DISTRIBUTION OF FALLOVER IN THE CARBOXYLASE REACTION AND FALLOVER-INDUCIBLE SITES AMONG RIBULOSE 1,5-BISPHOSPHATE CARBOXYLASE OXYGENASES OFPHOTOSYNTHETIC ORGANISMS/

The biphasic reaction course, fallover, of carboxylation catalysed by ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) has been kno wn as a characteristic of the enzyme from higher land plants. Fallover consists of hysteresis in the reaction seen during the initial severa l minutes and a very slow suicide inhibition by inhibitors formed from the substrate ribulose-1,5-bisphosphate (RuBP), This study examined t he relationship between occurrence of fallover and non-catalytic RuBP- binding sites, and the putative hysteresis-inducible sites (Lys-21 and Lys-305 of the large subunit in spinach RuBisCO) amongst RuBisCOs of a wide variety of photosynthetic organisms. Fallover could be detected by following the course of the carboxylase reaction at 1 mM RuBP and the non-catalytic binding sites by alleviation of fallover at 5 mM RuB P, RuBisCO from Euglena gracilis showed the same linear reaction cours e at both RuBP concentrations, indicating an association between an ab sence of fallover and an absence of the non-catalytic binding sites. T his was supported by the results of an equilibrium binding assay for t his enzyme with a transition state analogue, Green macroalgae and non- green algae contained the plant-type, fallover enzyme. RuBisCOs from C onjugatae, Closterium ehrenbergii, Gonatozygon monotaenium and Netrium digitus, showed a much smaller decrease in activity at 1 mM RuBP than the spinach enzyme and the reaction courses of these enzymes at 5 mM RuBP were almost linear. RuBisCO of a primitive type Conjugatae, Mesot aenium caldariorum, showed the same linear course at both RuBP concent rations, Sequencing of rbcL of these organisms indicated that Lys-305 was changed into arginine with Lys-21 conserved.