The roles of chalcogenides in O2 protection of H2ase active sites

Chloride- and Hydrosulfide-Bound 2Fe Complexes as Models of the Oxygen-Stable State of [FeFe] Hydrogenase

[FeFe] hydrogenases demonstrate remarkable catalytic efficiency in hydrogen evolution and oxidation processes. However, susceptibility of these enzymes to oxygen-induced degradation impedes their practical deployment in hydrogen-production devices and fuel cells. Recent investigations into the oxygen-stable (Hinact) state of the H-cluster revealed its inherent capacity to resist oxygen degradation. Herein, we present findings on Cl- and SH-bound [2Fe-2S] complexes, bearing relevance to the oxygen-stable state within a biological context. A characteristic attribute of these complexes is the terminal Cl-/SH- ligation to the iron center bearing the CO bridge. Structural analysis of the t-Cl demonstrates a striking resemblance to the Hinact state of DdHydAB and CbA5H. The t-Cl/t-SH exhibit reversible oxidation, with both redox species, electronically, being the first biomimetic analogs to the Htrans and Hinact states. These complexes exhibit notable resistance against oxygen-induced decomposition, supporting the potential oxygen-resistant nature of the Htrans and Hinact states. The swift reductive release of the Cl-/SH-group demonstrates its labile and kinetically controlled binding. The findings garnered from these investigations offer valuable insights into properties of the enzymatic O2-stable state, and key factors governing deactivation and reactivation conversion. This work contributes to the advancement of bio-inspired molecular catalysts and the integration of enzymes and artificial catalysts into H2-evolution devices and fuel-cell applications.

Selenium-More than Just a Fortuitous Sulfur Substitute in Redox Biology

Living organisms use selenium mainly in the form of selenocysteine in the active site of oxidoreductases. Here, selenium's unique chemistry is believed to modulate the reaction mechanism and enhance the catalytic efficiency of specific enzymes in ways not achievable with a sulfur-containing cysteine. However, despite the fact that selenium/sulfur have different physicochemical properties, several selenoproteins have fully functional cysteine-containing homologues and some organisms do not use selenocysteine at all. In this review, selected selenocysteine-containing proteins will be discussed to showcase both situations: (i) selenium as an obligatory element for the protein's physiological function, and (ii) selenium presenting no clear advantage over sulfur (functional proteins with either selenium or sulfur). Selenium's physiological roles in antioxidant defence (to maintain cellular redox status/hinder oxidative stress), hormone metabolism, DNA synthesis, and repair (maintain genetic stability) will be also highlighted, as well as selenium's role in human health. Formate dehydrogenases, hydrogenases, glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases will be herein featured.

Electrocatalytic Behavior of Tetrathiafulvalene (TTF) and Extended Tetrathiafulvalene (exTTF) [FeFe] Hydrogenase Mimics

TTF- and exTTF-containing [(μ-S₂)Fe₂(CO)₆] complexes have been prepared by the photochemical reaction of TTF or exTTF and [(μ-S₂)Fe₂(CO)₆]. These complexes are able to interact with PAHs. In the absence of air and in acid media an electrocatalytic dihydrogen evolution reaction (HER) occurs, similarly to analogous [(μ-S₂)Fe₂(CO)₆] complexes. However, in the presence of air, the TTF and exTTF organic moieties strongly influence the electrochemistry of these systems. The reported data may be valuable in the design of [FeFe] hydrogenase mimics able to combine the HER properties of the [FeFe] cores with the unique TTF properties.

Ethereal Hydroperoxides: Powerful Reagents for S-Oxygenation of Bridging Thiophenolate Functions

S-Oxygenation of thiophenolate bridges by ethereal hydroperoxides was studied. [Ni^II₂L^S(PhCO₂)]⁺ (1), where L^S = macrocyclic aminethiolate supporting ligand, is S-oxygenated readily in a mixed methanol/acetonitrile solution with ether/dioxygen at room temperature in the presence of daylight. The reactions were found to depend strongly on the choice of the ether. Uptake of two O atoms occurs in dioxane to give a mixed thiolate/sulfinate complex [Ni^II₂L^SO₂(PhCO₂)]⁺ (2) containing the rare five-membered Ni(μ_1,1-S)(μ_1,2-OS)Ni core. In tetrahydrofuran, four O atoms are taken up by 1 to generate the bis(sulfinate) species [Ni^II₂L^SO₄(PhCO₂)]⁺ (3). A mono-S-oxygenated sulfenate intermediate can be detected by electrospray ionization mass spectrometry. The oxygenation reactions proceed in high yields without complex disintegration and invariably provide μ_1,2-bridging sulfinates as established by spectroscopy (IR and UV/vis), X-ray crystallography, and accompanying density functional theory calculations. The oxygenation of the S atoms has a strong impact on the electronic structures of the nickel complexes. The monosulfinate complex 2 has an S = 2 ground state resulting from moderate ferromagnetic exchange coupling interactions (J = +15.7 cm^-1; H = -2JS₁S₂), while an antiferromagnetic exchange interaction in 3 shows the presence of a ground state with spin S = 0 (J = -0.56 cm^-1).

Selective cysteine-to-selenocysteine changes in a [NiFe]-hydrogenase confirm a special position for catalysis and oxygen tolerance

In [NiFe]-hydrogenases, the active-site Ni is coordinated by four cysteine-S ligands (Cys; C), two of which are bridging to the Fe(CO)(CN)2 fragment. Substitution of a single Cys residue by selenocysteine (Sec; U) occurs occasionally in nature. Using a recent method for site-specific Sec incorporation into proteins, each of the four Ni-coordinating cysteine residues in the oxygen-tolerant Escherichia coli [NiFe]-hydrogenase-1 (Hyd-1) has been replaced by U to identify its importance for enzyme function. Steady-state solution activity of each Sec-substituted enzyme (on a per-milligram basis) is lowered, although this may reflect the unquantified presence of recalcitrant inactive/immature/misfolded forms. Protein film electrochemistry, however, reveals detailed kinetic data that are independent of absolute activities. Like native Hyd-1, the variants have low apparent KMH2 values, do not produce H2 at pH 6, and display the same onset overpotential for H2 oxidation. Mechanistically important differences were identified for the C576U variant bearing the equivalent replacement found in native [NiFeSe]-hydrogenases, its extreme O2 tolerance (apparent KMH2 and Vmax [solution] values relative to native Hyd-1 of 0.13 and 0.04, respectively) implying the importance of a selenium atom in the position cis to the site where exogenous ligands (H-, H2, O2) bind. Observation of the same unusual electrocatalytic signature seen earlier for the proton transfer-defective E28Q variant highlights the direct role of the chalcogen atom (S/Se) at position 576 close to E28, with the caveat that Se is less effective than S in facilitating proton transfer away from the Ni during H2 oxidation by this enzyme.