Glutaredoxins (GRXs) are thiol-disulfide oxidoreductases abundant in prokaryotes, although little is understood of these enzymes from the domain Archaea. The numerous characterized GRXs from the domain Bacteria utilize a diversity of low-molecular-weight thiols in addition to glutathione as reductants. We report here the biochemical and structural properties of a GRX-like protein named methanoredoxin (MRX) from Methanosarcina acetivorans of the domain Archaea. MRX utilizes coenzyme M (CoMSH) as reductant for insulin disulfide reductase activity, which adds to the diversity of thiol protectants in prokaryotes. Cell-free extracts of M. acetivorans displayed CoMS-SCoM reductase activity that complements the CoMSH-dependent activity of MRX. The crystal structure exhibits a classic thioredoxin-glutaredoxin fold comprising three α-helices surrounding four antiparallel β-sheets. A pocket on the surface contains a CVWC motif, identifying the active site with architecture similar to GRXs. Although it is a monomer in solution, the crystal lattice has four monomers in a dimer of dimers arrangement. A cadmium ion is found within the active site of each monomer. Two such ions stabilize the N-terminal tails and dimer interfaces. Our modeling studies indicate that CoMSH and glutathione (GSH) bind to the active site of MRX similar to the binding of GSH in GRXs, although there are differences in the amino acid composition of the binding motifs. The results, combined with our bioinformatic analyses, show that MRX represents a class of GRX-like enzymes present in a diversity of methane-producing Archaea.
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