Dual avatars of E. coli grxB encoded Glutaredoxin 2 perform ascorbate recycling and ion channel activities

Abstract

Glutaredoxins (Grxs) are single-domain redox enzymes of the thioredoxin superfamily, and primarily function as glutathione (GSH) dependent disulphide reductases. Whereas, the E. coli Glutaredoxin 2 (EcGrx2) encoded by grxB has two conserved GST-fold domains, it still lacks a classical Grx-like functions. In this study, we show for the first time, that EcGrx2 exists in both soluble and membrane integrated forms. The soluble form associates with a previously unidentified GSH dependent dehydroascrobate (DHA) reductase, and the membrane integrated form possesses ion channel activities. Using enzyme kinetic data and structural data we unequivocally demonstrate that EcGrx2 recycles ascorbate (AsA) from DHA. This ability to recycle AsA is inhibited by Zinc (Zn2+). We also show that both wildtype and the E. coli grxB deletion mutant can be rescued from H2O2-induced oxidative stress using ascorbate as an antioxidant, which otherwise is only known as a carbon source in bacteria. Moreover, the grxB- mutant is susceptible to intracellular killing by ROS producing macrophages. We further discovered that EcGrx2 integrates into the native E. coli membrane and show that the purified soluble protein readily inserts into artificial lipid bilayer membrane and conducts ions in vitro. Our data demonstrates a highly conserved functional similarity among EcGrx2-orthologs and highlights that the utilization and subsequent recycling of ascorbate as an antioxidant by grxB harbouring gram-negative bacteria, including human pathogens, may provide a survival advantage under hostile oxidative environments.