A new record of Percursaria percursa (Ulvaceae, Ulvales) on the North Island, New Zealand

The filamentous green alga Percursaria percursa (Ulvaceae, Ulvales) was recorded for the first time on the North Island of New Zealand at mokoroa Estuary, Tauranga Harbour. This species is previously known within New Zealand from only two records, both from the South Island. In Tauranga Harbour, this species was restricted to anoxic estuarine sediments where mangrove forests had been mulched, and mulchate left in situ. Percursaria percursa was found intertwined with Ulva spp. and Rhizoclonium spp. Surveys of other North and South Island estuaries suggest that this alga, although occurring as part of nuisance green algal blooms in Tauranga Harbour, has only colonized human-impacted locations, and has not yet been observed in natural' estuarine ecosystems in New Zealand. As this species was found intertwined with other mat-forming filamentous green algae, it can easily be misidentified in the field, leading to both over- and under-reporting of species occurrence

A Multi-Heme Flavoenzyme as a Solar Conversion Catalyst

The enzyme flavocytochrome <i>c</i>₃ (fcc₃), which catalyzes hydrogenation across a CC double bond (fumarate to succinate), is used to carry out the fuel-forming reaction in an artificial photosynthesis system. When immobilized on dye-sensitized TiO₂ nanoparticles, fcc₃ catalyzes visible-light-driven succinate production in aqueous suspension. Solar-to-chemical conversion using neutral water as the oxidant is achieved with a photoelectrochemical cell comprising an fcc₃-modified indium tin oxide cathode linked to a cobalt phosphate-modified BiVO₄ photoanode. The results reinforce new directions in the area of artificial photosynthesis, in particular for solar-energy-driven synthesis of organic chemicals and commodities, moving away from simple fuels as target molecules

Selective Visible-Light-Driven CO₂ Reduction on a p‑Type Dye-Sensitized NiO Photocathode

We present a photocathode assembly for the visible-light-driven selective reduction of CO₂ to CO at potentials below the thermodynamic equilibrium in the dark. The photoelectrode comprises a porous p-type semiconducting NiO electrode modified with the visible-light-responsive organic dye P1 and the reversible CO₂ cycling enzyme carbon monoxide dehydrogenase. The direct electrochemistry of the enzymatic electrocatalyst on NiO shows that in the dark the electrocatalytic behavior is rectified toward CO oxidation, with the reactivity being governed by the carrier availability at the semiconductor–catalyst interface

Fast and Selective Photoreduction of CO₂ to CO Catalyzed by a Complex of Carbon Monoxide Dehydrogenase, TiO₂, and Ag Nanoclusters

Selective, visible-light-driven conversion of CO₂ to CO with a turnover frequency of 20 s^–1 under visible light irradiation at 25 °C is catalyzed by an aqueous colloidal system comprising a pseudoternary complex formed among carbon monoxide dehydrogenase (CODH), silver nanoclusters stabilized by polymethacrylic acid (AgNCs-PMAA), and TiO₂ nanoparticles. The photocatalytic assembly, which is stable over several hours and for at least 250000 turnovers of the enzyme’s active site, was investigated by separate electrochemical (dark) and fluorescence measurements to establish specific connectivities among the components. The data show (a) that a coating of AgNCs-PMAA on TiO₂ greatly enhances its ability as an electrode for CODH-based electrocatalysis of CO₂ reduction and (b) that the individual Ag nanoclusters interact directly and dynamically with the enzyme surface, most likely at exposed cysteine thiols. The results lead to a model for photocatalysis in which the AgNCs act as photosensitizers, CODH captures the excited electrons for catalysis, and TiO₂ mediates hole transfer from the AgNC valence band to sacrificial electron donors. The results greatly increase the benchmark for reversible CO₂ reduction under ambient conditions and demonstrate that, with such efficient catalysts, the limiting factor is the supply of photogenerated electrons

Unusual Reaction of [NiFe]-Hydrogenases with Cyanide

Cyanide reacts rapidly with [NiFe]-hydrogenases (hydrogenase-1 and hydrogenase-2 from Escherichia coli) under mild oxidizing conditions, inhibiting the electro­catalytic oxidation of hydrogen as recorded by protein film electrochemistry. Electrochemical, EPR, and FTIR measurements show that the final enzyme product, formed within a second (even under 100% H₂), is the resting state known as Ni–B, which contains a hydroxido-bridged species, Ni^III–μ­(OH)–Fe^II, at the active site. “Cyanide inhibition” is easily reversed because it is simply the reductive activation of Ni–B. This paper brings back into focus an observation originally made in the 1940s that cyanide inhibits microbial H₂ oxidation and addresses the interesting mechanism by which cyanide promotes the formation of Ni–B. As a much stronger nucleophile than hydroxide, cyanide binds more rapidly and promotes oxidation of Ni^II to Ni^III; however, it is quickly replaced by hydroxide which is a far superior bridging ligand

EPR Spectroscopic Studies of the Fe–S Clusters in the O₂‑Tolerant [NiFe]-Hydrogenase Hyd‑1 from Escherichia coli and Characterization of the Unique [4Fe–3S] Cluster by HYSCORE

The unusual [4Fe–3S] cluster proximal to the active site plays a crucial role in allowing a class of [NiFe]-hydrogenases to function in the presence of O₂ through its unique ability to undergo two rapid, consecutive one-electron transfers. This property helps to neutralize reactive oxygen species. Mechanistic details and the role of the medial and distal clusters remain unresolved. To probe the Fe–S relay, continuous wave and pulse electron paramagnetic resonance (EPR) studies were conducted on the O₂-tolerant hydrogenase from Escherichia coli (Hyd-1) and three variants with point mutations at the proximal and/or medial clusters. Reduction potentials of the proximal ([4Fe–3S]^5+/4+/3+) and medial ([3Fe–4S]^+/0) clusters were determined by potentiometry. The medial [3Fe–4S]^+/0 reduction potential is exceptionally high, implicating a mechanistic role in O₂-tolerance. Numerous experiments establish that the distal cluster has a ground state <i>S</i> > 1/2 in all three variants and indicate that this is also the case for native Hyd-1. Concurrent with the Hyd-1 crystal structure, EPR data for the ‘superoxidized’ P242C variant, in which the medial cluster is ‘magnetically silenced’, reveal two conformations of the proximal [4Fe–3S]⁵⁺ cluster, and X-band HYSCORE spectroscopy shows two ¹⁴N hyperfine couplings attributed to one conformer. The largest, <i>A</i>(¹⁴N) = [11.5,11.5,16.0] ± 1.5 MHz, characterizes the unusual bond between one Fe (Fe₄) and the backbone amide-N of cysteine-20. The second, <i>A</i>(¹⁴N) = [2.8,4.6,3.5] ± 0.3 MHz, is assigned to N_C19. The ¹⁴N hyperfine couplings are conclusive evidence that Fe₄ is a valence-localized Fe³⁺ in the superoxidized state, whose formation permits an additional electron to be transferred rapidly back to the active site during O₂ attack

Investigations by Protein Film Electrochemistry of Alternative Reactions of Nickel-Containing Carbon Monoxide Dehydrogenase

Protein film electrochemistry has been used to investigate reactions of highly active nickel-containing carbon monoxide dehydrogenases (CODHs). When attached to a pyrolytic graphite electrode, these enzymes behave as reversible electrocatalysts, displaying CO₂ reduction or CO oxidation at minimal overpotential. The O₂ sensitivity of CODH is suppressed by adding cyanide, a reversible inhibitor of CO oxidation, or by raising the electrode potential. Reduction of N₂O, isoelectronic with CO₂, is catalyzed by CODH, but the reaction is sluggish, despite a large overpotential, and results in inactivation. Production of H₂ and formate under highly reducing conditions is consistent with calculations predicting that a nickel-hydrido species might be formed, but the very low rates suggest that such a species is not on the main catalytic pathway

Mechanistic Exploitation of a Self-Repairing, Blocked Proton Transfer Pathway in an O₂‑Tolerant [NiFe]-Hydrogenase

Catalytic long-range proton transfer in [NiFe]-hydrogenases has long been associated with a highly conserved glutamate (E) situated within 4 Å of the active site. Substituting for glutamine (Q) in the O₂-tolerant [NiFe]-hydrogenase-1 from Escherichia coli produces a variant (E28Q) with unique properties that have been investigated using protein film electrochemistry, protein film infrared electrochemistry, and X-ray crystallography. At pH 7 and moderate potential, E28Q displays approximately 1% of the activity of the native enzyme, high enough to allow detailed infrared measurements under steady-state conditions. Atomic-level crystal structures reveal partial displacement of the amide side chain by a hydroxide ion, the occupancy of which increases with pH or under oxidizing conditions supporting formation of the superoxidized state of the unusual proximal [4Fe–3S] cluster located nearby. Under these special conditions, the essential exit pathway for at least one of the H⁺ ions produced by H₂ oxidation, and assumed to be blocked in the E28Q variant, is partially repaired. During steady-state H₂ oxidation at neutral pH (i.e., when the barrier to H⁺ exit via Q28 is almost totally closed), the catalytic cycle is dominated by the reduced states “Ni_a-R” and “Ni_a-C”, even under highly oxidizing conditions. Hence, E28 is not involved in the initial activation/deprotonation of H₂, but facilitates H⁺ exit later in the catalytic cycle to regenerate the initial oxidized active state, assumed to be Ni_a-SI. Accordingly, the oxidized inactive resting state, “Ni-B”, is not produced by E28Q in the presence of H₂ at high potential because Ni_a-SI (the precursor for Ni-B) cannot accumulate. The results have important implications for understanding the catalytic mechanism of [NiFe]-hydrogenases and the control of long-range proton-coupled electron transfer in hydrogenases and other enzymes

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Despite extensive studies on [NiFe]-hydrogenases, the mechanism by which these enzymes produce and activate H₂ so efficiently remains unclear. A well-known EPR-active state produced under H₂ and known as Ni-C is assigned as a Ni^III–Fe^II species with a hydrido ligand in the bridging position between the two metals. It has long been known that low-temperature photolysis of Ni-C yields distinctive EPR-active states, collectively termed Ni-L, that are attributed to migration of the bridging-H species as a proton; however, Ni-L has mainly been regarded as an artifact with no mechanistic relevance. It is now demonstrated, based on EPR and infrared spectroscopic studies, that the Ni-C to Ni-L interconversion in Hydrogenase-1 (Hyd-1) from Escherichia coli is a pH-dependent process that proceeds readily in the darkproton migration from Ni-C being favored as the pH is increased. The persistence of Ni-L in Hyd-1 must relate to unassigned differences in proton affinities of metal and adjacent amino acid sites, although the unusually high reduction potentials of the adjacent Fe–S centers in this O₂-tolerant hydrogenase might also be a contributory factor, impeding elementary electron transfer off the [NiFe] site after proton departure. The results provide compelling evidence that Ni-L is a true, albeit elusive, catalytic intermediate of [NiFe]-hydrogenases

How Formaldehyde Inhibits Hydrogen Evolution by [FeFe]-Hydrogenases: Determination by ¹³C ENDOR of Direct Fe–C Coordination and Order of Electron and Proton Transfers

Formaldehyde (HCHO), a strong electrophile and a rapid and reversible inhibitor of hydrogen production by [FeFe]-hydrogenases, is used to identify the point in the catalytic cycle at which a highly reactive metal-hydrido species is formed. Investigations of the reaction of Chlamydomonas reinhardtii [FeFe]-hydrogenase with formaldehyde using pulsed-EPR techniques including electron–nuclear double resonance spectroscopy establish that formaldehyde binds close to the active site. Density functional theory calculations support an inhibited super-reduced state having a short Fe–¹³C bond in the 2Fe subsite. The adduct forms when HCHO is available to compete with H⁺ transfer to a vacant, nucleophilic Fe site: had H⁺ transfer already occurred, the reaction of HCHO with the Fe-hydrido species would lead to methanol, release of which is not detected. Instead, Fe-bound formaldehyde is a metal-hydrido mimic, a locked, inhibited form analogous to that in which two electrons and only one proton have transferred to the H-cluster. The results provide strong support for a mechanism in which the fastest pathway for H₂ evolution involves two consecutive proton transfer steps to the H-cluster following transfer of a second electron to the active site