Exchange Properties of the Activator CO2 of Spinach Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

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Abstract
The exchange properties of the activator CO2 of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO2 on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO2. The exchange kinetics of the activator CO2 were monitored by activation of the enzyme to steady state in the presence of 12CO2, followed by addition of 14CO2 and determination of the amount of labeled CO2 trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants (Kobs) for exchange with both the purified enzyme (0.45 min−1) and in illuminated chloroplasts (0.18 min−1) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO2 was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation.