title: A paramagnetic species with unique EPR characteristics in the active site of heterodisulfide reductase from methanogenic archaea.
abstract: Heterodisulfide reductase (Hdr) from methanogenic archaea is an iron-sulfur protein that catalyses the reversible reduction of the heterodisulfide (CoM-S-S-CoB) of the methanogenic thiol coenzymes, coenzyme M (H-S-CoM) and coenzyme B (H-S-CoB). In EPR spectroscopic studies with the enzyme from Methanothermobacter marburgensis, we have identified a unique paramagnetic species that is formed upon reaction of the oxidized enzyme with H-S-CoM in the absence of H-S-CoB. This paramagnetic species can be reduced in a one-electron step with a midpoint-potential of -185 mV but not further oxidized. A broadening of the EPR signal in the 57Fe-enriched enzyme indicates that it is at least partially iron based. The g values (gxyz = 2.013, 1.991 and 1.938) and the midpoint potential argue against a conventional [2Fe-2S]+, [3Fe-4S]+, [4Fe-4S]+ or [4Fe-4S]3+ cluster. This species reacts with H-S-CoB to form an EPR silent form. Hence, we propose that only a half reaction is catalysed in the presence of H-S-CoM and that a reaction intermediate is trapped. This reaction intermediate is thought to be a [4Fe-4S]3+ cluster that is coordinated by one of the cysteines of a nearby active-site disulfide or by the sulfur of H-S-CoM. A paramagnetic species with similar EPR properties was also identified in Hdr from Methanosarcina barkeri.
authors.affiliation: Max-Planck-Institut für terrestrische Mikrobiologie, Marburg, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Madadi-Kahkesh
authors.forename: E C
authors.initials: EC
authors.lastname: Duin
authors.forename: S
authors.initials: S
authors.lastname: Heim
authors.forename: S P
authors.initials: SP
authors.lastname: Albracht
authors.forename: M K
authors.initials: MK
authors.lastname: Johnson
authors.forename: R
authors.initials: R
authors.lastname: Hedderich
mesh_terms: D000477:Alkylating Agents / Q000494:pharmacology; D000595:Amino Acid Sequence; D001483:Base Sequence; D020134:Catalytic Domain; D017931:DNA Primers / Q000235:genetics; D004578:Electron Spin Resonance Spectroscopy; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019605:Euryarchaeota / Q000201:enzymology* / Q000235:genetics; D017014:Methanobacterium / Q000201:enzymology / Q000235:genetics; D017021:Methanosarcina barkeri / Q000201:enzymology / Q000235:genetics; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010769:Phosphothreonine / Q000031:analogs & derivatives* / Q000737:chemistry; D021122:Protein Subunits; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics
chemical_list: D000477:Alkylating Agents; D017931:DNA Primers; D004791:Enzyme Inhibitors; D021122:Protein Subunits; D011994:Recombinant Proteins; C049349:7-mercaptoheptanoylthreonine phosphate; D010769:Phosphothreonine; D010088:Oxidoreductases; C066352:coenzyme M-7-merceptoheptanoylthreonine-phosphate-heterodisulfide hydrogenase; C087171:heterodisulfide reductase