2phosphoglycolate

2-phosphoglycolate, abbreviated 2-PG, is a phosphorylated two-carbon metabolite produced as a byproduct of RuBisCO’s oxygenase activity during oxygenic photosynthesis. When RuBisCO fixes O2 instead of CO2, one product is 2-phosphoglycolate alongside 3-phosphoglycerate. Because 2-PG can inhibit several enzymes, organisms that perform photosynthesis have a salvage pathway to metabolize it rather than allow its accumulation.

Metabolism and pathway: 2-PG is dephosphorylated by 2-phosphoglycolate phosphatase to glycolate. In peroxisomes, glycolate is oxidized to glyoxylate, generating hydrogen peroxide. Glyoxylate is then metabolized to form glycine, which, in mitochondria, is combined with another glycine by the glycine decarboxylase complex to yield serine, with CO2 and NH3 released. Serine is transported back to the chloroplast and converted through a short sequence back to a Calvin-cycle intermediate, effectively returning carbon to photosynthetic metabolism. Overall, photorespiration recycles carbon and salvages energy, but it reduces photosynthetic efficiency by dissipating ATP and reducing power and releasing CO2.

Distribution and significance: The pathway is widespread among plants, algae, and many cyanobacteria. Photorespiration becomes more prominent under low atmospheric CO2, high oxygen levels, heat, or drought. 2-PG and its salvage enzymes are of interest in agricultural research as potential targets to improve photosynthetic efficiency and crop yields. Some bacteria possess alternative or auxiliary routes for metabolizing 2-PG, reflecting broader carbon metabolism of glycolate-derived intermediates.