title: Catalytic mechanism of Sep-tRNA:Cys-tRNA synthase: sulfur transfer is mediated by disulfide and persulfide.
abstract: Sep-tRNA:Cys-tRNA synthase (SepCysS) catalyzes the sulfhydrylation of tRNA-bound O-phosphoserine (Sep) to form cysteinyl-tRNA(Cys) (Cys-tRNA(Cys)) in methanogens that lack the canonical cysteinyl-tRNA synthetase (CysRS). A crystal structure of the Archaeoglobus fulgidus SepCysS apoenzyme provides information on the binding of the pyridoxal phosphate cofactor as well as on amino acid residues that may be involved in substrate binding. However, the mechanism of sulfur transfer to form cysteine was not known. Using an in vivo Escherichia coli complementation assay, we showed that all three highly conserved Cys residues in SepCysS (Cys(64), Cys(67), and Cys(272) in the Methanocaldococcus jannaschii enzyme) are essential for the sulfhydrylation reaction in vivo. Biochemical and mass spectrometric analysis demonstrated that Cys(64) and Cys(67) form a disulfide linkage and carry a sulfane sulfur in a portion of the enzyme. These results suggest that a persulfide group (containing a sulfane sulfur) is the proximal sulfur donor for cysteine biosynthesis. The presence of Cys(272) increased the amount of sulfane sulfur in SepCysS by 3-fold, suggesting that this Cys residue facilitates the generation of the persulfide group. Based upon these findings, we propose for SepCysS a sulfur relay mechanism that recruits both disulfide and persulfide intermediates.
authors.affiliation: Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520, USA.
authors.forename: Yuchen
authors.initials: Y
authors.lastname: Liu
authors.forename: Patricia C
authors.initials: PC
authors.lastname: Dos Santos
authors.forename: Xiang
authors.initials: X
authors.lastname: Zhu
authors.forename: Ron
authors.initials: R
authors.lastname: Orlando
authors.forename: Dennis R
authors.initials: DR
authors.lastname: Dean
authors.forename: Dieter
authors.initials: D
authors.lastname: Söll
authors.forename: Jing
authors.initials: J
authors.lastname: Yuan
mesh_terms: D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry / Q000378:metabolism*; D019611:Archaeoglobus fulgidus / Q000201:enzymology; D055162:Biocatalysis; D017124:Conserved Sequence; D003545:Cysteine / Q000378:metabolism*; D004220:Disulfides / Q000378:metabolism*; D013058:Mass Spectrometry; D017015:Methanococcales / Q000201:enzymology; D008969:Molecular Sequence Data; D010768:Phosphoserine / Q000378:metabolism*; D013440:Sulfides / Q000378:metabolism*; D013455:Sulfur / Q000378:metabolism*
chemical_list: D004220:Disulfides; D013440:Sulfides; C051552:persulfides; D010768:Phosphoserine; D013455:Sulfur; D000604:Amino Acyl-tRNA Synthetases; D003545:Cysteine