title: The active site of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans. I. Light sensitivity and magnetic hyperfine interactions as observed by electron paramagnetic resonance.
abstract: The hydrogen-activating cluster (H cluster) in [FeFe]-hydrogenases consists of two moieties. The [2Fe]H subcluster is a (L)(CO)(CN)Fe(mu-RS2)(mu-CO)Fe(CysS)(CO)(CN) centre. The Cys-bound Fe is called Fe1, the other iron Fe2. The Cys-thiol forms a bridge to a [4Fe-4S] cluster, the [4Fe-4S]H subcluster. We report that electron paramagnetic resonance (EPR) spectra of the 57Fe-enriched enzyme from Desulfovibrio desulfuricans in the H(ox)-CO state are consistent with a magnetic hyperfine interaction of the unpaired spin with all six Fe atoms of the H cluster. In contrast to the inactive aerobic enzyme, the active enzyme is easily destroyed by light. The [2Fe]H subcluster in some enzyme molecules loses CO by photolysis, whereupon other molecules firmly bind the released CO to form the H(ox)-CO state giving rise to the so-called axial 2.06 EPR signal. Though not destroyed by light, the H(ox)-CO state is affected by it. As demonstrated in the accompanying paper [49] two of the intrinsic COs, both bound to Fe2, can be exchanged by extrinsic 13CO during illumination at 2 degrees C. We found that only one of the three 13COs, the one at the extrinsic position, gives an EPR-detectable isotropic superhyperfine interaction of 0.6 mT. At 30 K both the inhibiting extrinsic CO bound to Fe2 and one more CO can be photolysed. EPR spectra of the photolysed products are consistent with a 3d7 system of Fe with the formal oxidation state +1. The damaged enzyme shows a light-sensitive g = 5 signal which is ascribed to an S = 3/2 form of the [2Fe](H) subcluster. The light sensitivity of the enzyme explains the occurrence of the g = 5 signal and the axial 2.06 signal in published EPR spectra of nearly all preparations studied thus far.
authors.affiliation: Swammerdam Institute for Life Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands, asiem@science.uva.nl
authors.forename: Simon P J
authors.initials: SP
authors.lastname: Albracht
authors.forename: Winfried
authors.initials: W
authors.lastname: Roseboom
authors.forename: E Claude
authors.initials: EC
authors.lastname: Hatchikian
mesh_terms: D001665:Binding Sites; D002248:Carbon Monoxide / Q000737:chemistry; D003545:Cysteine / Q000737:chemistry; D045285:Desulfovibrio desulfuricans / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D008027:Light; D008280:Magnetics; D010782:Photolysis; D020795:Photophobia; D013438:Sulfhydryl Compounds / Q000737:chemistry
chemical_list: D007506:Iron-Sulfur Proteins; D013438:Sulfhydryl Compounds; D002248:Carbon Monoxide; D007501:Iron; C118830:iron hydrogenase; D006864:Hydrogenase; D003545:Cysteine