title: Assignment of the [4Fe-4S] clusters of Ech hydrogenase from Methanosarcina barkeri to individual subunits via the characterization of site-directed mutants.
abstract: Ech hydrogenase from Methanosarcina barkeri is a member of a distinct group of membrane-bound [NiFe] hydrogenases with sequence similarity to energy-conserving NADH:quinone oxidoreductase (complex I). The sequence of the enzyme predicts the binding of three [4Fe-4S] clusters, one by subunit EchC and two by subunit EchF. Previous studies had shown that two of these clusters could be fully reduced under 10(5) Pa of H2 at pH 7 giving rise to two distinct S1/2 electron paramagnetic resonance (EPR) signals, designated as the g = 1.89 and the g = 1.92 signal. Redox titrations at different pH values demonstrated that these two clusters had a pH-dependent midpoint potential indicating a function in ion pumping. To assign these signals to the subunits of the enzyme a set of M. barkeri mutants was generated in which seven of eight conserved cysteine residues in EchF were individually replaced by serine. EPR spectra recorded from the isolated mutant enzymes revealed a strong reduction or complete loss of the g = 1.92 signal whereas the g = 1.89 signal was still detectable as the major EPR signal in five mutant enzymes. It is concluded that the cluster giving rise to the g = 1.89 signal is the proximal cluster located in EchC and that the g = 1.92 signal results from one of the clusters of subunit EchF. The pH-dependence of these two [4Fe-4S] clusters suggests that they simultaneously mediate electron and proton transfer and thus could be an essential part of the proton-translocating machinery.
authors.affiliation: Max-Planck-Institute for terrestrial Microbiology, Marburg, Germany.
authors.forename: Lucia
authors.initials: L
authors.lastname: Forzi
authors.forename: Jürgen
authors.initials: J
authors.lastname: Koch
authors.forename: Adam M
authors.initials: AM
authors.lastname: Guss
authors.forename: Carl G
authors.initials: CG
authors.lastname: Radosevich
authors.forename: William W
authors.initials: WW
authors.lastname: Metcalf
authors.forename: Reiner
authors.initials: R
authors.lastname: Hedderich
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000235:genetics; D017021:Methanosarcina barkeri / Q000201:enzymology*; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000235:genetics; D021122:Protein Subunits / Q000737:chemistry; D017494:Proton Pumps
chemical_list: D007506:Iron-Sulfur Proteins; D021122:Protein Subunits; D017494:Proton Pumps; D010088:Oxidoreductases; C400203:Ech hydrogenase