RuBisCO catalyzes the first step of carbon fixation in the Calvin cycle. It attaches carbon dioxide to ribulose-1,5-bisphosphate (RuBP), a five-carbon molecule, producing an unstable six-carbon intermediate that immediately splits into two three-carbon molecules of 3-phosphoglycerate.
The enzyme’s full name is ribulose-1,5-bisphosphate carboxylase/oxygenase. Its carboxylase activity fixes carbon dioxide, while its oxygenase activity begins photorespiration when oxygen competes successfully for the active site. This makes RuBisCO both essential and inefficient: it is often described as one of the most abundant proteins on Earth, yet it works relatively slowly and can mistake oxygen for carbon dioxide.
RuBisCO operates in the chloroplast stroma during the light-independent reactions. The ATP and NADPH needed for later Calvin-cycle steps come from the light-dependent reactions. In C4 plants such as corn and sugarcane, an additional carbon-concentrating pathway raises carbon-dioxide levels around RuBisCO and reduces photorespiration.