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Ribulose-1,5-bisphosphate carboxylase/oxygenase, often abbreviated as RuBisCO, is an enzyme that plays a crucial role in photosynthesis. It is found in the chloroplasts of plants and algae, as well as in cyanobacteria. The primary function of RuBisCO is to catalyze the fixation of carbon dioxide from the atmosphere into organic molecules. This process is the first step in the Calvin cycle, the light-independent reactions of photosynthesis, which ultimately leads to the production of sugars.

RuBisCO's catalytic activity involves the addition of carbon dioxide to ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar. However, the enzyme also exhibits an oxygenase activity, where it can react with oxygen instead of carbon dioxide. This leads to photorespiration, a process that consumes oxygen and releases carbon dioxide, effectively reducing the efficiency of photosynthesis. The relative rates of carboxylation and oxygenation depend on the concentrations of CO2 and O2, as well as temperature.

Despite its critical role, RuBisCO is considered a relatively inefficient enzyme. Its slow catalytic rate and its propensity for oxygenation have led to extensive research aimed at improving its performance. Evolutionary adaptations, such as the formation of carbon-concentrating mechanisms in some plants, have evolved to mitigate the negative effects of photorespiration. Understanding RuBisCO's structure, function, and limitations is fundamental to comprehending plant productivity and exploring strategies for enhancing crop yields.