The Effect of SO32- and SO42- Ions on the Reactions of Ribulose Bisphosphate Carboxylase

Abstract

The effects of sulphite ion ($SO32-$) and sulphate ion ($SO42-$) on both the activation and the catalytic activities of ribulose- 1, 5-bisphosphate carboxylase (EC 4.1.1.39) were studied and compared to those of other effectors of the enzyme, particularly inorganic phosphate (P₁). The activation by CO₂ and Mg²⁺ of a slow activating form of the carboxylase in the presence of the two anions produced high specific activities with significant lower concentrations of CO₂ than normally required. This was due to stabilization of the ternary complex between the enzyme, CO₂ and Mg²⁺. With a rapidly activating species of enzyme, $SO32-$ and $SO42-$ caused only a small increase in activation with subsaturating CO₂. $SO32-$, $SO42-$ and P₁, with saturating concentrations of CO₂ also enhanced the catalytic activity above that achieved with CO₂ and Mg²⁺ alone; P₁ was the most effective of the anions, producing a 50% increase in the specific activity, both with the slow and rapidly activating species. $SO32-$ and $SO42-$ were potent inhibitors of the carboxylase and oxygenase reactions of the enzyme. $SO42-$ was a non-competitive inhibitor with respect to CO₂, and competitive/mixed with respect to ribulose-1, 5-bisphosphate. The time course of the carboxylase and oxygenase reactions in the presence of $SO32-$ were biphasic with inhibition apparent only in the second phase.