Control of the ribulose 1,5-bisphosphate carboxylase/oxygenase activity by the chloroplastic glutathione pool

• We examined the possibility of a redox regulation of Rubisco activity by glutathione.
• Direct access of glutathione to Rubisco critical cysteines is kinetically hindered.
• Reduced glutathione can reactivate oxidized Rubisco through intermediary thiols.
• Oxidized glutathione cannot inactivate Rubisco through intermediaries.
• Thus, the glutathione pool keeps Rubisco active under different redox environments.

The CO2-fixing activity of ribulose 1,5-bisphosphate carboxylase/oxygenase depends on the redox state of its cysteines. Disulfides like cystamine or 5,5′-dithio-bis(2-nitrobenzoic acid), but not oxidized glutathione, switch the enzyme to the inactive oxidized form. Conversely, thiols like cysteamine, cysteine, dithiotreitol or 2-mercaptoethanol, but not reduced glutathione, recover enzymatic activity after a previous oxidation. Direct regulation of the carboxylase activity by the chloroplastic glutathione pool is hindered by kinetic barriers impeding access to the critical residues. However, reduced glutathione can drive the recovery of activity by means of minute amounts of smaller intermediary thiol/disulfide exchangers. In contrast, oxidized glutathione does not inactivate the enzyme even in the presence of these intermediaries. This asymmetrical effect should help to maintain the enzyme in the active form in vivo.