As early as the 1920s, Otto Warburg observed that O₂ inhibits photosynthesis in C3 plants. However, it wasn't until the 1960s that mechanism of photorespiration was discovered. The reason is that Rubisco is a flawed enzyme which struggles to distinguish between O₂ and CO₂. This flaw is also considered a relic in biological evolution. When Rubisco first appeared, there’s no such problem at all. The scarce O₂ on the air couldn't compete with CO₂ for active sites in enzyme. This fault became increasingly apparent as oxygen began to be released into atmosphere through photosynthesis of cyanobacteria. Nonetheless, organisms have continuously mitigated this drawback during evolution. The carboxylation activity of Rubisco in higher plants is several times higher than prokaryotic bacteria.
Rubisco has dual catalytic properties. When CO₂ is abundant, it promotes the combination of CO₂ and pentose called RuBP. If O₂ is abundant and CO₂ is scarce, the Calvin cycle is inhibited and photorespiration is initiated. The unstable intermediate formed by RuBP and oxygen quickly breaks down into phosphoglycerate (PGA) and phosphoglycolate (PG). The former enters dark reaction, while PG can’t be metabolized by Calvin cycle. High concentrations of PG are toxic to plants, so plant cells must find some means to deal with them promptly.
PG dephosphorylate to a two-carbon compound, glycolate, in the chloroplast. It’s why the photorespiration is also called Glycolate pathway or C2 cycle. Then, O₂ is consumed to synthesize intermediate glyoxylate and final product glycine in peroxisomes. Glycine is transported to mitochondria where it is catalyzed into serine to release CO₂ and ammonium ions. Peroxisomes and chloroplasts consume NADPH and ATP to reconvert serine into PGA for the Calvin cycle.
Advantages and disadvantages: Is photorespiration really useless?
It's similar to cellular aerobic respiration that consumes O₂ and releases CO₂, but photorespiration not only produce zero energy, it also consumes high-energy compounds ATP and NADPH. Despite Rubisco's weaker oxygenation rate, the O₂ in atmosphere far exceeds CO₂. Especially in hot and dry environments, C3 plants close their stomata to prevent CO₂ from entering and O₂ from leaving. Therefore, photorespiration inevitably occurs in C3 plants strongly. Rubisco, like a flawed machine, continuously produces defective products in certain environments. Glycolate or the C2 cycle acts as a helpless remedial measure to convert glycolate into nutrients that plants can recycle. Nevertheless, a portion of carbon is still lost to atmosphere as CO₂.
The Glycolate Pathway weakens photosynthesis to reduce crop yields significantly. About 30%-50% of saccharide is lost. So, can we boost photosynthesis efficiency by inhibiting it? Experiments with rice have shown that reducing photorespiration by 30% can increase rice yield. However, if it’s further inhibited, photosynthesis decreases.
More and more people now believe that the Glycolate Pathway is positive because it may be a protective mechanism for photosynthesis. Especially under conditions of strong light and insufficient CO₂, the light reaction often produce far excessive ATP and NADPH for carbon anabolism. If these high-energy compounds aren't consumed in other biochemical reactions, reactive oxygen species will damage chloroplasts. The Glycolate Pathway also converts excess O₂ into CO₂ to drive the Calvin cycle and maintain balanced metabolism, especially when stomata are closed. Additionally, amino acids from photorespiration can serve as raw materials for polypeptides or proteins. For example, glycine can increase the content of glutathione in chloroplasts and cytoplasm, an antioxidant that absorbs free radicals and regulates the activity of various photosynthetic enzymes.