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. RuBisCO folding and assembly commences when the nascent large subunit folds with the help of the chaperonin GroEL-GroES. Requires homodimeric RuBisCO chaperone RbcX2 to assemble into RbcL8 octamers, making RbcL8-(RbcX2)8. The exposed C-terminus of RbcL binds in a cleft in RbcX2 (PubMed:17574029). RbcX2 is displaced by RbcS; as RbcX2 is removed RbcS mediates the ordering of an internal RbcL loop (Thr-63-Leu-69) in a catalytically active conformation (By similarity). Interacts with accumulation factor Raf1; dimeric Raf1 acts after chaperonin GroEL-GroES, binding to RbcL(2) leading to an RbcL8-Raf1(8) complex. RbcS displaces Raf1, resulting in holoenzyme formation (Probable). 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.