Relationship between the Nucleo-Cytoplasm Composition and the Kinetic Properties of Ribulose-l,5-bisphosphate Carboxylase/Oxygenase of Rice

Ribulose-l,5-bisphosphate carboxylase/oxygenase (RubisCO, EC 4.1.1.39) was purified from leaves of rice (Oryza sativa), including four fertile cultivars and six male-sterile nuclear substitution lines: genome of cultivars in O. sativa L. f. spontanea cytoplasm and in O. sativa subsp. indica cv. Gambiaka Kokum cytoplasm, respectively. These RubisCO enzymes were divided into, two categories: (a) RubisCO with identical large subunits but different small subuinits, (b) RubisCO with variable large subunits and identical small subunits. Specific activities for both the carboxylation and oxygenation reactions of RubisCO were determined under standard conditions of activation. It was found that the Vmax(CO2) remained constant, but Km(O2) changed greatly, ranging from 223 μmol/L to 371 μmol/L in (a) group. The small subunits had significant effect on Km(O2) and on Vmax(O2)/Km(O2). No significant variation in Km(CO2), Vmax(O2), and the specificky factor were detected among the RubisCO with varied small subunits. Significant variation in Km(O2) and specificity factor were detected among the enzymes with varied large subunits, which also showed an important effect on Km(CO2) and Vmax(O2). RubisCO with heterologous large subunits had higher carboxylase activity and specificity factor than those with homologous large subunits.