RuBisCO
The enzyme that begins carbon fixation by adding carbon dioxide to RuBP. It can also react with oxygen, initiating photorespiration.
Rubisco is short for ribulose-1,5-bisphosphate carboxylase/oxygenase. In the Calvin cycle, its carboxylase activity adds carbon dioxide to RuBP, producing compounds that the plant can use to build carbohydrates.
The same active site can bind oxygen. That oxygenase reaction begins photorespiration Photorespiration A carbon-recycling pathway initiated when Rubisco reacts with oxygen instead of carbon dioxide, consuming energy and releasing some previously fixed carbon. , which consumes energy and releases some fixed carbon. Carbon dioxide and oxygen availability, temperature, and the enzyme's kinetics influence which reaction occurs.
Rubisco has a relatively slow catalytic rate, so C3 leaves produce it in large quantities. It can account for a substantial share of leaf protein and nitrogen.