Activation of Ribulosebisphosphate Carboxylase/Oxygenase at Physiological CO(2) and Ribulosebisphosphate Concentrations by Rubisco Activase.

The enzyme-catalyzed activation of ribulosebisphosphate carboxylase/oxygenase (rubisco) was investigated in an illuminated reconstituted system containing thylakoid membranes, rubisco, ribulosebisphosphate (RuBP), MgCl(2), carbonic anhydrase, catalase, the artificial electron acceptor pyocyanine, and partially purified rubisco activase. Optimal conditions for light-induced rubisco activation were found to include 100 micrograms per milliliter rubisco, 300 micrograms per milliliter rubisco activase, 3 millimolar RuBP, and 6 millimolar free Mg(2+) 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 K(act)(CO(2)) of rubisco activation in the reconstituted system was 4 micromolar CO(2), compared to a K(act)(CO(2)) of 25 to 30 micromolar CO(2) for the previously reported spontaneous CO(2)/Mg(2+) activation mechanism. The activation process characterized here explains the high degree of rubisco activation at the physiological concentrations of 10 micromolar CO(2) and 2 to 4 millimolar RuBP found in intact leaves, conditions which lead to almost complete deactivation of rubisco in vitro.