Photorespiration is the light‐dependent release of carbon dioxide initiated by ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in oxygen‐producing photosynthetic organisms. It occurs because oxygen can substitute for carbon dioxide in the first reaction of the photosynthetic carbon dioxide‐fixation process, causing the idle synthesis of phosphoglycolate. Phosphoglycolate is scavenged in the photorespiratory C2 cycle, which is an essential auxiliary metabolic pathway that allows photosynthesis in oxygen‐containing environments. Three out of four misdirected carbon atoms are recovered and the fourth is released as photorespiratory carbon dioxide. Absolute rates vary in different organisms and they also depend on environmental conditions, mainly oxygen, carbon dioxide and temperature. They are highest in C3 plants and much reduced in other organisms, such as C4 plants, algae and cyanobacteria. In the presence of oxygen, phosphoglycolate production is unavoidable and cannot be eliminated.

Key concepts:

  • Nearly all photosynthetic activity on earth is oxygen‐producing photosynthesis. Dictated by the biochemistry of carbon dioxide fixation, this unavoidably comes with the production of phosphoglycolate.

  • When assessed by mass flow, excelled only by photosynthesis, phosphoglycolate production constitutes the second‐most important process in the land‐based biosphere.

  • The scavenging of phosphoglycolate was a condition for the evolution of cyanobacteria, algae and plants.

  • Oxygenic photosynthesis is only possible on the condition of adequate photorespiratory metabolism and life on earth would look very different without it.

  • Oxygenic photosynthesis is coupled to photorespiratory carbon dioxide losses and this reduces competitiveness of C3 plants in warm environments. Gene technology‐assisted breeding hopes to reduce these losses in crops.

Keywords: C2 cycle; carbon dioxide; glycolate; oxygen; photorespiration; photosynthesis; Rubisco

Figure 1.

The plant photorespiratory C2 cycle spans three organelles: the chloroplast, the peroxisome and the mitochondrion. The enzymes of the core cycle are Rubisco, phosphoglycolate phosphatase (PGP), glycolate oxidase (GOX), serine‐glyoxylate aminotransferase (SGT), glutamate‐glyoxylate aminotransferase (GGT), glycine decarboxylase (GDC), serine hydroxymethyltransferase (SHMT), peroxisomal hydroxypyruvate reductase (HPR1) and glycerate kinase (GLYK). Catalase (CAT) detoxifies hydrogen peroxide. A cytosolic hydroxypyruvate reductase (HPR2) supports HPR1 when the peroxisomal malate dehydrogenase (pMDH) does not provide NADH rapidly enough for hydroxypyruvate reduction. Photorespiratory ammonia is captured by glutamine synthetase (GS2). The produced glutamine is then used by ferredoxin‐dependent glutamate synthase (GOGAT) to recycle 2‐oxoglutarate into fresh glutamate for peroxisomal transamination.

Figure 2.

Photorespiration is embedded into whole cell metabolism and manifold interactions with other metabolic pathways exist. For example, SHMT and other enzymes are strongly inhibited by 5‐formyl‐THF, which is produced in considerable amounts by SHMT itself. Cellular metabolism would rapidly break down if this noxious compound would not be detoxified and recycled to THF. This requires four folate‐interconverting enzymes, 5,10‐CH2THF dehydrogenase combined with 5,10‐methenyl‐THF cyclohydrolase in a bifunctional enzyme (E1), 5‐formyl‐THF cycloligase (E2) and 10‐formyl‐THF deformylase (E3). If E3 is blocked, plants accumulate massive amounts of glycine in normal air and need elevated carbon dioxide to survive (Collakova et al., ).

Figure 3.

Cyanobacteria recycle phosphoglycolate via two partially redundant pathways, a plant‐like C2 cycle (metabolites in blue, enzymes in red) and the bacterial glycerate pathway (black route). Both pathways start with phosphoglycolate phosphatase (PGP) and glycolate dehydrogenase (GLCDH). The glycerate pathway circumvents the glycine‐to‐serine conversion by directly converting glyoxylate into glycerate using tartronic semialdehyde synthase (TSS) and tartronic semialdehyde reductase (TSR). Enzymes of the plant‐like branch are serine‐glyoxylate aminotransferase (SGT), glutamate‐glyoxylate aminotransferase (GGT), glycine decarboxylase (GDC), serine hydroxymethyltransferase (SHMT) and hydroxypyruvate reductase (HPR). Only few advanced cyanobacteria have a plant‐type 3PGA‐forming glycerate kinase (GLYK, green route); all others use 2PGA‐forming glycerate kinases (GK) in combination with phosphoglyceromutase (PGM) for the generation of 3PGA. Some cyanobacteria can also completely decompose glyoxylate to carbon dioxide (grey route) via oxalate decarboxylase (ODC) and formate dehydrogenase (FDH). Cyanobacterial mutants without functioning phosphoglycolate metabolism cannot survive in normal environments (Eisenhut et al., ).



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Bauwe, Hermann(Feb 2010) Photorespiration. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001292.pub2]