Studies on Molecular Mechanisms for Carboxylation of RuBisCO toward High Efficiency

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In these three years from 1998 to 2000, we could determine several threedimensional structures of RuBisCO, ribulose bis-phosphate carboxylase/oxygenase. Based on these structures, new structural principal to control the enzymatic function of RuBisCO was proposed.RuBisCO catalyses both reactions of carboxylation and oxygenation. Therefore, it is important to increase carboxylation/oxygenation ratio to design carboxylase with higher efficiency. RuBisCO has divalent magnesium ion on its active center. When the ion is replaced by manganese, the carboxylation activity decreases. In order to find some structural factor in these functional change, the crystal structure of manganese binding spinach RuBisCO.There exists so-called loop-6 with flexible structure near the active site of RuBisCO.The loop takes open structure when the substrate ribulose bis-phosphate is not near the active site, while it takes closed structure when the substrate is near the active site. The loop has lysine residue in the middle and fix the reaction intermediate formed from substrate and carbon dioxide. RuBisCO obtained from red algae Galdieria partita was found to have closed structure even though it has no reaction intermediate analogue in its active center. From the detailed comparison of the structure with other RuBisCOs, a new interaction was found to participates the open-close control of loop-6.RuBisCO of green algae Chlamydomonas reinhardtii has been well investigated in its enzymatic functions, while little of its structure was known. We have determined its three-dimensional structure for the first time and found the little difference in its large subunit, while marked difference in its small subunit resulted in the narrow solvent channel formed in the center of RuBisCO molecule.