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 in the photorespiration process. Both reactions occur simultaneously and in competition at the same active site. Although the small subunit is not catalytic it is essential for maximal activity. Heterohexadecamer of 8 large and 8 small subunits. This cyanobacterium makes beta-type carboxysomes. RuBisCO folding and assembly commences when the nascent large subunit folds with the help of chaperonin GroEL-GroES. Both RbcX and Raf1 help folded RbcL release from the chaperonin and dimerize (Probable). Dimeric Raf1 binds to RbcL(2) leading to an RbcL8-Raf1(8) complex. RbcS displaces Raf1, resulting in holoenzyme formation (By similarity). 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 small chain family.