Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides. Class I RNRs are composed of two homodimeric proteins, α2 and β2. The class Ia E. coli β2 contains ...dinuclear, antiferromagnetically coupled iron centers and one tyrosyl free radical, Y122•/β2. Y122• acts as a radical initiator in catalysis. Redox-linked conformational changes may accompany Y122 oxidation and provide local control of proton-coupled electron transfer reactions. To test for such redox-linked structural changes, FT-IR spectroscopy was employed in this work. Reaction-induced difference spectra, associated with the reduction of Y122• by hydroxyurea, were acquired from natural abundance, 2H4 tyrosine, and 15N tyrosine labeled β2 samples. Isotopic labeling led to the assignment of a 1514 cm−1 band to the υ19a ring stretching vibration of Y122 and of a 1498 cm−1 band to the υ7a CO stretching vibration of Y122•. The reaction-induced spectra also exhibited amide I bands, at 1661 and 1652 cm−1. A similar set of amide I bands, with frequencies of 1675 and 1651 cm−1, was observed when Y• was generated by photolysis in a pentapeptide, which matched the primary sequence surrounding Y122. This result suggests that reduction of Y122• is linked with structural changes at nearby amide bonds and that this perturbation is mediated by the primary sequence. To explain these data, we propose that a structural perturbation of the amide bond is driven by redox-linked electrostatic changes in the tyrosyl radical aromatic ring.
Isotope studies show that the Vostok ice core consists of ice refrozen from Lake Vostok water, from 3539 meters below the surface of the Antarctic ice sheet to its bottom at about 3750 meters. ...Additional evidence comes from the total gas content, crystal size, and electrical conductivity of the ice. The Vostok site is a likely place for water freezing at the lake-ice interface, because this interface occurs at a higher level here than anywhere else above the lake. Isotopic data suggest that subglacial Lake Vostok is an open system with an efficient circulation of water that was formed during periods that were slightly warmer than those of the past 420,000 years. Lake ice recovered by deep drilling is of interest for preliminary investigations of lake chemistry and bedrock properties and for the search for indigenous lake microorganisms. This latter aspect is of potential importance for the exploration of icy planets and moons.
Pluripotent embryonic stem cells (ESCs) were first isolated nearly three decades ago from mice, yet efficient ESC isolation has been limited to rodents and primates to date. We report a novel and ...robust technique for isolating ESCs from mammalian pre-implantation embryos by altering the epigenotype of embryonic explants and using pressed zona pellucida-free blastocysts. We first examined this technique for murine ESC derivation. Compared with controls, murine ESCs were efficiently derived when explants were exposed to 1μM 5-azacytidine, an epigenetic modifier that causes DNA demethylation (56.1% vs 31.6%; P < 0.01). Mouse ESCs stained positively for alkaline phosphatase, expressed markers of pluripotency including Oct4, Rex1 and SSEA1 and formed teratomas when injected into Severe Combined Immuno-Deficient (SCID) mice. The approach was subsequently used for bovine ESC derivation. In bovine a higher concentration of 5-azacytidine (5 μM) was required to elicit a response. This technique resulted in up to 18 times more efficient isolation of pluripotent cells than traditional methods (71.4% vs 4.0%; P < 0.001). These putative bovine ESCs expressed OCT4, REX1 mRNA and SSEA-1 and SSEA-4 proteins; and were able to form embryoid bodies in vitro and teratomas when injected in Severe Combined Immuno Deficient (SCID) mice. This is the first report on derivation of ESCs with both in vitro and in vivo differentiation potential in a livestock species.
Recent models for water oxidation in photosystem II propose that His190 of the D1 polypeptide facilitates electron transfer from tyrosine YZ to P680 + by accepting the hydroxyl proton from YZ. To ...test these models, and to further define the role of D1-His190 in the proton-coupled electron transfer reactions of PSII, the rates of P680 + reduction, YZ oxidation, QA - oxidation, and YZ • reduction were measured in PSII particles isolated from several D1-His190 mutants constructed in the cyanobacterium Synechocystis sp. PCC 6803. These measurements were conducted in the absence and presence of imidazole and other small organic bases. In all mutants examined, the rates of P680 + reduction, YZ oxidation, and YZ • reduction after a single flash were slowed dramatically and the rate of QA - oxidation was accelerated to values consistent with the reduction of P680 + by QA - rather than by YZ. There appeared to be little correlation between these rates and the nature of the residue substituted for D1-His190. However, in nearly all mutants examined, the rates of P680 + reduction, YZ oxidation, and YZ • reduction were accelerated dramatically in the presence of imidazole and other small organic bases (e.g., methyl-substituted imidazoles, histidine, methylamine, ethanolamine, and TRIS). In addition, the rate of QA - oxidation was decelerated substantially. For example, in the presence of 100 mM imidazole, the rate of electron transfer from YZ to P680 + in most D1-His190 mutants increased 26−87-fold. Furthermore, in the presence of 5 mM imidazole, the rate of YZ • reduction in the D1-His190 mutants increased to values comparable to that of Mn-depleted wild-type PSII particles in the absence of imidazole. On the basis of these results, we conclude that D1-His190 is the immediate proton acceptor for YZ and that the hydroxyl proton of YZ remains bound to D1-His190 during the lifetime of YZ •, thereby facilitating the reduction of YZ •.
The effects of iron limitation on photosystem II (PSII) composition and photochemical energy conversion efficiency were studied in the unicellular chlorophyte alga Dunaliella tertiolecta. The quantum ...yield of photochemistry in PSII, inferred from changes in variable fluorescence normalized to the maximum fluorescence yield, was markedly lower in iron-limited cells and increased 3-fold within 20 h following the addition of iron. The decrease in the quantum yield of photochemistry was correlated with increased fluorescence emission from the antenna. In iron-limited cells, flash intensity saturation profiles of variable fluorescence closely followed a cumulative one-hit Poisson model, suggesting that PSII reaction centers are energetically isolated, whereas in iron-replete cells, the slope of the profile was steeper and the calculated probability of energy transfer between reaction centers increased to 0.6. Immunoassays revealed that in iron-limited cells the reaction center proteins, D1, CP43, and CP47, were markedly reduced relative to the peripheral light-harvesting Chl-protein complex of PSII, whereas the alpha subunit of cytochrome b559 was about 10-fold higher. Spectroscopic analysis established that the cytochrome b559 peptide did not contain an associated functional heme. We conclude that the photochemical conversion of absorbed excitation energy in iron-limited cells is limited by the number of photochemical traps per unit antenna
Here, we report a systematic measurement of cumulants, Cn, for net-proton, proton, and antiproton multiplicity distributions, and correlation functions, κn, for proton and antiproton multiplicity ...distributions up to the fourth order in Au+Au collisions at √sNN = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, and 200 GeV. The Cn and κn are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y, and transverse momentum, pT. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010–2017) at the BNL Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity (|y| < 0.5) and transverse momentum 0.4 < pT < 2.0GeV/c, using the STAR detector at RHIC. We observe a nonmonotonic energy dependence (√sNN = 7.7–62.4 GeV) of the net-proton C4/C2 with the significance of 3.1σ for the 0–5% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with √sNN. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ2, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ4, of protons plays a role in determining the energy dependence of proton C4/C1 below 19.6 GeV, which cannot be understood by the effect of baryon number conservation.
Iron–sulfur clusters in type I reaction centers Vassiliev, Ilya R.; Antonkine, Mikhail L.; Golbeck, John H.
BBA - Bioenergetics,
10/2001, Letnik:
1507, Številka:
1
Book Review, Journal Article
Recenzirano
Odprti dostop
Type I reaction centers (RCs) are multisubunit chlorophyll–protein complexes that function in photosynthetic organisms to convert photons to Gibbs free energy. The unique feature of Type I RCs is the ...presence of iron–sulfur clusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic phototrophs is the best-studied Type I RC. It is comprised of an interpolypeptide 4Fe–4S cluster, F
X, that bridges the PsaA and PsaB subunits, and two terminal 4Fe–4S clusters, F
A and F
B, that are bound to the PsaC subunit. In this review, we provide an update on the structure and function of the bound iron–sulfur clusters in Type I RCs. The first new development in this area is the identification of F
A as the cluster proximal to F
X and the resolution of the electron transfer sequence as F
X→F
A→F
B→soluble ferredoxin. The second new development is the determination of the three-dimensional NMR solution structure of unbound PsaC and localization of the equal- and mixed-valence pairs in F
A
− and F
B
−. We provide a survey of the EPR properties and spectra of the iron–sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summarize new information about the kinetics of back-reactions involving the iron–sulfur clusters.
Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone, the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp. PCC 6803 by comparison with ...genes encoding enzymes of the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 microE m(-2) s(-1)), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 microE m(-2) s(-1)). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mutant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement of polypeptides, show photoreduction of F(A) and F(B) at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A(1) site, where it functions as an efficient cofactor in electron transfer from A(0) to the iron-sulfur clusters.
Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME)—an ...electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ( Δγ) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy (v2) . We report here differential measurements of the correlator as a function of the pair invariant mass (minv) in 20–50% centrality Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider. Strong resonance background contributions to Δγ are observed. At large minv where this background is significantly reduced, the Δγ value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the v2 background shape as a function of minv . We extract a v2-independent and minv -averaged signal Δγsig = (0.03 ± 0.06 ± 0.08 ) × 10 -4, or (2 ± 4 ± 5)% of the inclusive Δγ(minv > 0.4 GeV/c2) = ( 1.58 ± 0.02 ± 0.02 ) × 10-4, within pion pT = 0.2 –0.8 GeV/c and averaged over pseudorapidity ranges of -1 < η < - 0.05 and 0.05 < η < 1. This represents an upper limit of 0.23 × 10 -4 , or 15% of the inclusive result, at 95% confidence level for the minv -integrated CME contribution.
Here we present high-precision measurements of elliptic, triangular, and quadrangular flow v2, v3, and v4, respectively, at midrapidity for identified hadrons π, p, K, φ, Ks, Λ as a function of ...centrality and transverse momentum in Au+Au collisions at the center-of-mass energy √sNN = 200 GeV. We observe similar vn trends between light and strange mesons which indicates that the heavier strange quarks flow as strongly as the lighter up and down quarks. The number-of-constituent-quark scaling for v2, v3, and v4 is found to hold within statistical uncertainty for 0–10%, 10–40%, and 40–80% collision centrality intervals. The results are compared to several viscous hydrodynamic calculations with varying initial conditions, and could serve as an additional constraint to the development of hydrodynamic models.
PDF
