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. In Western blots an approximately 220 kDa polyprotein and 2 smaller proteins of about 55 and 52 kDa are detected, suggesting the polyprotein may be cleaved at one end of the linker and then at the other end to give mature RuBisCO. 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). Synthesized as an approximately 220 kDa polyprotein, imported into chloroplasts and subsequently cleaved into 4 mature RuBisCO proteins. Only the 2 middle RuBisCO copies are complete in this entry. There are estimated to be at least 10 copies of this polyprotein gene. 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.