Activation of Ribulosebisphosphate Carboxylase/Oxygenase at Physiological CO2 and Ribulosebisphosphate Concentrations by Rubisco Activase

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Abstract
The enzyme-catalyzed activation of ribulosebisphosphate carboxylase/oxygenase (rubisco) was investigated in an illuminated reconstituted system containing [spinach] thylakoid membranes, rubisco, ribulosebisphosphate (RuBP), MgCl2, carbonic anhydrase, catalase, the artificial electron acceptor pyocyanine, and partially purified rubisco activase. Optimal conditions for light-induced rubisco activation were found to invlude 100 micrograms per milliliter rubisco, 300 micrograms per milliliter rubisco activase, 3 millimolar RuBP, and 6 millimolar free Mg2+ at pH 8.2. The half-time for rubisco activation was 2 minutes, and was 4 minutes for rubisco deactivation. The rate of rubisco deactivation was identical in the presence and absence of activase. The Kact(CO2) of rubisco activation in the reconstituted system was 4 micromolar CO2 compared to a Kact(CO2) of 25 to 30 micromolar CO2 for the previously reported spontaneous CO2/Mg2+ activation mechanism. The activation process characterized here explains the high degree of rubisco activation at the physiological concentrations of 10 micromolar CO2 and 2 to 4 millimolar RuBP found in intact leaves, conditions which lead to almost complete deactivation of rubisco in vitro.