C2 cycle or photorespiration

One of the inherent properties of enzyme-rubisco is oxygenation. Rubisco catalyses both carboxylation as well as oxygenation and active site for the both the substrate is same. Both CO₂ as well as O₂ compete each other for the active site. In prevailing atmospheric concentration of CO₂ (21%) and O₂ (0.04%) rubisco performs both carboxylation as well as oxygenation. In carboxylation rubisco adds CO₂ to the ribulose 1,5 bisphosphate via so-called Calvin-Benson cycle and fixes CO₂ into 3 phosphoglycerate (a three carbon monosaccharide).
During oxygenation, rubisco adds O₂ to the ribulose 1,5 bisphosphate breaking it into 3 phosphoglycerate and 2 phosphoglycolate. This is what we know as ‘photorespiration’. To recover the carbon lost in 2 phosphoglycolate, plants have well developed mechanism so-called ‘C2 cycle’ of ‘photorespiratory pathway’. C2 cycle in higher plants and algae involves three cell organelles: chloroplast, peroxisome and mitochondria. The C2 cycle is elucidate in the figure given bellow:

C2 cycle or photorespiratory pathway

Efficiency of C2 cycle

Photorespiratory pathway efficiently recovers three carbons out of 4 carbon lost in the form of 2 molecules of 2 phosphoglycolate. Thus, 75% loss of carbon is recovered via C2 cycle and 25 % carbon is lost because of oxygenase property of rubisco. During the C2 cycle all the intermediates are regenerated, but extra ATP and reduced ferredoxin is required in recovery process. These losses reduce the efficiency of C3 plants and decrease the productivity of plants.

Photorespiration decrease the productivity of C3 plants in warm climates

In tropical regions the efficiency of C3 plants is further reduced because of the high temperature. High temperature has three effects on carbon fixation:

Increasing temperature increases the oxygenation property of rubisco more than carboxylation.
As the temperature increases, solubility of CO₂ in is reduced, and.
At high temperature, plants close their stomata to reduce water loss in transpiration, which again reduce the CO₂ concentration in the vicinity of rubisco.

All the three events further reduce the productivity of C3 plants in hot environments.

Chloroplast can be engineered to reduce the photorespiratory loss

Cyanobacteria have mechanism of proteobacteria, through which they recover all the carbon loss during photorespiration. In higher plants and algae however, no such mechanism is reported. Researches are going on to incorporate the mechanism through the genetic engineering, which can completely convert 2 phosphoglycolate into CO₂ in the chloroplast. If it is achieved CO₂ would be utilized in the C3 cycle in the chloroplast and CO₂ fixation efficiency would be enhanced.

First published on 13-04-2021
Last updated on 29-04-2021