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

Modulation of Ligand-Field Parameters by Heme Ruffling in Cytochromes <i>c</i> Revealed by EPR Spectroscopy

Electron paramagnetic resonance (EPR) spectra of variants of <i>Hydrogenobacter thermophilus</i> cytochrome <i>c</i>₅₅₂ (<i>Ht c</i>-552) and <i>Pseudomonas aeruginosa</i> cytochrome <i>c</i>₅₅₁ (<i>Pa c</i>-551) are analyzed to determine the effect of heme ruffling on ligand-field parameters. Mutations introduced at positions 13 and 22 in <i>Ht c</i>-552 were previously demonstrated to influence hydrogen bonding in the proximal heme pocket and to tune reduction potential (<i>E</i>_m) over a range of 80 mV [Michel, L. V.; Ye, T.; Bowman, S. E. J.; Levin, B. D.; Hahn, M. A.; Russell, B. S.; Elliott, S. J.; Bren, K. L. <i>Biochemistry</i> <b>2007</b>, <i>46</i>, 11753–11760]. These mutations are shown here to also increase heme ruffling as <i>E</i>_m decreases. The primary effect on electronic structure of increasing heme ruffling is found to be a decrease in the axial ligand-field term Δ/λ, which is proposed to arise from an increase in the energy of the d_<i>xy</i> orbital. Mutations at position 7, previously demonstrated to influence heme ruffling in <i>Pa c</i>-551 and <i>Ht c</i>-552, are utilized to test this correlation between molecular and electronic structure. In conclusion, the structure of the proximal heme pocket of cytochromes <i>c</i> is shown to play a role in determining heme conformation and electronic structure

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

X‑ray Absorption Spectroscopy Reveals an Organometallic Ni–C Bond in the CO-Treated Form of Acetyl-CoA Synthase

Acetyl-CoA synthase (ACS) is a key enzyme in the Wood–Ljungdahl pathway of anaerobic CO₂ fixation, which has long been proposed to operate by a novel mechanism involving a series of protein-bound organometallic (Ni–CO, methyl–Ni, and acetyl–Ni) intermediates. Here we report the first direct structural evidence of the proposed metal–carbon bond. We describe the preparation of the highly active metal-replete enzyme and near-quantitative generation of the kinetically competent carbonylated intermediate. This advance has allowed a combination of Ni and Fe K-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure experiments along with density functional theory calculations. The data reveal that CO binds to the proximal Ni of the six-metal metallocenter at the active site and undergoes dramatic structural and electronic perturbation in forming this organometallic Ni–CO intermediate. This direct identification of a Ni–carbon bond in the catalytically competent CO-bound form of the A cluster of ACS provides definitive experimental structural evidence supporting the proposed organometallic mechanism of anaerobic acetyl-CoA synthesis

Transient B₁₂-Dependent Methyltransferase Complexes Revealed by Small-Angle X‑ray Scattering

In the Wood–Ljungdahl carbon fixation pathway, protein–protein interactions between methyltransferase (MeTr) and corrinoid iron–sulfur protein (CFeSP) are required for the transfer of a methyl group. While crystal structures have been determined for MeTr and CFeSP both free and in complex, solution structures have not been established. Here, we examine the transient interactions between MeTr and CFeSP in solution using anaerobic small-angle X-ray scattering (SAXS) and present a global analysis approach for the deconvolution of heterogeneous mixtures formed by weakly interacting proteins. We further support this SAXS analysis with complementary results obtained by anaerobic isothermal titration calorimetry. Our results indicate that solution conditions affect the cooperativity with which CFeSP binds to MeTr, resulting in two distinct CFeSP/MeTr complexes with differing oligomeric compositions, both of which are active. One assembly resembles the CFeSP/MeTr complex observed crystallographically with 2:1 protein stoichiometry, while the other best fits a 1:1 CFeSP/MeTr arrangement. These results demonstrate the value of SAXS in uncovering the rich solution behavior of transient protein interactions visualized by crystallography

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

How Light-Harvesting Semiconductors Can Alter the Bias of Reversible Electrocatalysts in Favor of H₂ Production and CO₂ Reduction

The most efficient catalysts for solar fuel production should operate close to reversible potentials, yet possess a bias for the fuel-forming direction. Protein film electrochemical studies of Ni-containing carbon monoxide dehydrogenase and [NiFeSe]-hydrogenase, each a reversible electrocatalyst, show that the electronic state of the electrode strongly biases the direction of electrocatalysis of CO₂/CO and H⁺/H₂ interconversions. Attached to graphite electrodes, these enzymes show high activities for both oxidation and reduction, but there is a marked shift in bias, in favor of CO₂ or H⁺ reduction, when the respective enzymes are attached instead to n-type semiconductor electrodes constructed from CdS and TiO₂ nanoparticles. This catalytic rectification effect can arise for a reversible electrocatalyst attached to a semiconductor electrode if the electrode transforms between semiconductor- and metallic-like behavior across the same narrow potential range (<0.25 V) that the electrocatalytic current switches between oxidation and reduction