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 (By similarity). 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. Homodimer. The basic functional RuBisCO is composed of a large chain homodimer in a 'head-to-tail' conformation. In contrast to form I RuBisCO, the form II RuBisCO are composed solely of large subunits (By similarity). Belongs to the RuBisCO large chain family. Type II subfamily.