Involvement of sulphydryl groups in glutamine synthetase activity fromRhodobacter capsulatusE1F1

Abstract

Glutamine synthetase (GS) from the phototrophic bacterium Rhodobacter capsulatus E1F1 (formerly known as Rhodopseudomonas capsulata E1F1) contains 9 SH-groups per subunit. These SH-groups reacted with sulphydryl group reagents resulting in an inhibition of the transferase activity of GS. Inhibited GS was recovered up to 90% of its initial value by treatment with thiols such as l-cysteine, dithiothreitol, or 2-mercaptoethanol. However, dithioerythritol exhibited, at concentrations above 0.5 mM, a significant inhibition of the transferase activity. Glutamine and glutamate protected against the inactivation by sulphydryl group reagents, whereas ADP or ATP enhanced the velocity of inactivation considerably. Mn2+-dependent GS was unaffected in vivo by light-dark transitions, which rules out a redox photoregulation of SH-groups required for activity.