183 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). RBCS-4 Ribulose bisphosphate carboxylase small chain 4, chloroplastic MASSLMSNAATTMAAATTTAQANMVAPFNGLKSISAFPVTRKNNDITSVASNGGRVQCMQVWPPLGMKKFETLSYLPPLSEESLLKEVQYLLNNGWVPCLEFEPTHGFVYREHGNTPGYYDGRYWTMWKLPMFGCTDPSQVVAELEEAKKAYPEAFIRIIGFDNVRQVQCVSFIAYKPASYGA RBS4_MESCR