14 research outputs found
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><sub>3</sub> (fcc<sub>3</sub>), which catalyzes hydrogenation
across a Cī»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 TiO<sub>2</sub> nanoparticles, fcc<sub>3</sub> 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 fcc<sub>3</sub>-modified indium tin oxide cathode
linked to a cobalt phosphate-modified BiVO<sub>4</sub> 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 CO<sub>2</sub> Reduction on a pāType Dye-Sensitized NiO Photocathode
We present a photocathode
assembly for the visible-light-driven
selective reduction of CO<sub>2</sub> 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 CO<sub>2</sub> 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 semiconductorācatalyst
interface
Fast and Selective Photoreduction of CO<sub>2</sub> to CO Catalyzed by a Complex of Carbon Monoxide Dehydrogenase, TiO<sub>2</sub>, and Ag Nanoclusters
Selective, visible-light-driven
conversion of CO<sub>2</sub> to
CO with a turnover frequency of 20 s<sup>ā1</sup> under visible
light irradiation at 25 Ā°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 TiO<sub>2</sub> nanoparticles. The photocatalytic
assembly, which is stable over several hours and for at least 250000
turnovers of the enzymeās 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 TiO<sub>2</sub> greatly enhances its ability
as an electrode for CODH-based electrocatalysis of CO<sub>2</sub> 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 TiO<sub>2</sub> mediates hole transfer from the
AgNC valence band to sacrificial electron donors. The results greatly
increase the benchmark for reversible CO<sub>2</sub> 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 electroĀcatalytic
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% H<sub>2</sub>), is
the resting state known as NiāB, which contains a hydroxido-bridged
species, Ni<sup>III</sup>āĪ¼Ā(OH)āFe<sup>II</sup>, at the active site. āCyanide inhibitionā is easily
reversed because it is simply the reductive activation of NiāB.
This paper brings back into focus an observation originally made in
the 1940s that cyanide inhibits microbial H<sub>2</sub> oxidation
and addresses the interesting mechanism by which cyanide promotes
the formation of NiāB. As a much stronger nucleophile than
hydroxide, cyanide binds more rapidly and promotes oxidation of Ni<sup>II</sup> to Ni<sup>III</sup>; however, it is quickly replaced by
hydroxide which is a far superior bridging ligand
EPR Spectroscopic Studies of the FeāS Clusters in the O<sub>2</sub>āTolerant [NiFe]-Hydrogenase Hydā1 from Escherichia coli and Characterization of the Unique [4Feā3S] Cluster by HYSCORE
The unusual [4Feā3S] cluster proximal to the active
site plays a crucial role in allowing a class of [NiFe]-hydrogenases
to function in the presence of O<sub>2</sub> 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 FeāS relay, continuous wave and pulse electron paramagnetic
resonance (EPR) studies were conducted on the O<sub>2</sub>-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 ([4Feā3S]<sup>5+/4+/3+</sup>) and medial ([3Feā4S]<sup>+/0</sup>) clusters
were determined by potentiometry. The medial [3Feā4S]<sup>+/0</sup> reduction potential is exceptionally high, implicating a mechanistic
role in O<sub>2</sub>-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 āsuperoxidizedā P242C variant, in which the
medial cluster is āmagnetically silencedā, reveal two
conformations of the proximal [4Feā3S]<sup>5+</sup> cluster,
and X-band HYSCORE spectroscopy shows two <sup>14</sup>N hyperfine
couplings attributed to one conformer. The largest, <i>A</i>(<sup>14</sup>N) = [11.5,11.5,16.0] Ā± 1.5 MHz, characterizes
the unusual bond between one Fe (Fe<sub>4</sub>) and the backbone
amide-N of cysteine-20. The second, <i>A</i>(<sup>14</sup>N) = [2.8,4.6,3.5] Ā± 0.3 MHz, is assigned to N<sub>C19</sub>. The <sup>14</sup>N hyperfine couplings are conclusive evidence
that Fe<sub>4</sub> is a valence-localized Fe<sup>3+</sup> in the
superoxidized state, whose formation permits an additional electron
to be transferred rapidly back to the active site during O<sub>2</sub> 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 CO<sub>2</sub> reduction
or CO oxidation at minimal overpotential. The O<sub>2</sub> sensitivity
of CODH is suppressed by adding cyanide, a reversible inhibitor of
CO oxidation, or by raising the electrode potential. Reduction of
N<sub>2</sub>O, isoelectronic with CO<sub>2</sub>, is catalyzed by
CODH, but the reaction is sluggish, despite a large overpotential,
and results in inactivation. Production of H<sub>2</sub> 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 O<sub>2</sub>āTolerant [NiFe]-Hydrogenase
Catalytic long-range
proton transfer in [NiFe]-hydrogenases has long been associated with
a highly conserved glutamate (E) situated within 4 Ć
of the active
site. Substituting for glutamine (Q) in the O<sub>2</sub>-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 [4Feā3S]
cluster located nearby. Under these special conditions, the essential
exit pathway for at least one of the H<sup>+</sup> ions produced by
H<sub>2</sub> oxidation, and assumed to be blocked in the E28Q variant,
is partially repaired. During steady-state H<sub>2</sub> oxidation
at neutral pH (i.e., when the barrier to H<sup>+</sup> exit via Q28
is almost totally closed), the catalytic cycle is dominated by the
reduced states āNi<sub>a</sub>-Rā and āNi<sub>a</sub>-Cā, even under highly oxidizing conditions. Hence,
E28 is not involved in the initial activation/deprotonation of H<sub>2</sub>, but facilitates H<sup>+</sup> exit later in the catalytic
cycle to regenerate the initial oxidized active state, assumed to
be Ni<sub>a</sub>-SI. Accordingly, the oxidized inactive resting state,
āNi-Bā, is not produced by E28Q in the presence of H<sub>2</sub> at high potential because Ni<sub>a</sub>-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
ŠŠ¾Š»ŃŃŠµŠ²ŠøŠŗ. 1941. ā 052
Despite extensive studies on [NiFe]-hydrogenases,
the mechanism
by which these enzymes produce and activate H<sub>2</sub> so efficiently
remains unclear. A well-known EPR-active state produced under H<sub>2</sub> and known as Ni-C is assigned as a Ni<sup>III</sup>āFe<sup>II</sup> 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 darkīøproton
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 FeāS centers in this O<sub>2</sub>-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 <sup>13</sup>C ENDOR of Direct FeāC 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 electronānuclear
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 Feā<sup>13</sup>C bond in the 2Fe subsite. The adduct forms when HCHO is available
to compete with H<sup>+</sup> transfer to a vacant, nucleophilic Fe
site: had H<sup>+</sup> 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 H<sub>2</sub> evolution involves two consecutive proton transfer
steps to the H-cluster following transfer of a second electron to
the active site