Characterization of glutathione amide reductase from Chromatium gracile - Identification of a novel thiol peroxidase (Prx/Grx) fueled by glutathione amide redox cycling

Among the Chromatiaceae, the glutathione derivative gamma -L-glutamyl-L-cys teinylglycine amide, or glutathione amide, was reported to be present in fa cultative aerobic as well as in strictly anaerobic species, The gene (garB) encoding the central enzyme in glutathione amide cycling, glutathione amid e reductase (GAR), has been isolated from Chromatium gracile, and its genom ic organization has been examined. The garB gene is immediately preceded by an open reading frame encoding a novel 27.5-kDa chimeric enzyme composed o f one N-terminal peroxiredoxin-like domain followed by a glutaredoxin-like C terminus. The 27.5-kDa enzyme was established in vitro to be a glutathion e amide-dependent peroxidase, being the first example of a prokaryotic low molecular mass thiol-dependent peroxidase, Amino acid sequence alignment of GAR with the functionally homologous glutathione and trypanothione reducta ses emphasizes the conservation of the catalytically important redox-active disulfide and of regions involved in binding the FAD prosthetic group and the substrates glutathione amide disulfide and NADH, By establishing Michae lis constants of 97 and 13.2 muM for glutathione amide disulfide and NADH, respectively (in contrast to K-m values of 6.9 mM for glutathione disulfide and 1.98 mM for NADPH), the exclusive substrate specificities of GAR have been documented. Specificity for the amidated disulfide cofactor partly can be explained by the substitution of Arg-37, shown by x-ray crystallographi c data of the human glutathione reductase to hydrogen-bond one of the gluta thione glycyl carboxylates, by the negatively charged Glu-21, On the other hand, the preference for the unusual electron donor, to some extent, has to rely on the substitution of the basic residues Arg-218, His-219, and Arg-2 24, which have been shown to interact in the human enzyme with the NADPH 2' -phosphate group, by Leu-197, Glu-198, and Phe-203, We suggest GAR to be th e newest member of the class I flavoprotein disulfide reductase family of o xidoreductases.