Rubisco is an important enzyme in the Calvin cycle, allowing the fixation of atmospheric carbon. This ensures the number of carbon atoms is maintained over the cycles.
Rubisco is very poor as an enzyme, having a low affinity for CO2 (one of the substrates) and a slow rate. This means that around 50% of leaf protein is rubisco, as high concentrations are required to match the supply of ATP and NADPH.
In cellular conditions, rubisco’s activity is favourable, with a negative $$\Delta\text{G}$$.