11 research outputs found
Orientations of Iron–Sulfur Clusters F<sub>A</sub> and F<sub>B</sub> in the Homodimeric Type‑I Photosynthetic Reaction Center of <i>Heliobacterium modesticaldum</i>
Orientations of the F<sub>A</sub> and F<sub>B</sub> iron–sulfur
(FeS) clusters in a structure-unknown type-I homodimeric heriobacterial
reaction center (hRC) were studied in oriented membranes of the thermophilic
anaerobic photosynthetic bacterium <i>Heliobacterium modesticaldum</i> by electron paramagnetic resonance (EPR), and compared with those
in heterodimeric photosystem I (PS I). The Rieske-type FeS center
in the cytochrome <i>b/c</i> complex showed a well-oriented
EPR signal. Illumination at 14 K induced an F<sub>B</sub><sup>–</sup> signal with <i>g</i>-axes of <i>g</i><sub><i>z</i></sub> = 2.066, <i>g</i><sub><i>y</i></sub> = 1.937, and <i>g</i><sub><i>x</i></sub> = 1.890, tilted at angles of 60°, 60°, and 45°, respectively,
with respect to the membrane normal. Chemical reduction with dithionite
produced an additional signal of F<sub>A</sub><sup>–</sup>,
which magnetically interacted with F<sub>B</sub><sup>–</sup>, with <i>g</i><sub><i>z</i></sub> = 2.046, <i>g</i><sub><i>y</i></sub> = 1.942, and <i>g</i><sub><i>x</i></sub> = 1.911 at 30°, 60°, and
90°, respectively. The angles and redox properties of F<sub>A</sub><sup>–</sup> and F<sub>B</sub><sup>–</sup> in hRC resemble
those of F<sub>B</sub><sup>–</sup> and F<sub>A</sub><sup>–</sup>, respectively, in PS I. Therefore, F<sub>A</sub> and F<sub>B</sub> in hRC, named after their <i>g</i>-value similarities,
seem to be located like F<sub>B</sub> and F<sub>A</sub>, not like
F<sub>A</sub> and F<sub>B</sub>, respectively, in PS I. The reducing
side of hRC could resemble those in PS I, if the names of F<sub>A</sub> and F<sub>B</sub> are interchanged with each other
Menaquinone as the Secondary Electron Acceptor in the Type I Homodimeric Photosynthetic Reaction Center of <i>Heliobacterium modesticaldum</i>
The type I photosynthetic reaction
center (RC) of heliobacteria
(hRC) is a homodimer containing cofactors almost analogous to those
in the plant photosystem I (PS I). However, its three-dimensional
structure is not yet clear. PS I uses phylloquinone (PhyQ) as a secondary
electron acceptor (A<sub>1</sub>), while the available evidence has
suggested that menaquinone (MQ) in hRC has no function as A<sub>1</sub>. The present study identified a new transient electron spin-polarized
electron paramagnetic resonance (ESP-EPR) signal, arising from the
radical pair of the oxidized electron donor and the reduced electron
acceptor (P800<sup>+</sup>MQ<sup>–</sup>), in the hRC core
complex and membranes from <i>Heliobacterium modesticaldum</i>. The ESP signal could be detected at 5–20 K upon flash excitation
only after prereduction of the iron–sulfur center, F<sub>X</sub>, and was selectively lost by extraction of MQ with diethyl ether.
MQ was suggested to be located closer to F<sub>X</sub> than PhyQ in
PS I based on the simulation of the unique A/E (A, absorption; E,
emission) ESP pattern, the reduction/oxidation rates of MQ, and the
power saturation property of the static MQ<sup>–</sup> signal.
The result revealed the quinone usage as the secondary electron acceptor
in hRC, as in the case of PS I
Experimental and Theoretical Mutation of Exciton States on the Smallest Type‑I Photosynthetic Reaction Center Complex of a Green Sulfur Bacterium Chlorobaclum tepidum
The
exciton states on the smallest type-I photosynthetic reaction
center complex of a green sulfur bacterium Chlorobaculum
tepidum (GsbRC) consisting of 26 bacteriochlorophylls a (BChl a) and four chlorophylls a (Chl a) located on the homodimer of two
PscA reaction center polypeptides were investigated. This analysis
involved the study of exciton states through a combination of theoretical
modeling and the genetic removal of BChl a pigments
at eight sites. (1) A theoretical model of the pigment assembly exciton
state on GsbRC was constructed using Poisson TrESP (P-TrESP) and charge
density coupling (CDC) methods based on structural information. The
model reproduced the experimentally obtained absorption spectrum,
circular dichroism spectrum, and excitation transfer dynamics, as
well as explained the effects of mutation. (2) Eight BChl a molecules at different locations on the GsbRC were selectively
removed by genetic exchange of the His residue, which ligates the
central Mg atom of BChl a, with the Leu residue on
either one or two PscAs in the RC. His locations are conserved among
all type-I RC plant polypeptide, cyanobacteria, and bacteria amino
acid sequences. (3) Purified mutant-GsbRCs demonstrated distinct absorption
and fluorescence spectra at 77 K, which were different from each other,
suggesting successful pigment removal. (4) The same mutations were
applied to the constructed theoretical model to analyze the outcomes
of these mutations. (5) The combination of theoretical predictions
and experimental mutations based on structural information is a new
tool for studying the function and evolution of photosynthetic reaction
centers
Experimental and Theoretical Mutation of Exciton States on the Smallest Type‑I Photosynthetic Reaction Center Complex of a Green Sulfur Bacterium Chlorobaclum tepidum
The
exciton states on the smallest type-I photosynthetic reaction
center complex of a green sulfur bacterium Chlorobaculum
tepidum (GsbRC) consisting of 26 bacteriochlorophylls a (BChl a) and four chlorophylls a (Chl a) located on the homodimer of two
PscA reaction center polypeptides were investigated. This analysis
involved the study of exciton states through a combination of theoretical
modeling and the genetic removal of BChl a pigments
at eight sites. (1) A theoretical model of the pigment assembly exciton
state on GsbRC was constructed using Poisson TrESP (P-TrESP) and charge
density coupling (CDC) methods based on structural information. The
model reproduced the experimentally obtained absorption spectrum,
circular dichroism spectrum, and excitation transfer dynamics, as
well as explained the effects of mutation. (2) Eight BChl a molecules at different locations on the GsbRC were selectively
removed by genetic exchange of the His residue, which ligates the
central Mg atom of BChl a, with the Leu residue on
either one or two PscAs in the RC. His locations are conserved among
all type-I RC plant polypeptide, cyanobacteria, and bacteria amino
acid sequences. (3) Purified mutant-GsbRCs demonstrated distinct absorption
and fluorescence spectra at 77 K, which were different from each other,
suggesting successful pigment removal. (4) The same mutations were
applied to the constructed theoretical model to analyze the outcomes
of these mutations. (5) The combination of theoretical predictions
and experimental mutations based on structural information is a new
tool for studying the function and evolution of photosynthetic reaction
centers
Experimental and Theoretical Mutation of Exciton States on the Smallest Type‑I Photosynthetic Reaction Center Complex of a Green Sulfur Bacterium Chlorobaclum tepidum
The
exciton states on the smallest type-I photosynthetic reaction
center complex of a green sulfur bacterium Chlorobaculum
tepidum (GsbRC) consisting of 26 bacteriochlorophylls a (BChl a) and four chlorophylls a (Chl a) located on the homodimer of two
PscA reaction center polypeptides were investigated. This analysis
involved the study of exciton states through a combination of theoretical
modeling and the genetic removal of BChl a pigments
at eight sites. (1) A theoretical model of the pigment assembly exciton
state on GsbRC was constructed using Poisson TrESP (P-TrESP) and charge
density coupling (CDC) methods based on structural information. The
model reproduced the experimentally obtained absorption spectrum,
circular dichroism spectrum, and excitation transfer dynamics, as
well as explained the effects of mutation. (2) Eight BChl a molecules at different locations on the GsbRC were selectively
removed by genetic exchange of the His residue, which ligates the
central Mg atom of BChl a, with the Leu residue on
either one or two PscAs in the RC. His locations are conserved among
all type-I RC plant polypeptide, cyanobacteria, and bacteria amino
acid sequences. (3) Purified mutant-GsbRCs demonstrated distinct absorption
and fluorescence spectra at 77 K, which were different from each other,
suggesting successful pigment removal. (4) The same mutations were
applied to the constructed theoretical model to analyze the outcomes
of these mutations. (5) The combination of theoretical predictions
and experimental mutations based on structural information is a new
tool for studying the function and evolution of photosynthetic reaction
centers
Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800<sup>+</sup> Menaquinone<sup>–</sup> Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
Function/location
of menaquinone (MQ) was studied in the photosynthetic
reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs.
The spin-polarized electron paramagnetic resonance signals of light-induced
radical pair species, which are made of oxidized electron donor bacteriochlorophyll <i>g</i> (P800<sup>+</sup>) and reduced menaquinone (MQ<sup>–</sup>) or iron–sulfur cluster (F<sub>X</sub><sup>–</sup>), were measured in the oriented membranes of Hbt.
modesticaldum at cryogenic temperature. The spectral
shape of transient electron spin-polarized signal of P800<sup>+</sup>F<sub>X</sub><sup>–</sup> radical pair state varied little
with respect to the direction of the external magnetic field. It suggested
a dominant contribution of the spin evolution on the precursor primary
radical pair P800<sup>+</sup>A<sub>0</sub><sup>–</sup> state
with the larger isotropic magnetic exchange interaction <i>J</i> than the anisotropic dipole interaction <i>D</i>. The
pure P800<sup>+</sup>MQ<sup>–</sup> signal was simulated by
subtracting the effects of spin evolution during the electron-transfer
process. It was concluded that the <i>J</i> value of the
P800<sup>+</sup>MQ<sup>–</sup> radical pair is negative with
an amplitude almost comparable to |<i>D</i>|. It is in contrast
to a positive and small <i>J</i> value of the P700<sup>+</sup>PhyQ<sup>–</sup> state in photosystem I (PS I). The results
indicate similar but somewhat different locations/binding sites of
quinones between hRC and PS I
Lipid-Controlled Stabilization of Charge-Separated States (P<sup>+</sup>Q<sub>B</sub><sup>–</sup>) and Photocurrent Generation Activity of a Light-Harvesting–Reaction Center Core Complex (LH1-RC) from <i>Rhodopseudomonas palustris</i>
The
photosynthetic light-harvesting–reaction center core
complex (LH1-RC) is a natural excitonic and photovoltaic device embedded
in a lipid membrane. In order to apply LH1-RCs as a biohybrid energy-producing
material, some important issues must be addressed, including how to
make LH1-RCs function as efficiently as possible. In addition, they
should be characterized to evaluate how many active LH1-RCs efficiently
work in artificial systems. We report here that an anionic phospholipid,
phosphatidylglycerol (PG), stabilizes the charge-separated state (a
photooxidized electron donor and reduced quinone pair, P<sup>+</sup>Q<sub>B</sub><sup>–</sup>) of LH1-RC (from <i>Rhodopseudomonas
palustris</i>) and enhances its activity in photocurrent generation.
Steady-state fluorometric analysis demonstrated that PG enhances the
formation of the P<sup>+</sup>Q<sub>B</sub><sup>–</sup> state
at lower irradiances. The photocurrent generation activity was analyzed
via Michaelis–Menten kinetics, revealing that 38% of LH1-RCs
reconstituted into the PG membrane generated photocurrent at a turnover
frequency of 46 s<sup>–1</sup>. PG molecules, which interact
with LH1-RC in vivo, play the role of an active effector component
for LH1-RC to enhance its function in the biohybrid system
Oxygen-Evolving Porous Glass Plates Containing the Photosynthetic Photosystem II Pigment–Protein Complex
The
development of artificial photosynthesis has focused on the
efficient coupling of reaction at photoanode and cathode, wherein
the production of hydrogen (or energy carriers) is coupled to the
electrons derived from water-splitting reactions. The natural photosystem
II (PSII) complex splits water efficiently using light energy. The
PSII complex is a large pigment–protein complex (20 nm in diameter)
containing a manganese cluster. A new photoanodic device was constructed
incorporating stable PSII purified from a cyanobacterium Thermosynechococcus vulcanus through immobilization
within 20 or 50 nm nanopores contained in porous glass plates (PGPs).
PSII in the nanopores retained its native structure and high photoinduced
water splitting activity. The photocatalytic rate (turnover frequency)
of PSII in PGP was enhanced 11-fold compared to that in solution,
yielding a rate of 50–300 mol e<sup>–</sup>/(mol PSII·s)
with 2,6-dichloroindophenol (DCIP) as an electron acceptor. The PGP
system realized high local concentrations of PSII and DCIP to enhance
the collisional reactions in nanotubes with low disturbance of light
penetration. The system allows direct visualization/determination
of the reaction inside the nanotubes, which contributes to optimize
the local reaction condition. The PSII/PGP device will substantively
contribute to the construction of artificial photosynthesis using
water as the ultimate electron source
Light-Driven Hydrogen Production by Hydrogenases and a Ru-Complex inside a Nanoporous Glass Plate under Aerobic External Conditions
Hydrogenases
are powerful catalysts for light-driven H<sub>2</sub> production using
a combination of photosensitizers. However, except
oxygen-tolerant hydrogenases, they are immediately deactivated under
aerobic conditions. We report a light-driven H<sub>2</sub> evolution
system that works stably even under aerobic conditions. A [NiFe]-hydrogenase
from <i>Desulfovibrio vulgaris</i> Miyazaki F was immobilized
inside nanoporous glass plates (PGPs) with a pore diameter of 50 nm
together with a ruthenium complex and methyl viologen as a photosensitizer
and an electron mediator, respectively. After immersion of PGP into
the medium containing the catalytic components, an anaerobic environment
automatically established inside the nanopores even under aerobic
external conditions upon irradiation with solar-simulated light; this
system constantly evolved H<sub>2</sub> with an efficiency of 3.7
μmol H<sub>2</sub> m<sup>–2</sup> s<sup>–1</sup>. The PGP system proposed in this work represents a promising first
step toward the development of an O<sub>2</sub>-tolerant solar energy
conversion system
Molecular Assembly of Zinc Chlorophyll Derivatives by Using Recombinant Light-Harvesting Polypeptides with His-tag and Immobilization on a Gold Electrode
LH1-α
and -β polypeptides, which make up the light-harvesting
1 (LH1) complex of purple photosynthetic bacteria, along with bacteriochlorophylls,
have unique binding properties even for various porphyrin analogs.
Herein, we used the porphyrin analogs, Zn-Chlorin and the Zn-Chlorin
dimer, and examined their binding behaviors to the LH1-α variant,
which has a His-tag at the C-terminus (MBP-rubα-YH). Zn-Chlorin
and the Zn-Chlorin dimer could bind to MBP-rubα-YH and form
a subunit-type assembly, similar to that from the native LH1 complex.
These complexes could be immobilized onto Ni-nitrilotriacetic acid-modified
Au electrodes, and the cathodic photocurrent was successfully observed
by photoirradiation. Since Zn-Chlorins in this complex are too far
for direct electron transfer from the electrode, a contribution of
polypeptide backbone for efficient electron transfer was implied.
These findings not only show interesting properties of LH1-α
polypeptides but also suggest a clue to construct artificial photosynthesis
systems using these peptide materials