RuBisCOs

RuBisCOs, or ribulose-1,5-bisphosphate carboxylase/oxygenase, are a family of enzymes that catalyze the fixation of atmospheric carbon dioxide in photosynthetic organisms and some chemoautotrophs. The enzyme catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP) with CO2 to yield two molecules of 3-phosphoglycerate, the first stable products of the Calvin cycle. In many conditions, RuBisCO also catalyzes a competing oxygenation reaction where O2 is added to RuBP, producing one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate, which must be recycled in photorespiration. This dual activity makes RuBisCO relatively slow and inefficient under ambient air, particularly at higher temperatures or when CO2 levels are low relative to O2.

RuBisCO exists in several forms. Form I is common in plants, algae, and cyanobacteria and forms a large holoenzyme composed of eight large subunits (RbcL) and eight small subunits (RbcS), assembled as L8S8. Form II is found in certain bacteria and dinoflagellates and tends to assemble as dimers or tetramers of large subunits lacking small subunits. Form III occurs in archaea and some bacteria. Form IV, Rubisco-like proteins, are related enzymes that generally lack carboxylase/oxygenase activity and participate in other metabolic pathways.

In plants, the catalytic subunits are encoded by rbcL in chloroplasts and the small subunits by rbcS in the nucleus, imported into the chloroplast. RuBisCO activity is modulated by RuBisCO activase, an ATP-dependent chaperone that remodels inhibited active sites. Kinetics vary with temperature and CO2/O2 concentrations; plants display relatively high CO2/O2 specificity factor, but photorespiration remains a limiting factor.

RuBisCO is one of the most abundant enzymes on Earth and a major determinant of photosynthetic efficiency in C3 plants. Its slow turnover and oxygenase side activity have motivated efforts to enhance carbon fixation through breeding, genetic engineering, or by introducing CO2-concentrating mechanisms, with the aim of increasing crop yields and reducing greenhouse gas release.