Photorespiration involves several enzymes across multiple cellular compartments (chloroplast, peroxisome, and mitochondrion). While Rubisco initiates the process, other enzymes are essential for completing the photorespiratory cycle.
Here's a breakdown of the key enzymes involved:
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Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco): This is the first enzyme and the one that initiates photorespiration. Rubisco oxygenates ribulose-1,5-bisphosphate (RuBP) instead of carboxylating it, leading to the formation of phosphoglycolate.
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Phosphoglycolate Phosphatase (PGP): Located in the chloroplast, PGP removes a phosphate group from phosphoglycolate, producing glycolate.
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Glycolate Oxidase (GOX): Found in the peroxisome, GOX oxidizes glycolate to glyoxylate, producing hydrogen peroxide (H2O2) as a byproduct.
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Glutamate:Glyoxylate Aminotransferase (GGT): Also in the peroxisome, GGT transfers an amino group from glutamate to glyoxylate, forming glycine.
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Glycine Decarboxylase Complex (GDC): Located in the mitochondrion, GDC catalyzes the decarboxylation and deamination of two glycine molecules, yielding serine, carbon dioxide (CO2), and ammonia (NH3). This is a major step where CO2 is released during photorespiration.
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Serine Hydroxymethyltransferase (SHMT): Also in the mitochondrion, SHMT converts serine to hydroxymethyl serine, which is then converted back into glycine.
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Serine:Glyoxylate Aminotransferase (SGT): Located in the peroxisome, SGT transfers an amino group from serine to glyoxylate, forming hydroxypyruvate.
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Hydroxypyruvate Reductase (HPR): In the peroxisome, HPR reduces hydroxypyruvate to glycerate, using NADH.
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Glycerate Kinase (GLK): Located in the chloroplast, GLK phosphorylates glycerate to 3-phosphoglycerate (3-PGA), which can then re-enter the Calvin cycle.
In summary, while Rubisco initiates photorespiration, enzymes such as phosphoglycolate phosphatase, glycolate oxidase, glutamate:glyoxylate aminotransferase, glycine decarboxylase complex, serine hydroxymethyltransferase, serine:glyoxylate aminotransferase, hydroxypyruvate reductase, and glycerate kinase are all essential for the process.
