Photorespiration

The enzyme RuBisCO, introduced in the section on the Calvin cycle as the key CO2-fixing enzyme of the C3 pathway, has one important biochemical limitation: its full name, ribulose bisphosphate carboxylase-oxygenase, already reveals that it is not perfectly specific for CO2. Alongside its carboxylase activity (adding CO2 to RuBP), RuBisCO also possesses a genuine competing oxygenase activity, in which it instead adds a molecule of O2 to RuBP. Because CO2 and O2 physically compete for the same active site on the enzyme, which reaction actually occurs at any given moment depends on the relative concentrations of the two gases available to the enzyme: when the internal CO2:O2 ratio within the leaf falls -- as it does, for example, under bright light and high temperature, conditions that cause stomata to partially close and reduce fresh CO2 entry while photosynthetic O2 continues to accumulate -- the oxygenase reaction becomes proportionally more frequent.

When RuBisCO does add O2 to RuBP instead of CO2, the reaction yields one molecule of the usual three-carbon PGA (which can still re-enter the Calvin cycle normally) together with one molecule of a two-carbon compound called phosphoglycolate. The metabolic pathway that then processes this phosphoglycolate is called photorespiration (sometimes also called the C2 cycle, or the oxidative photosynthetic carbon cycle), and it earns the name "respiration" because, like true mitochondrial respiration, it consumes O2 and releases CO2 -- but it does so in the light, and it is triggered directly by the photosynthetic machinery's own enzyme, making it, in effect, the exact chemical opposite of ordinary net photosynthesis running alongside it.

Photorespiration is metabolically unusual in that it is distributed across three separate organelles working in sequence. It begins in the chloroplast, where RuBisCO's oxygenase reaction generates phosphoglycolate as just described. This phosphoglycolate is then transported to the peroxisome, where it is converted first to glycolate and then oxidised, through further steps, into glyoxylate and finally into the amino acid glycine, consuming molecular oxygen along the way. The glycine produced is then transported a second time, now to the mitochondrion, where two molecules of glycine are combined to form one molecule of the amino acid serine, releasing one molecule of CO2 in the process -- the CO2-releasing step that gives the whole pathway its "respiratory" character. The serine produced is eventually transported back toward the peroxisome and chloroplast, where it re-enters the carbon pool. …