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. In this organism the plastid is the result of a secondary endosymbiosis event, and thus is found within the endomembrane system, necessitating a complex targeting process. 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). It is not clear if this is the true start of the protein or not; this is probably part of a polyprotein. This may be first cotranslationally imported into the ER up to a stop-transfer signal, so that the N-terminal region of the transit peptide is in the lumen of the ER while the rest of the protein remains in the cytoplasm. Maintaining this topology, proteins are directed to the Golgi and sorted into vesicles that will fuse with the outermost plastid membrane, exposing the transit peptide to the Toc/Tic apparatus, which draws the entire protein across the remaining membranes (By similarity). Belongs to the RuBisCO large chain family. Type II subfamily. Note that unlike other eukaryotes, peridinin-containing dinoflagellates have a nuclear-encoded chloroplast-targeted form II RuBisCO.