RuBisCO

Ribulose-1,5-bisphosphate carboxylase oxygenase, better known as RuBisCO, is an enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle. Carbon fixation is how the atoms of atmospheric carbon dioxide are made available to organisms in the form of energy-rich molecules such as glucose. RuBisCO splits 6-C molecules into two equal parts.
RuBisCO is very important in biology and ecology because it catalyzes the primary chemical reaction by which inorganic carbon permanently enters the biosphere.
RuBisCO is also the most abundant protein in leaves and the most abundant protein on Earth. It accounts for 50% of soluble leaf protein (20-30% of total leaf nitrogen) and 30% of soluble leaf protein in plants (5-9% of total leaf nitrogen).
Given its important role in the biosphere, there are currently efforts to genetically engineer crop plants to contain more efficient RuBisCO.