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The iron-sulfur cluster-free hydrogenase (Hmd) from methanogenic archaea harbors an iron-containing cofactor of yet unknown structure. X-ray absorption spectroscopy of the active, as isolated enzyme from Methanothermobacter marburgensis (mHmd) and of the active, reconstituted enzyme from Methanocaldococcus jannaschii (jHmd) revealed the presence of mononuclear iron with two CO, one sulfur and one or two N/O in coordination distance. In jHmd, the single sulfur ligand is most probably provided by Cys 176 , as deduced from a comparison of the activity and of the x-ray absorption and Mössbauer spectra of the enzyme mutated in any of the three conserved cysteines. In the isolated Hmd cofactor, two CO, one sulfur, and two nitrogen/oxygen atoms coordinate the iron, the sulfur ligand being most probably provided by mercaptoethanol, which is absolutely required for the extraction of the iron-containing cofactor from the holoenzyme and for the stabilization of the extracted cofactor. In active mHmd holoenzyme, the number of iron ligands increased by one when one of the Hmd inhibitors (CO or KCN) were present, indicating that in active Hmd, the iron contains an open coordination site, which is proposed to be the site of H 2 interaction.Hydrogenases are enzymes that catalyze the reversible oxidation of molecular hydrogen (1). Their structure and catalytic mechanism are of considerable applied interest as models for the development of efficient catalysts for hydrogen-fueled processes. Despite intensive efforts, however, the understanding of how hydrogenases react with H 2 is only in its infancy.Two of the three known types of hydrogenases are ironsulfur proteins that contain, besides one or several iron-sulfur clusters, a dinuclear metal center, either [NiFe] or [FeFe], which was shown to be the site of H 2 reaction. Both dinuclear hydrogenases catalyze the reversible formation of 2e Ϫ and 2Hϩ from H 2 , the electrons being transferred one by one to electron acceptors via the iron-sulfur clusters. One iron ion in both dinuclear centers is coordinated by sulfur, CO, and cyanide ligands, as supported by crystal structures of the [NiFe]-hydrogenase (2, 3) and of the [FeFe]-hydrogenase (4 -7). During the catalytic cycle, the iron ion in the [NiFe] center remains in a low spin ferrous state and is not redoxactive (8, 9). In the case of the [FeFe]-hydrogenases, the oxidation states of the dinuclear iron center are still under discussion; however, the involvement of a low spin Fe(II) has been predicted based on the spectroscopic and DFT studies (9 -11). Despite similarities in structures and properties, [FeFe]-and [NiFe]-hydrogenases are phylogenetically not related.[FeFe]-hydrogenases are found in bacteria and eucarya, whereas [NiFe]-hydrogenases are found in bacteria and archaea. For recent reviews on these two types of hydrogenases, see Frey (9), Zhou et al.

The oncoprotein E7 of human papilloma viruses (HPV) is involved in the pathogenesis and maintenance of human cervical cancers. The most prevalent HPV types found in cervix carcinomas are HPV16, 18 and 45. The structure of the E7 dimer from HPV45 (PDB 2F8B) was determined by nuclear magnetic resonance spectroscopy. Each monomer comprises an unfolded N-terminus and a well-structured C-terminal domain with a b1b2a1b3a2 topology representing a unique zinc-binding fold found only for E7. Dimerization occurs through the a1/a1 0 helices and intermolecular b-sheet formation but excludes the zinc-binding sites. E7 is reported to interact with a number of cellular proteins (e.g. pRb, p21 CIP1 ). Binding of a peptide derived from the C-terminus of p21CIP1 to the Cterminal domain of E7 was characterized by monitoring chemical shift perturbations of the amide groups of E7. This provides direct evidence that a shallow groove situated between a1 and b1 of the E7 C-terminal domain is interacting with the C-terminus of p21 CIP1. Intriguingly, this binding site overlaps with the low-affinity binding site on E7 for the C-domain of pRb.

Methylmercury is among the most potentially toxic species to which human populations are exposed, both at high levels through poisonings and at lower levels through consumption of fish and other seafood. However, the molecular mechanisms of methylmercury toxicity in humans remain poorly understood. We used synchrotron X-ray absorption spectroscopy (XAS) to study mercury chemical forms in human brain tissue. Individuals poisoned with high levels of methylmercury species showed elevated cortical selenium with significant proportions of nanoparticulate mercuric selenide plus some inorganic mercury and methylmercury bound to organic sulfur. Individuals with a lifetime of high fish consumption showed much lower levels of mercuric selenide and methylmercury cysteineate. Mercury exposure did not perturb organic selenium levels. These results elucidate a key detoxification pathway in the central nervous system and provide new insights into the appropriate methods for biological monitoring.