Rubisco

RuBisCO, or ribulose-1,5-bisphosphate carboxylase/oxygenase, is the primary enzyme of carbon fixation in photosynthesis. In the Calvin cycle, it catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) with carbon dioxide to form two molecules of 3-phosphoglycerate. The enzyme can also catalyze the oxygenation of RuBP, producing one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate, a reaction that triggers photorespiration and reduces photosynthetic efficiency.

RuBisCO is found in the chloroplasts of plants and algae and in many autotrophic bacteria. In cyanobacteria it operates within carboxysomes, microcompartments that concentrate CO2 near the enzyme. RuBisCO exists in several forms. Form I, common in plants, algae, and cyanobacteria, typically assembles as a L8S8 holoenzyme. Form II, found in some bacteria and dinoflagellates, lacks small subunits. Form III is found in archaea, and Form IV, RuBisCO-like proteins, do not catalyze the carboxylation reaction.

Kinetically, RuBisCO is relatively slow and exhibits relatively low affinity for CO2. Its specificity factor for CO2 versus O2 (Sc/o) is typically in the range of 60–100 in higher plants, and turnover numbers for carboxylation are on the order of a few s−1 under physiological conditions. The balance between CO2 fixation and oxygenation is sensitive to atmospheric CO2 and O2 levels and to temperature.

RuBisCO is one of the most abundant proteins on Earth, often representing a substantial fraction of leaf soluble protein. Because its efficiency limits photosynthesis, researchers pursue strategies to improve its kinetics, reduce photorespiration, or concentrate CO2 around the enzyme, including engineering RuBisCO variants and incorporating carbon-concentrating mechanisms into crops.