title: Electronic structure of the unique [4Fe-3S] cluster in O2-tolerant hydrogenases characterized by 57Fe Mossbauer and EPR spectroscopy.
abstract: Iron-sulfur clusters are ubiquitous electron transfer cofactors in hydrogenases. Their types and redox properties are important for H(2) catalysis, but, recently, their role in a protection mechanism against oxidative inactivation has also been recognized for a [4Fe-3S] cluster in O(2)-tolerant group 1 [NiFe] hydrogenases. This cluster, which is uniquely coordinated by six cysteines, is situated in the proximity of the catalytic [NiFe] site and exhibits unusual redox versatility. The [4Fe-3S] cluster in hydrogenase (Hase) I from Aquifex aeolicus performs two redox transitions within a very small potential range, forming a superoxidized state above +200 mV vs. standard hydrogen electrode (SHE). Crystallographic data has revealed that this state is stabilized by the coordination of one of the iron atoms to a backbone nitrogen. Thus, the proximal [4Fe-3S] cluster undergoes redox-dependent changes to serve multiple purposes beyond classical electron transfer. In this paper, we present field-dependent (57)Fe-Mössbauer and EPR data for Hase I, which, in conjunction with spectroscopically calibrated density functional theory (DFT) calculations, reveal the distribution of Fe valences and spin-coupling schemes for the iron-sulfur clusters. The data demonstrate that the electronic structure of the [4Fe-3S] core in its three oxidation states closely resembles that of corresponding conventional [4Fe-4S] cubanes, albeit with distinct differences for some individual iron sites. The medial and distal iron-sulfur clusters have similar electronic properties as the corresponding cofactors in standard hydrogenases, although their redox potentials are higher.
authors.affiliation: Max-Planck Institut für Chemische Energiekonversion, D-45470 Mülheim an der Ruhr, Germany. mxp65@psu.edu
authors.forename: Maria-Eirini
authors.initials: ME
authors.lastname: Pandelia
authors.forename: Dmytro
authors.initials: D
authors.lastname: Bykov
authors.forename: Robert
authors.initials: R
authors.lastname: Izsak
authors.forename: Pascale
authors.initials: P
authors.lastname: Infossi
authors.forename: Marie-Thérèse
authors.initials: MT
authors.lastname: Giudici-Orticoni
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Frank
authors.initials: F
authors.lastname: Neese
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lubitz
mesh_terms: D000595:Amino Acid Sequence; D001419:Bacteria / Q000201:enzymology*; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D006864:Hydrogenase / Q000737:chemistry* / Q000235:genetics; D007501:Iron / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010084:Oxidation-Reduction; D016415:Sequence Alignment; D013056:Spectrophotometry, Ultraviolet; D015204:Spectroscopy, Mossbauer / Q000379:methods*; D013455:Sulfur / Q000737:chemistry*
chemical_list: D013455:Sulfur; D007501:Iron; C043296:nickel-iron hydrogenase; D006864:Hydrogenase