Photorespiration and the C4 and CAM Pathways:- Photorespiration Results from Rubisco’s Oxygenase Activity
Rubisco is not absolutely specific for CO2 as a substrate. Molecular oxygen (O2) competes with CO2 at the active site, and about once in every three or four turnovers, rubisco catalyzes the condensation of O2 with ribulose 1,5-bisphosphate to form 3-phosphoglycerate and 2-phosphoglycolate (Fig. 1), a metabolically useless product. This is the oxygenase activity referred to in the full name of the enzyme: ribulose 1,5-bisphosphate carboxylase/oxygenase. The reaction with O2results in no fixation of carbon and appears to be a net liability to the cell; salvaging the carbons from 2-phosphoglycolate (by the pathway outlined below) consumes significant amounts of cellular energy and releases some previously fixed CO2. Given that the reaction with oxygen is deleterious to the organism, why did the evolution of rubisco produce an active site unable to discriminate well between CO2 and O2? Perhaps much of this evolution occurred before the time, about 2.5 billion years ago, when pro duction of O2 by photosynthetic organisms started to raise the oxygen content of the atmosphere. Before that time, there was no selective pressure for rubisco to dis criminate between CO2 and O2. The Km for CO2 is about 9 M, and that for O2 is about 350 M. The modern at mosphere contains about 20% O2 and only 0.04% CO2, so an aqueous solution in equilibrium with air at room temperature contains about 250 MO2 and 11 MCO2— concentrations that allow significant O2 “fixation” by rubisco and thus a significant waste of energy. The temperature dependence of the solubilities of O2 and CO2 is such that at higher temperatures, the ratio of O2 to CO2 in solution increases. In addition, the affinity of rubisco for CO2 decreases with increasing temperature, exacerbating its tendency to catalyze the wasteful oxygenase reaction. And as CO2 is consumed in the assimilation re actions, the ratio of O2 to CO2 in the air spaces of a leaf increases, further favoring the oxygenase reaction.
FIGURE 1 Oxygenase activity of rubisco. Rubisco can incorporate O2 rather than CO2 into ribulose 1,5-bisphosphate. The unstable intermediate thus formed splits into 2-phosphoglycolate and 3-phosphoglycerate, which can reenter the Calvin cycle.
