosphorylation of 2-PG catalyzed by phosphoglycolate phosphatase (PGP). Glycolate diffuses into the matrix of peroxisomes, where it is oxidized to glyoxylate by glycolate oxidase (GOX) concomitant with the production of hydrogen peroxide (H2O2). Glyoxylate is transaminated by two aminotransferases, Ser:glyoxylate and Glu:glyoxylate aminotransferase (SGT and GGT), which ideally cooperate at a 1:1 stoichiometry. The resulting Gly is the substrate of two mitochondrial enzymes: Glycine decarboxylase (GDC) decomposes the amino acid to CO2, NH3, and NADH and transfers a C1 unit to 5,10-methylene tetrahydrofolate (THF). Serine hydroxymethyl transferase (SHMT) attaches this methylene unit to the second Gly molecule to produce Ser. Serine diffuses back to leaf peroxisomes, where the amino group is removed by SGT to yield hydroxypyruvate, which is reduced by NADH provided by peroxisomal malate dehydrogenase (pMDH) to form glycerate. Stromal glycerate kinase (GLYK) catalyzes the final phosphorylation step of the photorespiratory cycle to produce the Calvin cycle intermediate 3-PGA (from Reumann and Weber, 2006).
AUXILIARY ROLES: Gln synthase and Fd-dependent Glu synthase (GOGAT) are included in this SS due to their function in re-assimilation of photorespiratory ammonia generated by Glycine decarboxylase; pMDH provides reducing equivalents, imported into leaf peroxisomes in the form of malate.
In cyanobacteria photorespiratory 2-phosphoglycolate metabolism is not yet clear, but it appears that in these organisms phosphoglycolate is metabolized by the cooperative action of plant-like C2 cycle and the bacterial-type glycerate pathway (Eisenhut et al., 2006). SS under construction
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| Diagram | Functional Roles | Subsystem Spreadsheet | Description | Additional Notes | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Subsystem still under construction. It has been created by Andreas Weber, whose exquisite contribution we gratefully acknowledge; it is currently being curated by SvetaG. Photorespiration (oxidative photosynthetic C2 cycle) is a salvage pathway for 2-phosphoglycolate (2-PG), the product of the oxygenase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), to the Calvin cycle intermediate phosphoglycerate. Photorespiration is a light-dependent process reminiscent of mitochondrial respiration regarding its gas exchange, because O2 is taken up and CO2 released (reviewed in Reumann and Weber, 2006). In plants this pathway is highly compartmentalized and involves reactions in chloroplasts, peroxisomes, and mitochondria (see diagram). The H2O2-producing enzyme glycolate oxidase, catalase, and several aminotransferases of the photorespiratory cycle are located in peroxisomes, with catalase representing the major constituent of the peroxisomal matrix in photosynthetic tissues. Identification of all the enzymes involved in this process has recently been completed, but very little is known still about the metabolite transporters for the exchange of photorespiratory intermediates between the organelles involved, and about the regulation of this pathway (Reumann and Weber, 2006). In brief, the photorespiratory reactions first continue in the chloroplast stroma by dephosphorylation of 2-PG catalyzed by phosphoglycolate phosphatase (PGP). Glycolate diffuses into the matrix of peroxisomes, where it is oxidized to glyoxylate by glycolate oxidase (GOX) concomitant with the production of hydrogen peroxide (H2O2). Glyoxylate is transaminated by two aminotransferases, Ser:glyoxylate and Glu:glyoxylate aminotransferase (SGT and GGT), which ideally cooperate at a 1:1 stoichiometry. The resulting Gly is the substrate of two mitochondrial enzymes: Glycine decarboxylase (GDC) decomposes the amino acid to CO2, NH3, and NADH and transfers a C1 unit to 5,10-methylene tetrahydrofolate (THF). Serine hydroxymethyl transferase (SHMT) attaches this methylene unit to the second Gly molecule to produce Ser. Serine diffuses back to leaf peroxisomes, where the amino group is removed by SGT to yield hydroxypyruvate, which is reduced by NADH provided by peroxisomal malate dehydrogenase (pMDH) to form glycerate. Stromal glycerate kinase (GLYK) catalyzes the final phosphorylation step of the photorespiratory cycle to produce the Calvin cycle intermediate 3-PGA (from Reumann and Weber, 2006). AUXILIARY ROLES: Gln synthase and Fd-dependent Glu synthase (GOGAT) are included in this SS due to their function in re-assimilation of photorespiratory ammonia generated by Glycine decarboxylase; pMDH provides reducing equivalents, imported into leaf peroxisomes in the form of malate. In cyanobacteria photorespiratory 2-phosphoglycolate metabolism is not yet clear, but it appears that in these organisms phosphoglycolate is metabolized by the cooperative action of plant-like C2 cycle and the bacterial-type glycerate pathway (Eisenhut et al., 2006). SS under construction References 1. S. Reumann, Andreas P.M. Weber. 2006. Plant peroxisomes respire in the light: Some gaps of the photorespiratory C2 cycle have become filled—Others remain. Biochimica et Biophysica Acta 1763 (2006) 1496 – 1510 2. Eisenhut et al., M. Hagemann. 2006. The Plant-Like C2 Glycolate Cycle and the Bacterial-Like Glycerate Pathway Cooperate in Phosphoglycolate Metabolism in Cyanobacteria. Plant Physiology, 142:333–342 3. Boldt R, Edner C, Kolukisaoglu U, Hagemann M, Weckwerth W, Wienkoop S, Morgenthal K, Bauwe H. 2005. D-glycerate 3-kinase, the last unknown enzyme in the photorespiratory cycle in Arabidopsis, belongs to a novel kinase family. Plant Cell, 17(8):2413-20. 4. Igarashi D, Miwa T, Seki M, Kobayashi M, Kato T, Tabata S, Shinozaki K, Ohsumi C. 2003. Identification of photorespiratory glutamate:glyoxylate aminotransferase (GGAT) gene in Arabidopsis. Plant J, 33(6):975-87. 5. Voll LM, Jamai A, Renne P, Voll H, McClung CR, Weber AP. 2006. The photorespiratory Arabidopsis shm1 mutant is deficient in SHM1. Plant Physiol, 140(1):59-66 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
