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

Effect of a Dispersion of Interfacial Electron Transfer Rates on Steady State Catalytic Electron Transport in [NiFe]-hydrogenase and Other Enzymes

Redox enzymes can be adsorbed onto electrode surfaces such that there is a rapid and efficient direct electron transfer (ET) between the electrode and the enzyme's active site, along with high catalytic activity. In an idealized way, this may be analogous to protein−protein ET or, more significantly, the nonrigid interface between different domains of membrane-bound enzymes. The catalytic current that is obtained when substrate is added to the solution is directly proportional to the enzyme's turnover rate and its dependence on the electrode potential reports on the energetics and kinetics of the entire catalytic cycle. Although the current is expected to reach a limiting value as the electrode potential is varied to increase the driving force, a residual slope in voltammograms is often observed. This slope is significant, as it is unexpected from all simple considerations of electrochemical kinetics. A particularly remarkable result is obtained in experiments carried out with the [NiFe]-hydrogenase from Allochromatium vinosum:  this enzyme displays high catalytic activity for hydrogen oxidation and is easily studied up to 60 °C, at which temperature the current−potential response becomes completely linear over a range of more than 0.5 V. The explanation for this effect is that the enzyme molecules are not adsorbed in a homogeneous manner but vary in their degree of ET coupling with the electrode, i.e., through there being many slightly different orientations. Under conditions in which interfacial ET becomes rate-limiting, i.e., when turnover number is high at elevated temperatures, the current−potential response reflects the superposition of numerous electrochemical rate constants. This is highly relevant in the interpretation of the catalytic electrochemistry of enzymes

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

Rapid and Efficient Electrocatalytic CO₂/CO Interconversions by <i>Carboxydothermus </i><i>hydrogenoformans</i> CO Dehydrogenase I on an Electrode

The Ni-containing carbon monoxide dehydrogenase I from Carboxydothermus hydrogenoformans adsorbed on a pyrolytic graphite “edge” electrode catalyzes rapid CO2/CO interconversions at the thermodynamic potential

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

Efficient Electrocatalytic CO₂ Fixation by Nanoconfined Enzymes via a C3-to-C4 Reaction That Is Favored over H₂ Production

Reduction of CO2 and its direct entry into organic chemistry is achieved efficiently and in a highly visible way using a metal oxide electrode in which two enzyme catalysts, one for electrochemically regenerating reduced nicotinamide adenine dinucleotide phosphate and the other for assimilating CO2 and converting pyruvate (C3) to malate (C4), are entrapped within its nanopores. The resulting reversible electrocatalysis is exploited to construct a solar CO2 reduction/water-splitting device producing O2 and C4 with high faradaic efficiency

The Difference a Se Makes? Oxygen-Tolerant Hydrogen Production by the [NiFeSe]-Hydrogenase from <i>Desulfomicrobium baculatum</i>

Protein film voltammetry studies of the [NiFeSe]-hydrogenase from Desulfomicrobium baculatum show it to be a highly efficient H2 cycling catalyst. In the presence of 100% H2, the ratio of H2 production to H2 oxidation activity is higher than for any conventional [NiFe]-hydrogenases (lacking a selenocysteine ligand) that have been investigated to date. Although traces of O2 (≪ 1%) rapidly and completely remove H2 oxidation activity, the enzyme sustains partial activity for H2 production even in the presence of 1% O2 in the atmosphere. That H2 production should be partly allowed, whereas H2 oxidation is not, is explained because the inactive product of O2 attack is reductively reactivated very rapidly, but this requires a potential that is almost as negative as the thermodynamic potential for the 2H+/H2 couple. The study provides further encouragement and clues regarding the feasibility of microbial/enzymatic H2 production free from restrictions of anaerobicity

Electrochemical Investigations of the Interconversions between Catalytic and Inhibited States of the [FeFe]-Hydrogenase from <i>Desulfovibrio </i><i>desulfuricans</i>

Studies of the catalytic properties of the [FeFe]-hydrogenase from Desulfovibrio desulfuricans by protein film voltammetry, under a H2 atmosphere, reveal and establish a variety of interesting properties not observed or measured quantitatively with other techniques. The catalytic bias (inherent ability to oxidize hydrogen vs reduce protons) is quantified over a wide pH range:  the enzyme is proficient at both H2 oxidation (from pH > 6) and H2 production (pH 2, but the effect is much smaller than observed for [NiFe]-hydrogenases from Allochromatium vinosum or Desulfovibrio fructosovorans. Under anaerobic conditions and positive potentials, the [FeFe]-hydrogenase is oxidized to an inactive form, inert toward reaction with CO and O2, that rapidly reactivates upon one-electron reduction under 1 bar of H2. The potential dependence of this interconversion shows that the oxidized inactive form exists in two pH-interconvertible states with pKox = 5.9. Studies of the CO-inhibited enzyme under H2 reveals a strong enhancement of the rate of activation by white light at −109 mV (monitoring H2 oxidation) that is absent at low potential (−540 mV, monitoring H+ reduction), thus demonstrating photolability that is dependent upon the oxidation state