RuBisCO catalyzes two reactions: the carboxylation of D-ribulose 1,5-bisphosphate, the primary event in carbon dioxide fixation, as well as the oxidative fragmentation of the pentose substrate. Both reactions occur simultaneously and in competition at the same active site. 2 (2R)-3-phosphoglycerate + 2 H(+) = CO2 + D-ribulose 1,5-bisphosphate + H2O D-ribulose 1,5-bisphosphate + O2 = (2R)-3-phosphoglycerate + 2-phosphoglycolate + 2 H(+) Binds 1 Mg(2+) ion per subunit. Heterohexadecamer of 8 large chains and 8 small chains. The basic functional RuBisCO is composed of a large chain homodimer in a 'head-to-tail' conformation. In form I RuBisCO this homodimer is arranged in a barrel-like tetramer with the small subunits forming a tetrameric 'cap' on each end of the 'barrel'. Belongs to the RuBisCO large chain family. Type I subfamily.