Glycolate pathway is also known as the C2 cycle of photosynthesis or photorespiration or glycolate-glyoxylate metabolism. It reduces the photosynthesis efficiency of C3 plants. Glycolate metabolism is also found in unicellular green algae.
This cycle helps in removing 2-phosphoglycolate, a toxic metabolite produced by the oxygenation reaction of RuBisCO. It produces PGA eventually but ~25% of C is released as CO2 in the process and ATP is also utilised.
Key features of Glycolate Pathway
- The process competes with the photosynthesis process. It wastes some of the energy produced by photosynthesis.
- It occurs in Chloroplast, Peroxisomes and Mitochondria.
- The main enzyme of the Calvin cycle RuBisCO has an affinity for both CO2 and O2 and they both compete for binding with RuBisCO. Their binding is dependent on the concentration of both the species. At mild temperatures, RuBisCO has a higher affinity for carbon dioxide.
- The process initiates when O2 binds with RuBisCO. The enzyme adds oxygen to RuBP (Ribulose-1,5-bisphosphate), i.e. oxygenation of RuBPand forms 2-phosphoglycolate and 3PGA (3-phosphoglycerate). This occurs in the chloroplast.
- 3PGA produced at a reduced rate and enters the Calvin cycle.
- 2-phosphoglycolate is translocated to peroxisomes, where it is oxidised to glyoxylate by hydrogen peroxide (H2O2). The enzyme catalysing this reaction is Glycolate oxidase.
- Hydrogen peroxide is acted on by catalase, which breaks it to water and oxygen.
- Glyoxylate is converted to Glycine in peroxisomes by Glutamate-glyoxylate aminotransferase.
- Glycine is transported to mitochondria.
- In mitochondria, 2 Glycine molecules (2C) get converted to a Serine (3C). CO2 and NH3 are released in the process. This reaction is catalysed by an enzyme called Glycine decarboxylase.
- Serine is transported back to peroxisomes, where it is converted to Glycerate.
- Glycerate is transported back to the chloroplast, where it is phosphorylated to form 3PGA. ATP is used in this reaction. 3PGA enters the Calvin cycle.
It is important to note that RubisCO has a higher affinity for oxygen at increased temperatures. This results in a higher rate of photorespiration in hot and dry environments. To minimise photorespiration, C4 plants and CAM plants have derived different mechanisms for carbon fixation. Carbon dioxide gets accumulated around RuBisCO in these plants to suppress its oxygenation activity.
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