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. 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. Belongs to the RuBisCO large chain family. Type II subfamily.