A trinuclear Fe–Fe–Ni complex formed by ligand reshuffling

The title complex, dicarbonyl-3κ2 C-(μ3-3,6-dimethyl-3,6-diaza­octane-1,8-dithiol­ato-1:2:3κ7 S:S,N,N′,S′:S,S′)(μ2-3,6-di­methyl-3,6-diaza­octane-1,8-dithiol­ato-1:2κ5 S,N,N′,S′:S)-1,2-diiron(II)-3-nickel(0) [Fe2Ni(C8H18N2S2)2(CO)2], is the second example showing M(μ-SR)2Ni0(CO)2 coordination (M = any metal atom). Both FeII ions are five-coordinated in distorted trigonal–bipyramidal geometries by two N atoms and three S atoms. The Ni atom is four-coordinated in a distorted tetra­hedral geometry by two S atoms and two carbonyl ligands. One of the 3,6-dimethyl-3,6-diaza­octane-1,8-dithiol­ate ligands is disordered, the major component having a refined occupancy of 0.873 (2). The Fe⋯Fe distance is 3.0945 (3)Å and the Ni⋯Fe distance is 2.8505 (3) Å

The crystal structure of Fe₄S₄ quinolinate synthase unravels an enzymatic dehydration mechanism that uses tyrosine and a hydrolase-type triad.

International audienceQuinolinate synthase (NadA) is a Fe4S4 cluster-containing dehydrating enzyme involved in the synthesis of quinolinic acid (QA), the universal precursor of the essential nicotinamide adenine dinucleotide (NAD) coenzyme. A previously determined apo NadA crystal structure revealed the binding of one substrate analog, providing partial mechanistic information. Here, we report on the holo X-ray structure of NadA. The presence of the Fe4S4 cluster generates an internal tunnel and a cavity in which we have docked the last precursor to be dehydrated to form QA. We find that the only suitably placed residue to initiate this process is the conserved Tyr21. Furthermore, Tyr21 is close to a conserved Thr-His-Glu triad reminiscent of those found in proteases and other hydrolases. Our mutagenesis data show that all of these residues are essential for activity and strongly suggest that Tyr21 deprotonation, to form the reactive nucleophilic phenoxide anion, is mediated by the triad. NadA displays a dehydration mechanism significantly different from the one found in archetypical dehydratases such as aconitase, which use a serine residue deprotonated by an oxyanion hole. The X-ray structure of NadA will help us unveil its catalytic mechanism, the last step in the understanding of NAD biosynthesis

Crystal Structure of the Transcription Regulator RsrR Reveals a [2Fe-2S] Cluster Coordinated by Cys, Glu and His Residues

The recently discovered Rrf2 family transcriptional regulator RsrR coordinates a [2Fe-2S] cluster. Remarkably, binding of the protein to RsrR-regulated promoter DNA sequences is switched on and off through the facile cycling of the [2Fe-2S] cluster be-tween +2 and +1 states. Here, we report high resolution crystal structures of the RsrR dimer, revealing that the [2Fe-2S] cluster is asymmetrically coordinated across the RsrR monomer-monomer interface by two Cys residues from one subunit and His and Glu residues from the other. To our knowledge, this is the first example of a protein bound [Fe-S] cluster with three different amino acid side chains as ligands, and of Glu acting as ligand to a [2Fe-2S] cluster. Analyses of RsrR structures revealed a conformation-al change, centered on Trp9, which results in a significant shift in the DNA-binding helix-turn-helix region

Hydrogen-activation mechanism of [Fe] hydrogenase revealed by multi-scale modeling

When investigating the mode of hydrogen activation by [Fe] hydrogenases, not only the chemical reactivity at the active site is of importance but also the large-scale conformational change between the so-called open and closed conformations, which leads to a special spatial arrangement of substrate and iron cofactor. To study H2 activation, a complete model of the solvated and cofactor-bound enzyme in complex with the substrate methenyl-H4MPT+ was constructed. Both the closed and open conformations were simulated with classical molecular dynamics on the 100 ns time scale. Quantum-mechanics/molecular-mechanics calculations on snapshots then revealed the features of the active site that enable the facile H2 cleavage. The hydroxyl group of the pyridinol ligand can easily be deprotonated. With the deprotonated hydroxyl group and the structural arrangement in the closed conformation, H2 coordinated to the Fe center is subject to an ionic and orbital push-pull effect and can be rapidly cleaved with a concerted hydride transfer to methenyl-H4MPT+. An intermediary hydride species is not formed

Solar Water Splitting with a Hydrogenase Integrated in Photoelectrochemical Tandem Cells

Hydrogenases (H2ases) are benchmark electrocatalysts for H2 production, both in biology and (photo)catalysis in vitro. We report the tailoring of a p-type Si photocathode for optimal loading and wiring of H2ase through the introduction of a hierarchical inverse opal (IO) TiO2 interlayer. This proton-reducing Si j IO-TiO2 j H2ase photocathode is capable of driving overall water splitting in combination with a photoanode. We demonstrate unassisted (bias-free) water splitting by wiring Si j IO-TiO2 j H2ase to a modified BiVO4 photoanode in a photoelectrochemical (PEC) cell during several hours of irradiation. Connecting the Si j IO-TiO2 j H2ase to a photosystem II (PSII) photoanode provides proof of concept for an engineered Z-scheme that replaces the non-complementary, natural light absorber photosystem I with a complementary abiotic silicon photocathode