Oxidative inactivation of glutamine synthetase subunits

Escherichia coli glutamine synthetase (GS) was inactivated by a nonenzymic mixed-function oxidation system composed of ascorbate, O2, and Fe(III). Partial inactivation of GS by this system leads to the formation of hybrid GS molecules (dodecamers) composed of both active and inactive subunits. Subunit interactions in these hybrid molecules are weaker than in the native enzyme, as indicated by the kinetics of subunit dissociation in the presence of 4 M urea. Heterologous subunit interactions in these hybrid molecules do not affect the affinity of active subunits for glutamate. Incubation of partially adenylylated GS preparations (n = 6.7) with the ascorbate system in the absence of substrates leads to preferential oxidative inactivation of unadenylylated subunits, whereas incubation in the presence of ATP and glutamate leads to preferential inactivation of adenylylated subunits.