title: A cysteine-rich CCG domain contains a novel [4Fe-4S] cluster binding motif as deduced from studies with subunit B of heterodisulfide reductase from Methanothermobacter marburgensis.
abstract: Heterodisulfide reductase (HDR) of methanogenic archaea with its active-site [4Fe-4S] cluster catalyzes the reversible reduction of the heterodisulfide (CoM-S-S-CoB) of the methanogenic coenzyme M (CoM-SH) and coenzyme B (CoB-SH). CoM-HDR, a mechanistic-based paramagnetic intermediate generated upon half-reaction of the oxidized enzyme with CoM-SH, is a novel type of [4Fe-4S]3+ cluster with CoM-SH as a ligand. Subunit HdrB of the Methanothermobacter marburgensis HdrABC holoenzyme contains two cysteine-rich sequence motifs (CX31-39CCX35-36CXXC), designated as CCG domain in the Pfam database and conserved in many proteins. Here we present experimental evidence that the C-terminal CCG domain of HdrB binds this unusual [4Fe-4S] cluster. HdrB was produced in Escherichia coli, and an iron-sulfur cluster was subsequently inserted by in vitro reconstitution. In the oxidized state the cluster without the substrate exhibited a rhombic EPR signal (gzyx = 2.015, 1.995, and 1.950) reminiscent of the CoM-HDR signal. 57Fe ENDOR spectroscopy revealed that this paramagnetic species is a [4Fe-4S] cluster with 57Fe hyperfine couplings very similar to that of CoM-HDR. CoM-33SH resulted in a broadening of the EPR signal, and upon addition of CoM-SH the midpoint potential of the cluster was shifted to values observed for CoM-HDR, both indicating binding of CoM-SH to the cluster. Site-directed mutagenesis of all 12 cysteine residues in HdrB identified four cysteines of the C-terminal CCG domain as cluster ligands. Combined with the previous detection of CoM-HDR-like EPR signals in other CCG domain-containing proteins our data indicate a general role of the C-terminal CCG domain in coordination of this novel [4Fe-4S] cluster. In addition, Zn K-edge X-ray absorption spectroscopy identified an isolated Zn site with an S3(O/N)1 geometry in HdrB and the HDR holoenzyme. The N-terminal CCG domain is suggested to provide ligands to the Zn site.
authors.affiliation: Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch Strasse, D-35043 Marburg, Germany.
authors.forename: Nils
authors.initials: N
authors.lastname: Hamann
authors.forename: Gerd J
authors.initials: GJ
authors.lastname: Mander
authors.forename: Jacob E
authors.initials: JE
authors.lastname: Shokes
authors.forename: Robert A
authors.initials: RA
authors.lastname: Scott
authors.forename: Marina
authors.initials: M
authors.lastname: Bennati
authors.forename: Reiner
authors.initials: R
authors.lastname: Hedderich
mesh_terms: D020816:Amino Acid Motifs; D001665:Binding Sites; D003545:Cysteine / Q000737:chemistry; D004926:Escherichia coli; D007501:Iron / Q000378:metabolism*; D017013:Methanobacteriaceae / Q000201:enzymology* / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry; D013057:Spectrum Analysis; D013455:Sulfur / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism
chemical_list: D050505:Mutant Proteins; D021122:Protein Subunits; D013455:Sulfur; D007501:Iron; D010088:Oxidoreductases; C087171:heterodisulfide reductase; D015032:Zinc; D003545:Cysteine