Hydrogenases are the most active molecular catalysts for hydrogen production and uptake, and could therefore facilitate the development of new types of fuel cell. In FeFe-hydrogenases, catalysis ...takes place at a unique di-iron centre (the 2Fe subsite), which contains a bridging dithiolate ligand, three CO ligands and two CN(-) ligands. Through a complex multienzymatic biosynthetic process, this 2Fe subsite is first assembled on a maturation enzyme, HydF, and then delivered to the apo-hydrogenase for activation. Synthetic chemistry has been used to prepare remarkably similar mimics of that subsite, but it has failed to reproduce the natural enzymatic activities thus far. Here we show that three synthetic mimics (containing different bridging dithiolate ligands) can be loaded onto bacterial Thermotoga maritima HydF and then transferred to apo-HydA1, one of the hydrogenases of Chlamydomonas reinhardtii algae. Full activation of HydA1 was achieved only when using the HydF hybrid protein containing the mimic with an azadithiolate bridge, confirming the presence of this ligand in the active site of native FeFe-hydrogenases. This is an example of controlled metalloenzyme activation using the combination of a specific protein scaffold and active-site synthetic analogues. This simple methodology provides both new mechanistic and structural insight into hydrogenase maturation and a unique tool for producing recombinant wild-type and variant FeFe-hydrogenases, with no requirement for the complete maturation machinery.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cobalt (Co) is present in trace amounts in the environment but it can be toxic when it accumulates in cells. The question of how Co produces its toxic effects and how living organisms protect ...themselves from, and resist to, such a stress remains to be clarified. Studies pertaining to these issues were recently carried out in Escherichia coli and Salmonella enterica. Iron-sulfur proteins were identified as primary targets of Co ions. Perturbation of iron homeostasis, oxidative stress and possible effects on sulfur assimilation were noticed. Cells were found to respond by up-regulating genes involved in the biosynthesis of Fe-S clusters as well as genes involved in Co efflux. These data are summarized in this review article to provide a preliminary general view of Co toxicity mechanisms in these two bacterial models.
Hydrogen is a promising energy vector for storing renewable energies: obtained from water-splitting, in electrolysers or photoelectrochemical cells, it can be turned back to electricity on demand in ...fuel cells (FCs). Proton exchange membrane (PEM) devices with low internal resistance, high compactness and stability are an attractive technology optimized over decades, affording fast start-up times and low operating temperatures. However, they rely on the powerful catalytic properties of noble metals such as platinum, while lower cost, more abundant materials would be needed for economic viability. Replacing these noble metals at both electrodes has long proven to be a difficult task, so far incompatible with PEM technologies. Here we take advantage of newly developed bio-inspired molecular H
oxidation catalysts and noble metal-free O
-reducing materials, to fabricate a noble metal-free PEMFC, with an 0.74 V open circuit voltage and a 23 μW cm
output power under technologically relevant conditions. X-ray absorption spectroscopy measurements confirm that the catalysts are stable and retain their structure during turnover.
A series of Ru-Co dinuclear complexes have been synthesized and assayed as photocatalysts for the reduction of CO2 to CO in organic solvents. The Ru-Co complexes, with tris-diimine coordination at ...the Co ion, are the best supramolecular photocatalysts for CO2 reduction with a non-noble metal catalytic center reported so far.
Large‐scale implementation of electrochemical CO2 conversion to value‐added products is currently hampered by high electrolyzer cell voltages, resulting in low energy efficiency and high operating ...costs. Cell voltages are typically well above 3 V and need to be significantly lowered whilst maintaining current densities greater than 200 mA cm−2 to enable energy‐efficient CO2 electroreduction. This can be addressed through modification of the resistive components of the device to reduce energy consumption and lower operating costs. Electrodes, electrolyte solutions, and the separator between compartments, provide the largest contributions to the overall cell voltage, therefore decreasing their resistance can lower the electricity input required to drive effective CO2 conversion. Here, by careful analysis and tuning of the various sources of voltage drops within the cell, an optimized diaphragm‐based CO2R device is presented, which is able to operate at an industrially relevant current density of 200 mA cm−2 with an |Ecell| as low as 2.89 V, amongst the lowest reported values to date.
Through systematic alteration of key components of acidic CO2 electrolyzers, we were able to demonstrate production of multicarbon products at high current density and low cell voltage. Using a diaphragm in place of a membrane, altering the electrolyte composition and temperature, mixing the catholyte and anolyte, and reducing the distance between the electrodes, resulted in a stable, low‐voltage device.
The last biosynthetic step for 2-methylthio- N 6 -isopentenyl-adenosine (ms 2 i 6 A), present at position 37 in some tRNAs, consists of the methylthiolation of the isopentenyl-adenosine (i 6 A) ...precursor. In this work we have reconstituted in vitro the conversion of i 6 A to ms 2 i 6 A within a tRNA substrate using the iron-sulfur MiaB protein, S -adenosylmethionine (AdoMet), and a reducing agent. We show that a synthetic i 6 A-containing RNA corresponding to the anticodon stem loop of tRNA Phe is also a substrate. This study demonstrates that MiaB protein is a bifunctional system, involved in both thiolation and
methylation of i 6 A. In this process, one molecule of AdoMet is converted to 5â²-deoxyadenosine, probably through reductive cleavage and intermediate
formation ofa5â²-deoxyadenosyl radical as observed in other âRadical-AdoMetâ enzymes, and a second molecule of AdoMet is used
as a methyl donor as shown by labeling experiments. The origin of the sulfur atom is discussed.
FeFe Hydrogenases catalyze the reversible conversion of H
2
into electrons and protons. Their catalytic site, the H-cluster, contains a generic 4Fe–4S
H
cluster coupled to a 2Fe
H
subsite Fe
2
...(ADT)(CO)
3
(CN)
2
2−
, ADT = µ(SCH
2
)
2
NH. Heterologously expressed FeFe hydrogenases (apo-hydrogenase) lack the 2Fe
H
unit, but this can be incorporated through artificial maturation with a synthetic precursor Fe
2
(ADT)(CO)
4
(CN)
2
2−
. Maturation with a 2Fe complex in which the essential ADT amine moiety has been replaced by CH
2
(PDT = propane-dithiolate) results in a low activity enzyme with structural and spectroscopic properties similar to those of the native enzyme, but with simplified redox behavior. Here, we study the effect of sulfur-to-selenium (S-to-Se) substitution in the bridging PDT ligand incorporated in the FeFe hydrogenase HydA1 from
Chlamydomonas reinhardtii
using magnetic resonance (EPR, NMR), FTIR and spectroelectrochemistry. The resulting HydA1-PDSe enzyme shows the same redox behavior as the parent HydA1-PDT. In addition, a state is observed in which extraneous CO is bound to the open coordination site of the 2Fe
H
unit. This state was previously observed only in the native enzyme HydA1-ADT and not in HydA1-PDT. The spectroscopic features and redox behavior of HydA1-PDSe, resulting from maturation with Fe
2
(PDSe)(CO)
4
(CN)
2
2−
, are discussed in terms of spin and charge density shifts and provide interesting insight into the electronic structure of the H-cluster. We also studied the effect of S-to-Se substitution in the 4Fe–4S subcluster. The reduced form of HydA1 containing only the 4Fe–4Se
H
cluster shows a characteristic S = 7/2 spin state which converts back into the S = 1/2 spin state upon maturation with a 2Fe–PDT/ADT complex.
Protein-bound iron-sulfur (Fe-S) clusters, which are polynuclear combinations of iron and sulfur atoms, are among the most structurally and functionally versatile cofactors in biology. These clusters ...are ideal for electron-transfer and redox reactions because they can access various redox states, and particularly because the potentials of the corresponding redox couples can be finely tuned by the coordination environment, by the electronic properties of the protein site where they are anchored, and by hydrogen bonding.
No abstract available
and available online here Cet article fait partie du dossier thématique ci-dessous publié dans la revue OGST, Vol. 70, n°5, pp. 791-902 et téléchargeable ici
Ribonucleotide reductases (RNRs) catalyse the reduction of ribonucleotides to deoxyribonucleotides. They play a pivotal role in the regulation of DNA synthesis and are targets for antiproliferative ...drugs. Ribonucleotide reductases are unique enzymes in that they all require a protein radical for activity. Class I nonheme iron RNRs (mammals, plants, Escherichia coli) use a tyrosyl/cysteinyl radical pair, class II adenosylcobalamin RNRs (prokaryotes, archaea) a cysteinyl radical, class III iron-sulphur RNRs (facultative anaerobes) a glycyl radical. Here we describe the reactivity of these radicals with respect to the natural ribonucleotide substrates as well as to a variety of enzyme inhibitors, radical scavengers, nitric oxide, superoxide radicals and substrate analogues.
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